When looking at a US Geologic Survey (USGS) map the hiker will find benchmark symbols sprinkled across the topo. Benchmark and the many other symbols provide the details of a map. Symbols represent features such as mines, bridges, dams and many more items. To see a complete look at symbols visit the USGS site for more information.
Figure 1 Symbol examples from the USGS Topo Map Symbols web page.
Figure 2 Benchmarks on a topographic map.
A benchmark is control point on the map. Control points are positions of accurate measurement in terms of elevation and position (latitude and longitude.) Benchmarks are also known informally as “survey markers.” Originally, these markers were used in land surveying and by civil engineers for construction purposes. Benchmarks help to accurately determine location.
From www.mytopo.com’s frequently asked questions:
“A benchmark, abbreviated "BM," is a location whose elevation and horizontal position has been surveyed as accurately as possible. Benchmarks are designed for use as reference points, and are usually marked by small brass plates.”
Occasionally the hiker will find a benchmark plate in the backcountry. The image below is an example of the brass plate. These plates should not be tampered with and are not souvenirs to be taken home.
Figure 3 Brass benchmark found in the backcountry.
Note the elevation data found in the center of the plate. Importantly, the elevation information is measured in feet above sea level and not in relation to the adjacent topography. Wikipedia.com reports that over 740,000 benchmarks are dispersed around the United States.
Though elevation data is provided on the map, coordinate information (e.g., latitude and longitude, UTM) is not. It’s is up to the hiker to interpolate and determine the information through the use of a map tool.
Remember that the coordinate data provided on a topographic map is in degrees, minutes and seconds (GPS menu settings format: dd mm ss.s) while a new GPS is set at the factory to degrees minutes.minutes (GPS menu settings format: dd mm.m.)
Finding a benchmark can confirm your position on the map.
To improve you GPS skill level try “Benchmarking,” an activity similar to geocaching. The objective is to find the brass plates in the field. For more information visit Geocach.