DEM

A digital elevation model (DEM) is a grid of ground elevations. The U.S. Geological Survey (USGS) defines it as "a representation of the bare ground (bare earth) topographic surface of the Earth excluding trees, buildings, and any other surface objects." Each cell of the grid holds one elevation, so a DEM is raster data. See Raster Data.

How USGS DEMs are made

USGS says its DEMs "used to be derived primarily from topographic maps," and that those are "being systematically replaced with DEMs derived from high-resolution lidar and IfSAR (Alaska only) data." Lidar point clouds are "the foundational data for 3DEP in the conterminous U.S."

3DEP and The National Map

The 3D Elevation Program (3DEP) "is the elevation component of the National Map," and "All 3DEP products are available free of charge and without use restrictions." Its standard DEMs include:

• 1-meter: project-based, built from quality level 2 or better lidar, with horizontal coordinates "referenced to the Universal Transverse Mercator projection"
• 1/3 arc-second: seamless over the 48 conterminous states, Alaska, Hawaii, and U.S. territories. "Ground spacing is approximately 10 meters north/south, but variable east/west due to convergence of meridians with latitude."
• 1 arc-second: seamless over the conterminous U.S. and Alaska, with ground spacing of about 30 meters north/south
• 5-meter: IfSAR-derived, Alaska only, referenced to the Albers Equal Area Conical projection

USGS also notes that standard DEMs "contain flattened water surfaces."

DEM, DSM, and DTM

USGS separates bare-earth products from surface products. An IfSAR digital surface model (DSM) "contains the height of the highest surface on the ground including vegetation, man-made structures, and bare earth." A digital terrain model (DTM) "reflects the height of bare earth where the elevations of vegetation and man-made features have been removed."

What comes out of a DEM

Esri's ArcGIS Pro documentation notes that elevation values "can be used to derive slope, aspect, and watershed properties." Shaded relief and contours are built from the same grid.

Heights need a datum

Elevations are measured from a vertical datum. NOAA's National Geodetic Survey states: "The current vertical datum for the contiguous United States and Alaska is the North American Vertical Datum of 1988." See Datum.

Why it matters in mining claim research

Terrain is context for a mining claim map: slopes, drainages, and realistic access. A DEM supplies it at no cost. It does not change the legal description, and its distances are not survey distances. BLM's Manual of Surveying Instructions (2009) notes that "the Public Land Survey System datum is based upon measurements at actual average ground elevation along the line" (chapter VII), so a length measured on a grid or a map is not automatically the ground length of a mining claim line.