Geocaching for Science: How Treasure Hunters Help Keep America's Maps Accurate

Geocaching is a treasure hunt played with GPS: players navigate to a set of coordinates and search for the "geocache" hidden there, usually a container or marker. Some geocachers hunt for something more useful. When they find an old survey benchmark and report it, they perform a real public service.

A survey mark. Some caches are hard to spot, even with the coordinates in hand. Credit: NOAA.

What a benchmark is

A benchmark is a brass, bronze or aluminum disk set in bedrock or concrete — a kind of survey mark. Together, benchmarks form a network surveyors use to position other points precisely. That network makes sure everything lines up when people make maps, mark property boundaries, and plan, design and build structures, roads and bridges.

The system behind every map

Benchmarks are part of the National Spatial Reference System (NSRS), the foundation of all mapping and surveying in the United States. NOAA's National Geodetic Survey (NGS) defines and maintains it. For more than 200 years, NGS and the agencies before it have worked with public and private surveyors to set benchmarks across the country — more than 1.5 million of them between the 1920s and the 1980s.

Why NGS wants to hear from geocachers

Many benchmarks are nearly a century old. Over the years many have been disturbed and reset, and as many as half may have been destroyed by construction. So NGS wants to know when geocachers find one. A geocacher who locates a benchmark, measures its coordinates with a handheld GPS device and reports it to the National Geodetic Survey helps keep the nation's reference network accurate.

Sources

Based on "What is geocaching?" from NOAA's National Ocean Service, a work of the United States government in the public domain.