By Cindy A. Thatcher, Hans Karl Heidemann, Jason M. Stoker and Diane F. Eldridge, U.S. Geological Survey
The problem
Elevation data serve a wide range of work: geologic resource assessment, hazard mitigation, flood risk management, infrastructure and construction, and business. All of it benefits from high-resolution 3D elevation from lidar (light detection and ranging) and IfSAR (interferometric synthetic aperture radar).
The USGS 3D Elevation Program (3DEP) had been opportunity-oriented: it acquired lidar projects of all sizes, scattered across the country, depending on which partners answered an annual announcement. The result: gaps in the national elevation layer, or needless overlap between neighboring surveys.
The fix: a national index
3DEP is moving to a strategic, systematic plan for nationwide data. Announcing planned acquisitions in advance should also bring in more Federal and non-Federal partners and investment.
At its core is an authoritative national indexing scheme, which standardizes how elevation data are acquired and keeps investment aimed at 3DEP's goal: complete, high-resolution 3D elevation coverage of the conterminous United States, Hawaii and the U.S. territories.
| Where | Scheme |
|---|---|
| Alaska | already in use for IfSAR: 1-degree by 1-degree tiles, Alaska Albers Equal-Area Conic projection |
| Lower 48 States | 1-kilometer by 1-kilometer tiles — agreed by the 3DEP Working Group, an interagency group the USGS manages, for all future 3DEP collections |
| Hawaii and the territories | none yet — needed |
How the grid works
- Projection: Albers Equal Area (EPSG code 6350), with X, Y and Z in meters.
- Tile size: 1 square kilometer.
- Names: from the easting and northing of each tile's lower left corner, so tiles are easy to search — for example, w0002n0612.
- Groups: tiles can be grouped by county, State, hydrologic unit code and so on, but each tile belongs to one group only.
- Project boundaries: every new project is extended to whole tiles, and enlarged where needed so no gap is left between existing and planned lidar collections.
A lidar survey of the Atchafalaya Basin — a large wetland and swamp beside the Gulf of Mexico — flown in early 2013. Its original footprint (red) is extended to the nearest whole 1-km tile (yellow). Each black tile is 1 km². USGS.
From project area to whole tiles, then tiles added to close the gap with a neighboring survey. USGS.
Why not the U.S. National Grid
The Working Group chose this scheme over the U.S. National Grid because:
- its tiles are equal in area wherever they are; and
- it needs non-overlapping tiles of constant size, which in turn need one plane zone over each contiguous area — the conterminous States, Alaska, Hawaii, the territories. The U.S. National Grid does not meet that condition.
Why it pays: 3DEP cuts unit costs by pooling USGS and partner funding and by buying larger projects. Standard acquisition over larger areas gives users consistent, high-quality data and a better return on investment.
Rollout
- Once the scheme is public, stakeholders will be encouraged to use it for all lidar acquisitions.
- Fiscal Year 2018: a transition year.
- Fiscal Year 2019: full implementation — every 3DEP-supported project acquired and delivered in the national scheme, in the Albers Equal Area projection.
- If funding partners ask, data can also come in a local projection, such as UTM or State Plane. Informal talks with lidar vendors suggest that delivering both costs about 1 percent more.
- Training: by the 3DEP Working Group and the USGS network of National Map Liaisons.
What the USGS is doing now
On behalf of 3DEP's stakeholders, the USGS is:
- writing an implementation plan for the scheme;
- testing how to set project boundaries with the 1-km by 1-km tiles;
- creating outreach and educational materials;
- building new applications, starting with a web app that turns lidar acquisition footprints into the national tiles.
Questions: 3DEP@usgs.gov.
Sources
Based on The 3D Elevation Program National Indexing Scheme, by Cindy A. Thatcher, Hans Karl Heidemann, Jason M. Stoker and Diane F. Eldridge, USGS Fact Sheet 2017–3073, U.S. Geological Survey; a work of the United States government in the public domain. The map, the diagram and their captions are taken from the fact sheet's PDF, which the import had left out; the import also broke one paragraph around a stray heading, restored here.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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