PolyTrans' (USGS) DEM Geometry Import Converter

PolyTrans's DEM geometry import converter imports, manipulates and converts digital elevation model (DEM) data in the USGS format. This form of DEM data is available from the United States Geological Survey (USGS). Each data set describes the elevation of semi-square regions of land for various locations across the U.S.A., Alaska, Hawaii and some surrounding areas of Mexico and Canada. The most accurate DEM data sets are sampled every 30 metres (7.5 minute DEMs) while the least accurate are sampled every 3 arc seconds (for 1:250,000 scale DEMs). See below for a description of the various DEM dataset types.

Since these datasets are abundant and available freely via the Internet, this converter can be put to good use for creating realistic (and accurate) 3d landscape geometry.

Features of the DEM Converter

DEM datasets typically contain 60000 or more quadrilateral polygons, or 1200000 triangles (for a 258x258 resolution sample; the maximum DEM dataset size if 2050x2050 which would result in 4.2 million quadrilaterals or 8.4 million triangles). This is an enormous number of polygons for most 3d rendering programs so this DEM converter incorporates two unique options to overcome this problem:

  1. The converter can skip over samples in the dataset so that only every n-th sample is used. Rather than importing 258x258 samples, the converter imports 51x51 samples (for a skip factor of 5) which results in only 2601 quadrilateral polygons.
  2. Rather than store the entire DEM dataset in single object, the DEM converter breaks up the data into multiple smaller objects with a common parent. This has shown to be an effective method to speeding up the wireframe redraws of the DEM data (by a factor of 2 or 3), and makes interactive user movement of a 3d camera much faster since each sub-object is only a few hundred polygons. In addition, certain rendering programs (such as Okino's NuGraf renderer) use much less memory when many smaller objects are used rather than one large object with many polygons. By default each sub-object stores a maximum of 900 polygons; contrast this with other converters which lump all 120,000 polygons into a single object - few renderers will be able to render such a large object.
  3. A default 3d camera is added to the scene which views the DEM data from a pleasing angle.
  4. u/v texture coordinates are added to the imported data so that a bitmap image can be easily draped over the DEM data.
  5. The converter creates smoothed vertex normals for the DEM data so that it will appear to be smooth when rendered.