Digital Elevation Data

The term "DEM" (Digital Elevation Model) can refer either to a specific elevation file format, or to sources of elevation data in general.

File Formats

• USGS DEM
• DTED (US military)
• BT
• Bathymetry (Undersea elevation, includes CDF, HDF)
• other formats and sources (ASTER, PGM, DTM, GRD, ...)

Global Elevation Datasets (SRTM, GTOPO30, GLOBE..)

Caves, Overhangs, Non-heightfield Elevation

Regular Grid Quality Issues

• DEMs suffer from a large number of large number of problems, including gridding artifacts, contour artifacts, and mismatches with other data sources
• "sinks" or "pits"
  • for example, if you try to drape a river on a DEM, you may find that the river must run uphill!
  • things like lakes and depressions are valid sinks but when a sink develops where a stream should obviously flow, its a problem.
• in some cases you can improve a DEM by enforcing the constraints of other data (e.g. hydro)
• Quantifying and visualizing terrain fabric from digital elevation models, Peter Guth
• The Geomorphological Characterisation of Digital Elevation Models, Joseph Wood
  • describes a number of DEM problems: INTEGER ROUNDING, INTERPOLATION TERRACING, TERRAIN FLATTENING, PLANIMETRIC OFFSET
• Suzanne P. Wechsler's dissertation DEM Uncertainty: Evaluation and Effect on Topographic Parameters, now hosted on ESRI's site

Compressing Elevation Data

• it is not recommended to apply conventional lossy compression techniques to elevation grids
• Bryan Turner says:
  • "All of the lossy formats I've tried (JPEG, IFS/Fractal, Wavelet) produce some amount of distortion that is recognizable in 3D. Of those, Wavelets were the least problematic and JPEG was the worst. IFS could produce excellent results, or a mass of random garbage, depending on the data and the compression rate. JPEG consistently produced flattened square patches with steep dropoffs at the edge. Wavelets produced aliased versions of the heightmap at each level, but usually kept the overall shape of the terrain. In stress testing, it often flattened, or 'blurred' the terrain by eliminating steep areas and filling in crevices, rather than keeping their relative heights intact."
  • "All of the lossless formats I've used (Huffman, Adaptive Huffman, Level 3 Stat Model + Arithmetic Coding, LZ77, and LZW) are undesirable for paging unless they are used separately on a block. Otherwise finding the correct stretch of area to uncompress becomes a nightmare. I tried using them in-memory as well, but reading a compressed format on the fly is too slow."

Cell Registration

• is it good to know the cell registration of an elevation grid, though nearly all elevation grids have data which is cell-centered