Geographic Names Information System - 15 of 25

Geographic Names Information System FAQs - 25 Found

How accurate is the elevation data in the Geographic Names Information System Database? How was it measured?

The elevation data in GNIS are not official.
Only the geographic name and locative attributes are official.

The elevation data are from the seamless DEM layers of The National Map for the primary location of the feature (Coordinates Sequence = 1 in the Feature Detail Report).

The Primary coordinate values for communities are taken at the center of the 'original' community meaning the city hall, main post office, main intersection, etc. For other areal features, coordinates are taken at the approximate center, and for reservoirs at the dam. The primary coordinates for features classified as summit (all uplifted features), are recorded at the highest point and for linear features (stream, valley, and arroyo) at the mouth.

The elevation figures in the GNIS are not official and do not represent precisely measured or surveyed values. The data are interpolated from seamless raster elevation models for the given coordinates and might differ from elevations cited in other sources, including those published on USGS topographic maps. Published map data represent precisely surveyed points that often are marked by a benchmark or triangle on the map and a benchmark seal physically anchored into the ground at the site.

The variances between the GNIS elevation data and other sources generally arise from acceptable tolerances and will be most evident for features such as summits, where precision is of more concern, and where the local relief (rate of change of elevation) is more prominent. When the elevation figure is of particular note, for example the highest point in the State, then the actual elevation is recorded in the description field of the feature.

If the elevation figure for a particular feature seems significantly inaccurate, the feature coordinates might need adjusting and/or the elevation model data for those coordinates are not correct. For most purposes of general information, the elevation figures are sufficiently accurate. Efforts are continuously being made to improve the accuracy of both GNIS and DEM data, the results of which will be reflected at this site.

Tags: Maps, Coordinates, National Map, Topographic, Geospatial