The benchmarks on topographic maps published by the United States Geological Survey (USGS) are known control points that can be used as the starting point for land surveys. The locations of these benchmarks are known to a high degree of accuracy. Benchmarks are usually marked with small, permanently fixed brass disks.
Purpose
Benchmarks are placed by the National Geodetic Survey (NGS) to provide horizontal and vertical reference points for land surveyors. If an existing benchmark must be removed, the NGS should be notified so that a benchmark can be established at a new location nearby.
Horizontal Control
Benchmarks appearing on USGS topographic maps provide horizontal control with accurate locations. You can obtain the precise location of any benchmark, in latitude and longitude, from the National Geodetic Survey.
Vertical Control
Some benchmarks provide vertical as well as horizontal reference. The location data of vertical control points includes not only horizontal location (latitude-longitude and/or UTM X-Y) but also the benchmark's elevation relative to mean sea level.
Topographic Maps
Permanent benchmarks are identified on USGS topographic maps by an open triangle, sometimes with a superimposed cross. The name of a nearby geographic feature is often printed near the triangle. The notation for vertical control benchmarks also includes a number representing the benchmark's elevation relative to sea level in feet.
Tags: benchmark elevation, benchmark elevation relative, control points, elevation relative, Geodetic Survey
