Land use and land cover digital data

USGS Circular 895-A: Overview and USGS Activities

Questions regarding availability and ordering of US GeoData (all types of distributed by the u.s. Geological Survey) should be addressed to:

Technical questions and comments should be addressed to:

Any use of trade names and trademarks in this publication is for identification purposes only and does not constitute endorsement by the u.s. Geological Survey. USGS DIGITAL CARTOGRAPHIC DATA STANDARDS

User Services Section National Cartographic Information Center

507 National Center Reston, Virginia 22092

Chief, National Mapping Division

516 National Center Reston, Virginia B:

Digital Elevation Models C:

Digital Line Graphs from 1:24,000-Scale Maps D:

Digital Line Graphs from 1:2,000,000-Scale Maps E: Land Use and Land Cover Digital Data F:

Geographic Names Information System G:

Digital Line Graph Attribute Coding Standards

cartographic and geographic data produced and

Library of Congress catalog-card No. 83-600581

In recent years, the disciplines of cartography and geography have undergone a rapid and striking reorientation as the techniques for digital collection and manipulation of data have evolved from fledgling laboratory procedures into dominant and driving forces that now pervade the disciplines. niques have provided a variety of new and powerful capabilities to collect, manipulate, analyze, and display spatial data. However, this evolution also has introduced a number of new and complex problems. which is receiving particular attention at present, is the issue of digital cartographic data standards.

The developing digital cartographic and geographic techniques for over a decade and has taken significant steps to develop and define digital cartographic data that are being collected and archived in a national digital cartographic data base. standards are expressed in the form of specifications documents that were prepared to govern collection of the data and in the form of user guides that were prepared for distribution with the data.

In an effort to fulfill promulgation of Federal standards in the earth sciences, the documents have been assembled with explanatory text into this USGS Circular consisting of separately bound chapters. Circular describes some of the pertinent issues relating to digital cartographic data standards, cartographic data standards currently in use within the USGS National Mapping Division, and details USGS efforts to define national digital cartographic data standards.

Chapter A is an overview in which the major issues involved in developing digital cartographic data standards are discussed and the activities of the USGS related to digital cartographic data production and standards development are described in detail. that establish USGS in-house standards for the various types of digital cartographic data currently produced by the National Mapping Division--that is, planimetric data, digital land use and land cover data, and digital geographic names data.

This compendium of relevant material is prepared to serve as a benchmark and to assist ongoing efforts to establish acceptable standards and conventions for both Federal agencies and the public.

Dallas L. Peck Director,

Department of the Interior FOREWORD

One of the most pressing problems, and one

u.s. Geological Survey {USGS) has been actively

in-house standards governing the

Succeeding chapters comprise the pertinent documents lead agency requirements for

R. B. Southard Chief, National Mapping Division types of

digital This Circular is the result of the efforts of numerous individuals who have contributed to the research, development, and preparation of various digital cartographic and geographic standards for the National Mapping Division of the u.s. Geological Survey. The individuals named as chapter authors represent both the originators of the various concepts as well as the writers who expanded and clarified these ideas. either to the concepts or the writing, are of such magnitude as to warrant crediting as authors.

Atef A. Elassal was largely responsible for the original data structures and computer file formats that are used for the Digital Line Graphs and Digital Elevation Models. coding scheme was first developed by members of the Digital Applications Team under the direction of Robert B. McEwen. Geographic Names Information System was conceived and developed by Sam Stulberg and Roger L. Payne. Retrieval and Analysis System was developed by Robin G. Fegeas, K. Eric Anderson, Stephen c. Guptill, Cheryl A. Hallam, and William B. Mitchell. The small-scale Digital Line Graph data structure and attribute coding scheme was developed by Warren E. Schmidt and Michael A. Domaratz.

The Circular was compiled in part from various user guides and technical instructions of the National Mapping Division. These documents were originally prepared by several individuals; credit is acknowledged to G. Michael Callahan, A. Joan Szeide, William R. Allder, Vincent M. Caruso, Hugh w. Calkins, Donna Cedar-Southworth, and Cheryl A. Hallam. various guides, instructions, Circular format was performed with major assistance by Clark H. Cramer, Eloise R. Byrd, and Cynthia L. Cunningham.

We acknowledge these substantial contributions that have led to this publication. PREFACE

The compilation of the and other material into the

iv Foreword • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • iii Preface . • . . . • . . . . . • • • • • • • • • • . . . . . . . . . . . . . . . • . • • • • • • • • • • • • • • Abstract • • . • • • • . . . . . . • • . • . • • • • • . • • • • • • • • • • • • • • • • • • . • . . . • • • •

Source data characteristics Land use and land cover map ••••••••••••••••••••••••••••

Census county subdivision map •••••••••••••••••••••••••• Hydrologic unit map •••••••••••••••••••••••••••••••••••• Federal land ownership ••••••••••••••••••••••••••••••••• State land ownership map ••••••••••••••••••••••••••••••• GIRAS data structure elements •••••••••••••••••••••••••••••• Creation of the GIRAS files •••••••••••••••••••••••••••••••• Applications • . . • • • • • • . . . . • . • • • . • • • • . . . . • • • . • . • . . . • • . . • . . . . • Manipulation and analysis •••••••••••••••••••••••••••••• Display • . • • • • • . • • • • • . • • • . . • • • • • . • • • • • • • . . • • • • • • • . • • • • • .

The standard local UTM coordinate frame of reference ••• Map header . • . • • . • • • • • . • • . • • . . • • • • • • • • • • . . • • • . • • • . • . . . • •

Arc records subfile •••••••••••••••••••••••••••••••••••• 11 Coordinate subfile ••••••••••••••••••••••••••••••••••••• 11 Polygon records subfile •••••••••••••••••••••••••••••••• 12

Composite theme grid (CTG) data file format •••••••••••••••• 16 Binary CTG data file format •••••••••••••••••••••••••••• 16

Character composite theme grid (CTG) file format ••••••• 18

Appendix. Listing of CTG map header data • • • • • • • • • • • • • • • •.• • 21

Figure 1. Topological elements of a polygon map •••••••••• GIRAS file structure ••••••••••••••••••••••••••• 10 GIRAS map header structure ••••••••••••••••••••• 10 GIRAS section header structure ••••••••••••••••• 5. GIRAS arc records subfile •••••••••••••••••••••• 11 GIRAS coordinate subfile ·••••••••••••••••••••••

Binary CTG map header ••••••••••••••••••••••••••••• 17

Standard character-formatted CTG data file •••••••• 19 Character CTG map header •••••••••••••••••••••••••• 19

Topological elements of a polygon map •••••••••• 2.

GIRAS file structure ••••••••••••••••••••••••••• 3.

GIRAS map header structure ••••••••••••••••••••• 4.

GIRAS section header structure ••••••••••••••••• 5.

GIRAS arc records subfile •••••••••••••••••••••• 6.

GIRAS coordinate subfile ·•••••••••••••••••••••• 7.

a.

Table 1.

u.s. Geological Survey land use and land cover

Federal land ownership ••••••••••••••••••••••••••• 3.

Sample GIRAS data volumes from 1:250,000-scale

4. Map type codes for GIRAS data base •••••••••••••••
5. Sample text subfile for a Census County Page

GIRAS polygon records subfile •••••••••••••••••• GIRAS FAP subfile •••••••••••••••••••••••••••••• GIRAS FAP subfile creation ••••••••••••••••••••• GIRAS text record subfile ••••••••••••••••••••••

classification system for use with remote sensor data ••••••••••••••••••••••••••••••••••••

Subdivision GIRAS file ••••••••••••••••••••••••• 16

vi The undergoing a number of profound changes that center on the emerging influence of digital data for the preparation of cartographic materials and for use in geographic information systems. ments have led to the development by the USGS National Mapping Division of several documents that establish in-house digital cartographic standards.

In an effort to fulfill lead agency requirements for promulgation of Federal standards in the earth sciences, the documents have been edited and assembled with explanatory Circular. of the digital documents the digital cartographic data standards currently in use within the USGS, and details the efforts of the USGS related to the definition of national d_igital cartographic data standards. consists of several chapters; the first is a general overview, and each succeeding chapter is made up from documents that establish in-house standards for one of the various types of digital cartographic data currently produced. chapter 895-E, describes the Geographic Information Retrieval and Analysis System that is used in conjunction with the USGS land use and land cover classification system to encode, and analyze land use digital data.

Stephen c. Guptill, K. Eric Anderson, and Cheryl A. Hallam

This Circular describes some pertinent cartographic USGS Digital Cartographic Data Standards

LAND USE AND LAND COVER DIGITAL DATA

By Robin G. Fegeas, Robert w. Claire,

edit, manipuate, and land cover analysis

The u.s. Geological Survey (USGS) is currently producing land use and land cover maps and associated overlays for the entire United States. being digitized, edited, and incorporated into a digital data base. be available to the public graphic and digital form.

These maps will help satisfy a longstanding need for a consistent level of detail, standardization of categories, and consistent use of scales of compilation for a type of data used by government land use planners, land managers, and resource-management planners. this benchmark series of maps is completed, updating of the maps will provide a much-needed tool for analyzing trends, problems in local and regional areas throughout the Nation, and changes in land use patterns.

The set of land use and land cover and associated maps consists of land use and land cover, political units, hydrologic units, census county subdivisions, Federal land ownership, and State land ownership (optional).

The land use and land cover map is compiled to portray the Level II categories of the land use and land cover classification system Anderson and others (1976). categories of this land use and land cover classification system provide the user with a basic framework to which third- and fourth-level categories may be added.

The data will in both

documented by The Level II The associated maps portray either natural or administrative information and provide the user with the opportunity to utilize the land.use and land cover maps and data either individually or collectively to produce graphic or statistical data for the areas portrayed on the associated maps. structed in such a way that the graphical and statistical land use and land cover data can be related to other resource fields such as soils, geology, hydrology, and demography.

To provide the data in digital form, the Geographic Information Retrieval and Analysis System (GIRAS) veloped (Mitchell and others, 1977). data structure used in GIRAS to store the information is the end result of a series of evolving structures and, as such, reflects the judgment by the USGS concerning the presentation and format of polygonal data. to handle data in a grid cell form, data are also provided in a Composite Theme Grid (CTG) format.

The characteristics of the digital cartographic data base for land use and land cover and associated maps reflect the parameters used in compiling the maps. ping program is designed so that standard topographic maps at a 1:250,000 can be used as a base for compilation and reproduction. start of the mapping program, the USGS began compiling intermediate-scale topographic maps at a scale of 1 :100,000. The USGS anticipates preparing land use and land cover and associated maps at a scale of 1:100,000 and will release them to the open-file system when 1 :100,000- scale available.

The 1:250,000-scale mapping format is generally a quadrangle unit of 1° of latitude x 2° of longitude. scale mapping format has been established as a 30 x 60 quadrangle, normally a quarter of a 1:250,000-scale quadrangle. Both series use the Universal Transverse Mercator projection. This mapping system is con-

For those users better able

The land use and land cover map-

topographic map has been de-The

The 1:100,000- The basic purpose of this map is to provide land use and land cover data to be used either as data sources by themselves or in combination with the other data sets produced in the program. of the basic sources of land use compilation data is the NASA high-altitude U-2/ RB-57 aerial photo coverage, usually at scales 1:250,000-scale topographic map series is used as the base map for the compilation of the land use and land cover maps and the associated overlays, with the exception that the 1:100,000-scale topographic map base is used if that base map is available at the time the data set is released to the open file system by the USGS. and land cover data is performed on a film-positive base enlarged to a scale of approximately 1:125,000, ciated overlays are both compiled and digitized at a scale of 1:250,000.

Land use and land cover data compilation is based upon the classification system and definitions of Level II land use and cover shown in table 1.

All features are delineated by curved or straight lines that depict the actual boundaries of the areas (polygons) being described. depicting all Urban or Built-up Land (categories 11-17), Water (51-54), Confined Agricultural Land (24), and Strip Mines, Quarries, and Gravel Pits (75) is 4 hectare (ha). use and land cover have a minimum polygon size of 16 ha. Those sizes also are considered the minimum sizes to which polygons are digitized. Built-up Land and Water categories, the minimum width of a feature to be shown ie 200 m; that is, if a square with sides 200 m in length is delineated, the area will be 4 ha. consideration precludes the delineation of very narrow and very long 4-ha polygons, triangles or other polygons are acceptable if the base of the triangle or minimum width of the polygon is 200 m in length and if the area of the polygon is 4 ha. or Built-up Land and Water, the 16-ha LAND USE AND LAND COVER MAP

Although compilation of land use

For categories other than Urban One

The minimum size of polygons

All other categories of land

In the Urban or

Although the minimum-width Table 1.--u.s. Geological Survey Land Use

minimum size for delineation requires a minimum width polygon of 400 m. weight for delineating land use and land cover polygons and for neat lines is 0.10 mm at the production scales of 1:250,000 or 1:100,000. and Land Cover Classification System for Use with Remote Sensor Data

Line The political unit map provides a graphic portrayal of the county and State boundaries and is compiled using base maps at scales of either 1:250,000 or 1:100,000. litical unit maps is from the Bureau of the Census "County Places" and from the Geographic Identification Code Scheme (U.S. Bureau of the Census, 197 2a) and the County and City Data Book (U.S. Bureau of the Census, 1972b). The "County Subdivision--Townships and Places maps are used also to separate census county subdivisions into census tracts. litical subdivisions are encoded with a five-digit number in accordance with the Geographic with the exception that nontracted "independent cities" in Maryland, Missouri, Nevada, and Virginia are given an eight-digit code reflecting the State and city codes.

CENSUS COUNTY SUBDIVISION MAP

Subdivisions--Townships Source material for the po-

State and county po-

(u.s.

are encoded with an maps

Bureau of the entitled and

Minor The hydrologic unit map is based on the Hydrologic Unit Maps published by

the u.s.

Water Data Coordination, together with the list "Boundary descriptions and name of region, subregion, accounting units, and cataloging unit." units are encoded with an eight-digit number region subregion (second two digits), accounting unit (third two digits) , and cataloging unit (fourth two digits).

The USGS has the responsibility for researching, maps, plots, and other descriptive data related to Federal land ownership. Minimum size for the delineation is 16 ha. Ownership is encoded according to the agencies listed in table 2.

In instances in which the USGS has a cost-sharing cooperative agreement with a specific State, a map overlay showing an inventory of State-owned land is produced from data furnished by the State. Although this overlay is compiled to the same map base used for the other overlays, the polygons are encoded according to the referencing system used by the State.

The GIRAS digital data structure was designed to handle large quantities of m~p data Gf the polygon type. logical elements associated with polygon maps are shown in figure 1. A polygon is an area that is homogeneous in the characteristic being mapped. scribes a boundary either between two polygons or between a polygon and the outside of the map. arc begins at one node, or point common to three or more arcs (that is, an intersection), and ends at another node but HYDROLOGIC UNIT MAP

that (first two digits),

FEDERAL LAND OWNERSHIP MAP

STATE LAND OWNERSHIP MAP

GIRAS DATA STRUCTURE ELEMENTS Geological Survey Office of

obtaining, and formatting The hydrologic

Further defined, an hydrologic hydrologic

The topo-Table 2.--Federal Land Ownership

u.s. coast Guard

does not pass through any node. A special case is a simple island polygon totally surrounded by a larger polygon. For purposes of digitizing, an arbitrary point on the boundary of the island is chosen as the beginning and ending node of the arc. Each polygon label on the map is a code, not necessarily unique, that identifies or describes the polygon in which it is placed or to which it points.

EXPLANATION

Figure !.--Topological elements of a polygon map.

In the GIRAS structure, the common boundaries, or arcs, are digitized only once. The arcs are then linked together by editing software to form polygons.

POLYGON LABEL

Within the GIRAS data structure, the basic topological elements of a polygon map (arcs, nodes, and polygons) are all uniquely identified and cross-referenced to one another. an arc is given by a string of x,y points; the first point is its beginning (from) node,· and the last point is its ending (to) node. The sequence of points of an arc defines a direction that, since the arc separates two polygons, determines a polygon to the right and a polygon to the left. endpoint (first and (or) last) of three or more arcs (or the first and last endpoint of a one-arc island). polygon is spatially defined as the sequence of arcs that constitute its boundary, both external perimeter and any internal islands·

The data capture procedure involves the conversion of the source material into a digital format. GIRAS, the digitization process includes not only the initial conversion to digital form, but also the editing process by which clean or logically correct data files are produced.

In digitizing, lines are not tagged in any way, and all that is required is the intersections processing. cover maps consist entirely of polygons, the map is completely defined when each arc, including the arcs that are the boundaries of the map, has been digitized. that contains at least one attribute and a point inside the polygon (represented by an x,y coordinate pair) for each polygon of the map must be entered into the system. format at the end of the digitizing. Data are converted to the GIRAS format only when editing is finished.

After the necessary data have been captured, following steps to produce a GIRAS file:

1· Conversion of the data file to the standard editing format; The spatial location of

CREATION OF THE GIRAS FILES

capability (nodes) Since land use and land

Along with the line data, a file

These data are not in the GIRAS

the data A single node is the

to As defined by

go through A given

2. Data reduction, points not needed to define lines within a specified tolerance;
3. Splitting sections, if necessary;
4. Limited automatic editing and detection for the arc data;
5. Manual batch editing of line with returns to step 4 until are error free;
6. Merging of labeled polygon points with either further error detection or clean files;
7. Manual labels data with returns to steps 5, 6, or 7, if necessary; a. Edge matching of each section with neighboring sections and of each map with every available adjoining map;
8. Conversion of data files to standard GIRAS format; and
9. Transformation coordinates to system; where equals 10 m.

Spatial data in the GIRAS format can be applied to individual problems through manipulation such as:

1. Rotation, translation, and scaling of the coordinates; 2 • Conversion to geographic coordinates and from geographic coordinates to specified map projections;
2. Conversion from arc-segment polygon structure to grid cells of a specified size;
3. Conversion fonnat;
4. Production of area summary statistics from polygon or grid cell data;
5. Production of border (perimeter) and adjacency lengths of particular polygon types from the polygon data; and
6. Selection of a portion of a map for closer consideration by using procedures 1 through 6. the

MANIPULATION AND ANALYSIS of

to by elimination of

of data from table the UTM coordinate one coordinate unit

polygon The first two procedures in the list deal apply the data to needs of various users, it is often necessary to be able to rotate system and scale it to the desired size. Similarly, a facility to transform the data to another map projection is desirable, data exist on a different projection.

A number of existing data systems utilize data stored in grid cells. Thus, the ability to convert the polygon structure to a grid-cell format can be very useful.

The land use and land cover and associated map data files were converted to grid cells of a specific size and orientation to permit their addition to a existing data base. data to be used where the GIRAS data structure capability polygon and grid-cell formats provides a flexibility whereby the needs of more users can be met.

Area summaries for a GIRAS data set may be obtained directly from the file. However, if a further breakdown of the information (for example, land use within each political unit for a data set) is wanted, it may be derived more easily from grid-cell formatted information than from an arc-segment file. ciated maps as well as the land use and land cover map for a 1:250,000-scale quadrangle sheet are in GIRAS format, they may be converted to one file of grid-cell information (an explanation of the CTG data file format begins on page

16) from which the more detailed summary

As with the grid-cell format data, the GIRAS format data facilitate production of certain types of information. An environmental study might find it useful to know the length of the border between two noncompatible types of land use or land cover (procedure 6). if an area of industrial land borders a lake under study, the amount of lakeshore occupied by the industrial site with

inappropriate. to produce conversion.

For might be of interest. formation can be derived easily from the arc-segment GIRAS files.

While GIRAS stores polygons as the arcs of which they are composed, many information systems that deal with polygon data store those polygons as complete entities. two formats, conversion to a standard polygon format is necessary. this requires more storage than GIRAS' s format, it allows the use of a simpler set of software for plotting, perimeter calculation, and area calculation.

Another technique for extracting information from the GIRAS files is the selection of an area smaller than the standard 1° x 2° map data file (procedure 7).

It is often helpful to select data from the files for closer consideration. This windowing process reduces the amount of information handled by eliminating the portions of the map that are of no interest to the study. tion has been selected, any of procedures 1 through 6 may be applied.

Computer-generated graphics may be used to augme~t the manipulations and analyses computer-generated ~haded color plot of the Level II land use and land cover for a data set along with a summary of the land use gives the investigator a spatial perspective of the distribution of land use and land cover over the area mapped. If it would be more helpful to see only one or more of the land uses displayed, they can be specially selected. lengths of borders between two specific land uses or land covers are under study, a plot showing only those two land uses or land covers would be helpful.

The detailed summary of land use and land cover by county might be illustrated by placing the outline of the county over the land use and land cover map. pattern of land use and land cover for that county can be seen with the areas of the different land uses computed and displayed. To bridge the gap between the

Once this smaller por-

described This type of in-

The Large sets of complex spatial data, such as those handled by GIRAS, necessitate an efficient data structure. 3 shows some measure of GIRAS data volumes derived from records of data editing procedures. (two coordinates, x and y, per point) are those defining the arcs and reflect the

Table 3.--Sample GIRAS data volumes from

(maps evaluated)

Land Use and Land Cover

Coordinates (no.) 54 Arcs (no.) Polygons (no.) Arc length

Coordinates (no.) 47 Arcs (no.) Polygons (no.) Arc length (m)

Coordinates (no.) 45 Arcs (no.) Polygons (no.) Arc length (m)

Coordinates (no.) 45 Arcs (no.) Polygons (no.) Arc length (m)

Coordinates (no.) 33 Arcs (no.) Polygons (no.) Arc length (m)

The numbers of coordinates

(m)

Federal Land Ownership Average

The GIRAS file format is a specific physical implementation of the logical GIRAS data structure. designed to optimize storage requirements, transfer operations, and sequential processing. computing environment based on 32-bit words, a GIRAS file is organized sequentially as 32-byte logical factor is appropriate for the particular storage device (disk or tape) and computing environment use. record may consist of anywhere from 1 to 16 data-element fields, depending upon where, within the file, it is located. Each data-element field may contain only one of three types of data elements: (1) a 2-byte (16-bit) binary integer, (2) a 4-byte (32-bit) binary integer, or (3) a string of EBCDIC coded characters, 1-byte per character.

For ea.se of transfer to other computing environments, binary file format may be translated into a card image character (coded in either ASCII or EBCDIC) file format. byte binary record is expanded to an SO-character card image; a 2-byte binary integer field decimal-digit character field, and a 4- byte binary integer field is translated into a 10-decimal-digit character field. The EBCDIC coded characters within a binary-formatted The actual physical size of a GIRAS is basically a

records, As implemented in a

(S bits per byte)

file function of the

s

without expansion, allowing two sequentially adjacent 32-byte records containing character data to be converted into one SO-character card image record.

A GIRAS file, either in binary or character format, logically consists of six or more subfiles. The general structure of a GIRAS file is shown in figure 2; details are shown in figures 3-s. The first six subfiles always exist (in the order shown in figure 2); the seventh, the text subfile, may or may not exist for data released by the National Cartographic Information Center (NCIC); the eighth, the associated data subfile, is meant for user-attached data and is never present in NCIC-released data. A map

c

F

Figure 2.--GIRAS file structure. data file may need to be divided spatially into several parts (sections) for processing purposes. the second through sixth subfiles (fig. 2, B through F) may be repeated, one set of five subfiles per section. subfile (only one per GIRAS file) may follow the final section subfile.

A GIRAS file contains a number of different types of data elements--text, codes, identifiers, counters, poifters, and some derived measurement data. of these data elements will be explained under the descriptions of the various subfiles below. data, the coordinate data, needs an overall discussion here. stored within a GIRAS file are coded as 2-byte (16-bit) integers. frame of reference is defined in the map header of the file by a projection code (MPJ, see fig. 3) and six control points. For each of the control points, the internal x,y coordinates are equated with the geographic (latitude and longitude) coordinates of the point. of an internal coordinate unit is indicated by the map scale (MSC) value in the map header. denominator of a graphic plot of the GIRAS file, if the data were plotted at one internal coordinate unit per mil (0.001 in.) on the plot.

A GIRAS file containing data based on a standard USGS 1:250,000- or 1:100,000- scale quadrangle (such as the land use and land cover and associated map series) is routinely stored in a local (to the map) UTM coordinate system (MPJ = "1" for UTM). store full UTM coordinates (which may exceed 4,000,000 m), the nearest 100,000-m UTM grid intersection, west and south of all map control points, is used as a local origin (x,y = 0,0). Further, the resolution of an internal coordinate unit is set to 10 m (the MSC value dinate referencing system, a GIRAS file may store THE STANDARD LOCAL UTM COORDINATE FRAME OF REFERENCE

This value is the scale

Since 16 bits are not enough to

data For this reason,

covering a one type of

The resolution A text

327,680-m square, more than enough for 1:250,000- and 1 :100, 000-scale data.

The map header (fig. 3) contains a substantial amount of information, including the amount of data in the file, the date of the source material, title information, and ground control information. NP, and LFP represent totals of the corresponding values for each section of the map data file. sum of the NAS (number of arcs in a section) values for all sections of the map data file. estimate the length of the file before analysis. arc length tolerance (ATL) are values used during processing and editing of the data to eliminate spurious or unneeded data. coordinate units, of a corridor that was used to delete unnecessary points from each arc of the map and reflects the relative accuracy of the original graphic lines. length (in internal coordinate units) of any arc on the map. type codes (MTP) are listed in table 4.

Table 4.--Map type codes for GIRAS data

The map date (MDA) is the year of the source material used to make the map, which is usually not the same as the year The values in variables NA, NC,

The point tolerance (PTL) and

PTL is the width,

ATL is the m~n~mum allowable

Land Use and Land Cover Political Units Census County Subdivisions Hydrologic Units Federal Land Ownership (optional) State Land Ownership (optional)

For example, NA is the

These values can be used to

MAP HEADER

c.

SECTION HEADER

Figure 3.--GIRAS map header structure. the map or data set is published. The number of characters (NCH) in the title gives information to ease the reading of the map title (TITLE) that occupies the first NCH bytes of the final two records of the map header.

Because of computer constraints, the number of x,y coordinates (twice the number of arc points) of a section of a map dat? file is held to 32,000; the numbet of arcs, to 2,500; the number of polyg6ns, to 1,500; and the total length of a FAP (file of arcs by polygon) subfile, to 6, 000. Where these limits are exceeded within one map data file, the map area is broken into more than one section.

Four elements near the end of the section header (fig. 4; XMNS, YMNS, XMXS, and YMXS) are the coordinate limits of the section, and indicate the minimum and maximum x and y coordinate values within the section.

sE

c

Figure 4.--GIRAS section header structure.

Each record of the arc records subfile (fig. 5) contains a pointer (PLC) to the x,y coordinates that represent the arc in the coordinate subfile. one PLC value for each arc, and it represents the position within the coordinate subfile of the last coordinate of that arc. For example, if the first arc contained 6 points (12 coordinates), its PLC value would be 12, and if the second arc contained 8 points (16 coordinates), the PLC value would be 28 (12+16). this pattern, the last arc would have a PLC value equal to the total number (NCS) of coordinates in the coordinate subfile. Along with the PLC value, each record in the arc unique numbers (PL and PR) of the polygons that this arc separates and the types (PAL and PAR) of polygons between which this arc forms the border.

ARC RECORD

Length Name (bytes)

XMNA, YMNA XMXA, YMXA ALEN SN FN

Figure s.--GIRAS arc records subfile.

records subfile contains the

2 ea. Minimum x,y coordinates

2 ea. Maximum x,y coordinates Description

Arc number. Position of last arc coordinate in COORDINATE subfile. Polygon number of polygon to left of arc. Polygon number of polygon to right of arc. Attribute of polygon to left of

arc.

Attribute of polygon to right of arc.

in arc.

in arc. Arc length in coordinate units. Node number at beginning of arc. Node number at end of arc.

s F

The coordinate subfile (fig. 6) is simply a sequential listing of every x,y coordinate needed to represent the arcs of the map section. When a map is digitized, a series of x,y recorded for each arc of the map. though the direction taken by the digitizer during recording is not significant, that direction--once determined by digitizing--becomes important in the data structure. is referred to as the positive direction

Variable (bytes)

Yn

COORDINATE (PLC(I-1)+1)

COORDINATE (PLC(I))

The order within the COORDINATE subfile of the (x,y) points of a given arc determines the direction of the arc, and therefore the right and left of the arc. The first point in an arc (x,y) string is its "FROM" or ''START'' node; the last point in an arc (x,y) string is its "TO" or "END" node.

Figure 6.--GIRAS coordinate subfile.

The direction of recording

From coordinates is Al-

x integer coordinate value of point n y integer coordinate value of point n

is the x coordinate value of the first point of arc I. [PLC(O)=O]

is the y coordinate value of the last point of arc I.

RIGHT for the arc, and each arc can be referred to by a positive or negative representation of its arc identification number (AID). read from its starting node (SN) to the node (FN) at the end of the arc, it is represented as "+l" or 1. to be read from FN to SN (in reverse order from that stored in the Coordinate subfile), it may be represented by "-1." The need for this will become apparent when the FAP subfile is described.

To repeat, the order in which the arcs are recorded is not significant, but their order, once established, is important to the GIRAS structure. allows each arc to be accessed by using only the unique arc number (AID) and its PLC value. coordinate of an arc exist in the coordinate subfile. for arc 1 is 16, then the 17th element of the coordinate subfile will be the first coordinate of arc number 2. storage of the x,y coordinates in the smallest possible space.

The polygon records subfile (fig. 7) describes each polygon of the map section. (PID) is the unique number by which each polygon may be referenced. (equivalent in purpose to PLC in an arc record) points to a list, in the FAP subfile, of the arcs that comprise that polygon' s borders. dinate subfile (referenced by PLC), the FAP subfile is a information. tains the list of the numbers (AID's) of the arcs needed to create the boundary of the polygon. first for polygon 1, second for polygon 2, and so on for each polygon. gon 1 of a section were the three-sided polygon with attribute 76 in figure 1, it would be composed of three arcs, and the PLA value for it would be "3." gon 2 were the polygon in figure 1 labeled slightly more complex because the FAP subfile entry for this polygon must also No pointers to the beginning

The polygon identification number

42, For example, if arc 1 is

If the arc is

For example, if the PLC

As with the coor-

sequential list of For each polygon it con-

These values are listed

the situation would If poly-

c c

Length Name (bytes)

CX,CY 2 ea. x,y coordinates of an interior

XMNP, YMNP XMXP, YMXP PERL

Figure 7.--GIRAS polygon records subfile.

The record are the x,y coordinates (CX and CY) of a point inside the polygon. coordinates do not represent a centroid or center of mass. inside the boundary of the polygon that 2 ea. Minimum x,y coordinates of

2 ea. Maximum x,y coordinates of

In this case, the value of PLA

A detailed explanation of the

Polygon number. Position of last arc number of polygon in FAP subfile.

point (arbitrarily positioned). Polygon attribute. Area of polygon in coordinate units squared.

polygon.

polygon. Perimeter length of polygon in coordinate units. Number of islands contained within polygon. Number of the polygon containing this polygon, if it is an island.

(LFS in the section

entries The PLA value for

They define a point

w p

These was process process to tie the arcs together to form the polygon. and polygon area (AREA) also are included, as well as the minimum and maximum x and y coordinates (XMNP, YMNP, XMXP, YMXP) of the polygon. a polygon is useful information and is stored limits of the polygon allow the user to-- in a simple way--isolate a polygon from much of the rest of the map section. The perimeter length (PERL) of the polygon can be used along with the total area occupied by the polygon to compute measures of the compactness of the polygon.

The final two entries in the polygon record are the number of islands (NIW) contained within the polygon and the number (NIP) of the polygon that contains another polygon as an island. number of islands is helpful when used along with PLA to read the island entries in the FAP subfile. tifies a polygon totally surrounded by another easily be obtained by looking at the value of ATT in the record for polygon number NIP. pressed and small polygons to be eliminated or combined with larger surrounding polygons, NIP would show quickly which polygon number and type that small island would become.

The FAP (file of arcs by polygon) subfile (fig. 8) is the last subfile that exists separately for each map section. It is accessed by way of an entry (PLA) in the polygon records subfile. total length (LFS) of the FAP subfile is stored in the section header. stand the contents of the FAP subfile, it is helpful to know how its contents are used to construct a polygon. subfile consists of lists of arcs, one arc-list per polygon. polygon with no islands are listed in clockwise order around the polygon from an arbitrary starting point. ing point can be either the starting node (SN) or final node (FN) of the first arc recorded and used during the

The polygon attribute (ATT)

If the data were to be com-

FAP SUBFILE recalculation.

The NIP entry iden-

The arcs for a

F AP (PLA(I)) is the last arc bordering polygon I.

Within the FAP subfile, the identification numbers of the arcs constituting a given polygon are ordered clockwise around the perimeter of the polygon and counterclockwise around interior islands of the polygon.

A negative arc identification number indicates the polygon is to the left of the arc.

A zero FAP element indicates that the arc identification numbers following it,. and before the next zero in the subfile or the end of the FAP list, point to the arcs that constitute an interior island of the polygon.

Figure 8.--GIRAS FAP subfile.

in the list. If the starting point is SN for the arc (the polygon to the right of the arc), then the number of that arc is recorded in FAP as "+AID" or ''AID.'' If the starting point is FN (the polygon is to the left of the arc), then that arc is recorded as "-AID." In figure 9, the direction in which the arcs of a polygon were digitized (positive direction) is indicated by the filled arrows. The list of arcs composing the polygon begins at the node closest to the internal point (CX, CY), and the FAP entry for this polygon is "-1 4 3 -2."

* (CX,CY)

Figure 9.--GIRAS FAP subfile creation.

The FAP subfile contains a series of elements for each polygon whether or not that polygon is an island. gon is an island, a list of the arcs that

REMAINDER OF MAP

If the polymake up its outside boundary will also be listed in counterclockwise order in the FAP entry for the surrounding polygon in which it is an island.

This points out one factor not yet mentioned in describing the FAP subfile. In many maps there may be two types of islands--simple and complex. Examples of both island types can be seen in figure 1.

A simple island is a polygon that stands alone, totally surrounded by one other polygon and directly bordered by only that polygon. A complex island is a cluster of adjacent polygons that, as a group, other polygon. up a counterclockwise order--in the FAP subfile, only the arcs that compose the outside boundary of the entire cluster are recorded. the polygons that make up the cluster are not maintained because the cluster is considered to be an area to exclude from the polygon being described.

The FAP subfile is crucial to the GIRAS data structure because it is the way individual arcs can be described as part of the polygons they compose. this file the entire outline of a polygon can be obtained, including island areas within its boundary that are to be excluded. example, when computer-generated shaded color plots are made.

The text subfile in a GIRAS file is reserved as a place where numerical polygon codes are assigned textual labels. An individual GIRAS file includes only a single follows the data for the various map sections.

Every record in the text subfile consists of three elements--an attribute code, a hierarchy code, and a descriptor (fig. 10).

A text subfile record format of (A4, A2, 13A2,/16A2) was designed to be compatible with the standard binary representation of GIRAS files. text subfile entry comprises two GIRAS 32-byte records.

Included in the text subfile are records for general codes that provide are totally surrounded by one

The individual identities of

This information is used, for

text When the arcs that make island

Each logical Name (bytes)

Figure 10.--GIRAS text record subfile.

labels general categories under which specific polygon codes come. made to Level I and Level II land use and land cover categories. as "13" is known to be Industrial. digit '1' indicates that it falls under a broader category of Urban and Built-up Land. in the text subfile to provide definitions for these more general categories. For the above example, a text subf ile record with "10" in the polygon code (ATT) field would have the descriptor Urban and Built-up Land. positional purposes only and can be considered insignificant.)

The role of the hierarchy code is more appreciated Length

General code records also appear

DESC

Polygon code, right justified in a 10-digit field. A 5-digit parameter to indicate polygon code type (that is, specific, general, or special). A label or descriptor defining the polygon code, left justified, in a 58-character field.

Our sample land use and

lURBAN AND BUILT-UP LAND

(The "0" is for

other classification schemes utilized by GIRAS that involve multiple levels.

Text subfile records are arranged in ascending order by polygon code. quently, general codes of greater detail, which in turn precede codes explicitly referenced in the file. sidered an indentation-type structure, as illustrated in table 5 for a census unit subfile.

Special polygon codes are required where available codes are not suitable. The set of special codes used in GIRAS is defined at the end of the subfile, as also shown in table 5. codes were intentionally assigned extreme values (greater than 2,000,000,000) to force them to the bottom of the text subfile.

The type of map stored in a GIRAS file has an impact on the nature of the text subfile. and Federal ownership maps classification schemes that are attribute in nature. relatively limited number of possible codes (37 for land use and land cover and 26 for Federal ownership), and many polygons may have the same codes. subfiles for these types of GIRAS files list text records for all codes in the classification scheme. Consequently, all land use and land cover text subfiles are identical, as are Federal ownership text subfiles.

Census unit files, files, and hydrologic unit files, alternatively, utilize codes that serve as unique identifiers. have the same code. would not be feasible to list all possible codes. explicitly referenced in the file (and respective general and special codes) are defined in these types of GIRAS text subfiles.

The text subfiles for State ownership maps are not processed because coding schemes vary from situation to situation.

The length of the text subfile is given by the LTX parameter in the GIRAS map header. logical text subfile records (that is, polygon code definitions) and includes general and.special polygon codes. more

This ordering can be con-

Land use and land cover

LTX refers to the number of general

That is, there are a

Two polygons rarely codes precede

it Table 5.--Sample Census GIRAS file

The final GIRAS subfile is the associated data subfile that allows the user to store information of his own that text subfile for a County Subdivision

6PENNSYLVANIA 3FAYETTE COUNTY OLURERENE 3GREEN COUNTY OALEPPO

OWAYNE OWHITELEY 6WEST VIRGINIA 3BARBOUR COUNTY OBAKER OELK OPHILIPPI

6PITTSBURGH 2WASHINGTON OWASHINGTON 2WASHINGTON OWASHINGTON

6WHEELING 2MARSHALL OMARSHALL 2MARSHALL OMARSHALL

9SPECIAL CODES 2AREA UNDEFINED BY CODING SCHEME OLAND AREA UNDEFINED BY CODING SCHEME OWATER AREA UNDEFINED BY CODING SCHEME 2UNMAPPED AREA OUNMAPPED U.S. AREA OUNMAPPED NON-U.S. AREA pertains to the file (for example, population information that pertains to a particular census map). to this subfile provides a place in the map header subfile to store the number of records in the associated data file once it is created.

Digital data from all the overlays of a given quadrangle also are combined in a raster or grid-cell format as a Composite Theme Grid (CTG) file.

A CTG file is available in either binary or character format. format, the files are sequential and consist of fixed-length logical records, all of identical internal format, one grid cell per logical record. are actually a regular point sample. attribute codes at the center point of each cell are recorded from each overlay. The points are oriented to the UTM projection and are usually spaced 200 m apart in both east-west and north-south directions. ordered in the file by row from north to south, then within each row, by column west to east.

Header data, usually physically separate from the CTG data file, contain the following data necessary to complete the description of an individual CTG data file:

1· Title; 2.

Number overlays are represented;

3. Numbers of rows and columns; 4. UTM zone number; 5. UTM Easting of the west edge of cells; UTM Northing of the north edge of cells; and 1. Cell width in meters.

Each logical record of a binary CTG file is 52 bytes in length. <:onsists of 13, 32-bit (4-byte) binary integers in the following order:

COMPOSITE THEME GRID (CTG) DATA FILE FORMAT

BINARY CTG DATA FILE FORMAT The cell records are first

of overlays A pointer (NAD)

A record Bytes 1- 4

If a given overlay has not been digitized, the codes for that overlay will all be zero. .grid of cells (form-ing a UTM rectangle} covers the entire base map quadrangle, a "buffer zone" of cells with all zero attributes has binary CTG data file.

The header data associated with a binary CTG data file are stored in a physically modeled after the GIRAS fonnatted map header. byte logical records. the first four records are coded as either 16-bit (2-byte} or 32-bit (4-byte} binary integers. and sixth records is coded as a string of EBCDIC characters (one character per byte}.

Binary CTG Map Header

The header consists of six, 32- Row index, where 1 is the index of the northern-most row and index numbers increase by one for each row moving south (NOTE, due to a processing files in which the State ownership is not coded will have all zero row index numbers 1 the row index is then a sequential position of the record within the file}; Column index, where 1 is the index of the westernmost column and index numbers increase by one for each column moving east; Land use and land cover code; Political unit code; Hydrologic unit code; Census county subdivision or SMSA tract code; Federal code; State land ownership code 1 and Null (binary zeros} fields.

To be sure that a regular

The title in the fifth error,

All elements in of

the Record 1:

Record 2:

Record 3:

Bytes 1- 4 5- 8

31-32 Number of rows; Total number of cells x 2; Number of columns; Meaningless field licate point tolerance}; Cell length) in meters; Number merged; Map below} 1 Projection zone number; Map (should be "1" for UTM)~ Scale of a plot at one mil per cell width; and Source date of the land use overlay.

Minimum column index; Minimum row index; Maximum column index; Maximum row index; Column index for SW control point; Row index for SW control point; Column index for NW control point; Row index for NW control point; Column index for NC control point; Row index for NC control point; Column index for NE control point; Row index for NE control point; Column index for SE control point; Row index for SE control point; Column index for SC control point; and Row index for SC control point.

Latitude of SW control point; size

code Record 4:

Records 5 and 6:

Some further explanation · is needed for some of the elements in the CTG map header:

The map type code (in bytes 19-20 of the first overlays have been included in the CTG data file. by the addition (in base 10) of the separate GIRAS map type codes for each of the overlays:

Land use and land cover;

Political units;

Census and SMSA tracts; Hydrologic units; Federal land ownership; and State land ownership. Longitude of SW control point; Latitude of NW control point; Longitude of NW control point; Latitude of NC control point; Longitude of NC control point; Latitude of NE control point; and Longitude of NE control point.

Latitude of SE control point; Longitude of SE control point; Latitude of SC control point; Longitude of SC control point; UTM west edge of cells; UTM Northing north edge of cells; File Julian date); and Meaningless field of file creation).

Title in EBCDIC coding).

record)

The code is formed value

(a For example, the map type code for a combination overlays above would be 17; all six overlays combined have a map type code of 77.

The UTM Easting and Northing values given in the fourth record (bytes 17-24) are in whole meters and are values for the west and north edges of the cells, rather than the center point corner) Northing values for a given cell may

where XORG and YORG are the Easting and Northing values in bytes 17-24 of the fourth header record, and CW is the cell width in bytes 15-16 of the first header record.

The control points usually define the 1° x 2° (for 1:250,000-scale base maps) or 30 x 1° (for 1 :100,000- scale base maps) quadrangle on which the overlay data are based. latitude and longitude values are given as positive integers of the form DDDMMSS, where DDD is degrees, MM is minutes, and SS is seconds. Western longitude values are given as positive numbers, increasing in value from east to west. column values given for the control points are the indices for the cell whose center point is closest to the true position of the control point.

CHARACTER COMPOSITE THEME GRID (CTG)

For ease of transfer and readabiltiy, CTG files also are available in character-coded Each binary integer value or code has been converted to a number, right justified of cell.

(ASCII or EBCDIC)

(with leading blanks if needed)

(YORG+CW/2) - (row the

The row and four

format. within a fixed-length field of an so-character data files consist of fixed length so-character logical records, one grid cell per logical record. formats are available--one with row and column index numbers such as the binary CTG format and the second (and standard) format Northing values in place of row and column indices.

The row and column number format is a direct copy of a binary CTG file with each 4-byte binary integer represented as a teger. the binary CTG data records are not copied. in this integer fields--row and column indices followed by the six attribute codes in the same order as in the binary CTG data file.

Each logical record of a standard character-formatted CTG data file is SO characters in length and consists of nine decimal integ~rs, right justified (with leading fields:

Bytes 1- 3 record.

10-digit character-formatted in-The null fields (bytes 33-52) of

Each character CTG data record format

Standard Character-Formatted CTG Data File

blanks)

41-50 51-60 UTM zone,

UTM value should be the same in every given first byte will always be a blank for zones in the northern hemisphere; U'IM whole sample point of the cell; UTM Northing whole sample point of the cell; Blank; Land use and land cover attribute code; Political State/county) code; USGS hydrologic unit code; Census county subdivision or SMSA tract code;

of file);

in If a given overlay category has not been included within the file, the codes for that category will be zero Since some misregistration of map overlays occurs, some of the cells along. the edges of the 1:250,000- or 1:100,000- scale quadrangle may have codes for some overlays, but not others (the "other" code(s) character CTG data file will have only those cell records for which at least one of the categories is coded. that, since the 1:250,000- and 1:100,000- scale quadrangles do not form perfect rectangles in the UTM projection (lines of latitude curve and lines of longitude converge), a variable number of cell records will exist for any given row or column.

A listing of the CTG map header data, which is meant to be delivered to a user with the CTG data file on tape, lists all the elements (see the list under the description of a binary CTG map header) of the header (but not in exactly the same order as in an actual CTG map header).

A sample listing of CTG map header data character conversion program CTGBTA) is shown in the Appendix. 61-70

will be zero).

Character CTG Map Header

leading blanks).

(as produced by the Federal agency code; and State land ownership code.

(1) as a separate

In either of the

(O).

binary-to-Anderson, J. R., J. T., and Witmer, R. land use and land cover classification system for sensor data: Professional Paper 964, 28 p. Mitchell,

w. B.,

Anderson, K. E., Fegeas, R. G., and Hallam, c. A., 1977, GIRAS--A geographic information analysis system for handling land use and Geological Survey Professional Paper 1059, 16 P• REFERENCES

land Hardy, E. E.,

cover E., 1976, A

s.

data: u.s. Bureau of the Census, 1972a, Census

c.,

u.s.

tracts--1970 census and housing: Census Final through PHC(l)-241. each SMSA.]

book, 1972: Census, 1020 P•

tion code scheme--1970 census of population and housing: the Census Reports PHC(R)-1 through PHC(R)-4. [One volume for each of the u.s. regions: NE, NC, s, and w.]

U.S. Bureau of the Reports

u.s.

and city Bureau of

u.s. Bureau of PHC(l)-1

THE FILE CONSISTS OF 80 CHARACTER RECORDS, ONE GRID CELL PER RECORD.

UTM ZONE, EASTING, AND NORTHING VALUES ARE PART OF EACH CTG DATA RECORD AS THE FIRST THREE INTEGERS, RIGHT JUSTIFIED IN BYTES 1-3, 4-11, AND 12-19.

BYTES 21-80 OF EACH RECORD CONTAIN THE USGS 10-DIGIT INTEGER CODES, RIGHT JUSTIFIED WITHIN 10-BYTE FIELDS, FROM THE FOLLOWING OVERLAYS, IN ORDER:

LAND USE/LAND COVER, POLITICAL UNIT, HYDROLOGIC UNIT, CENSUS SUBDIVISION/TRACT, FEDERAL LAND OWNERSHIP, AND STATE LAND OWNERSHIP.

ONLY RECORDS WITH AT LEAST ONE NON-ZERO ATTRIBUTE ARE PART OF THE FILE. (A VARIABLE NUMBER OF RECORDS EXIST FOR A GIVEN ROW OR COLUMN.)

Listing of CTG Map Header Data

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