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:
- 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.
- 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.
- A default 3d camera is added to the scene which views the DEM data from a
pleasing angle.
- u/v texture coordinates are added to the imported data so that a bitmap image
can be easily draped over the DEM data.
- The converter creates smoothed vertex normals for the DEM data so that
it will appear to be smooth when rendered.