NuGraf's (USGS) DEM Geometry Import Converter

The NuGraf Rendering System's DEM geometry import converter is a rather complex mechanism that 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 for rendering in NuGraf or for export to other 3d modeling packages such as 3D Studio. As an example, the following is a hidden line rendering (using NuGraf) of the Mount St. Helens dataset:

and the following is a simple shaded rendering of the same dataset (being visited by "Al" the gangster):

Even though this converter was initially created for an internal project at Okino Computer Graphics, it has nonetheless become one of the more popular import converters provided along with the NuGraf Rendering System partly due to its uniqueness and its ability to process very large DEM datasets. Some typical commerical uses of this converter in industry have been to create a large and detailed 3d landscape for a virtual reality game (subsequently manipulated in 3D Studio), used for urban planning studies of several towns in California and used as an interactive tool to visualize USGS DEM data in general.

The following Internet sites provide more information about USGS DEM data:

	http://nsdi.usgs.gov/nsdi/products/dem.html
	http://nsdi.usgs.gov/nsdi/maps/dem1deg.HTML

And the following Internet sites contain vast amounts of USGS DEM data:

	ftp://spectrum.xerox.com/ds9/map/dem
	ftp://edcftp.cr.usgs.gov/pub/data/DEM/250
	http://nsdi.usgs.gov/nsdi/wais/maps/dem1deg.HTML
	http://sun1.cr.usgs.gov/doc/edchome/ndcdb/ndcdb.html

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.

Dialog Box Options

DEM Height scaling factor

This option scales the height of the DEM data. It default to 1.0. Values greater than 1.0 will make the DEM data higher while values between 0.0 and 1.0 will make the DEM data shorter.

DEM Data skip factor (mesh quality)

This option determines the quality of the imported DEM data (it directly controls how many polygons will be used to approximate the input DEM data). THIS IS AN IMPORTANT CONTROL PARAMETER!! A value of 1 results in the highest quality mesh while higher values (2, 3, 4, etc) result in lower quality, but at the benefit of reducing the number of polygons in the input data. This number will cause the converter to 'skip' over every n-th input sample. For example, if the input dataset size is 258x258 samples, and the skip factor is set to 4, then the converter will actually read in the data as if it were of size 65x65 (258/4 = 65). This will produce 4225 polygons (65x65) instead of 66565 polygons. A value of 2 or 3 (16641 polygons to 7396 polygons) will produce good results for a final rendering, while values of 5 to 8 will produce small datasets ideal for fast previews (2704 polygons to 1024 polygons).

Maximum size of sub-objects

By default the DEM data will be cut up into several smaller sub- objects rather than having all of the DEM data clumped together into one huge object. This option controls how many polygons will be put into each sub-object. The default is 30 which will cause 900 polygons (30x30) to be stored in each sub-object.

List interesting header information from the DEM file

If the checkbox is enabled (check-marked) then the converter will print out information about the imported DEM data including the following information:

The DEM data description from the file,
The number of profiles which is the number of lines of sample data in the X direction,
The projection mapping type (geographic, UTM or state plane),
The actual geographic coordinates of the DEM dataset's four corners,
The minimum and maximum elevations.

List statistics after loading the DEM data

If the checkbox is enabled (check-marked) then the converter will print out the number of objects and polygons created.

Add default 2d texture coordinates

If the checkbox is enabled (check-marked) then u/v texture coordinates will be added to the imported dataset. These texture coordinates will allow a 2d bitmap image to be easily mapped to the surface of the data. Please note that the texture coordinates are aligned with the mathematical bounding quadrilateral of the dataset, not the actual physical edges of the data (this is because the physical edges of the data are not square or precise).

Add a default camera that views the data

If the checkbox is enabled (check-marked) then the converter will add a default camera to the scene which views the DEM data at a pleasing angle.

Create only one object (for 3D Studio)

If the checkbox is enabled (check-marked) then one single object is created for all of the imported data rather than having the data broken up into multiple smaller sub-objects (the default). THIS OPTION SHOULD BE USED FOR DATA DESTINED FOR 3D STUDIO because 3D Studio requires all polygons to be inside a single object so that its smoothing algorithm will work properly; if multiple objects are used then the vertex normals will not be the same where the sub-objects meet and hence "cracks" may appear at the junctions.

Center data about the origin (0,0,0)

If the checkbox is enabled (check-marked) then the DEM data will be centered about the origin (0,0,0). Please note that if the input DEM dataset uses the "Geographic" or "State Plane" grid types then the DEM data will always be centered about the origin; this may cause a problem if you want to import two DEM datasets and have them sit side-by-side; in this case you will have to physically move the two datasets so that they are side-by-side.

Overview of the DEM Dataset Types

DEM elevation data spacing varies from 30 meters for 7.5-minute DEMs to 3 arc seconds for 1:250,000 scale maps. All DEM data are similar in logical data structure and are ordered from south to north in profiles that are ordered from west to east.

7.5-minute DEM data are produced in 7.5-minute units which correspond to USGS 7.5-minute topographic quadrangle map series. 7.5-minute DEM data consist of a regular array of elevations referenced horizontally on the Universal Transverse Mercator (UTM) coordinate system of the North American Datum of 1927 (NAD 27). These data are stored as profiles with 30-meter spacing along and between each profile.
15-minute DEM data correspond to USGS 15-minute topographic quadrangle map series in Alaska. The unit sizes in Alaska vary depending on the latitudinal location of the unit. 15-minute DEM data consist of a regular array of elevation referenced horizontally to the geographic (latitude/longitude) coordinate system of North American Datum 1927 (NAD 27). The spacing between elevations along profiles is 2 arc seconds of latitude by 3 arc seconds of longitude.
30-minute DEM data cover 30-minute by 30-minute areas which correspond to the east half or west half of the USGS 30- by 60- minute topographic quadrangle map series for the conterminous United States and Hawaii. Each 30-minute unit is produced and distributed as four 15- by 15-minute cells. 30-minute DEM data have the same characteristics as the 15-minute DEM data except that the spacing of elevations along and between each profile is 2 arc seconds.
1-degree DEM data are produced by the Defense Mapping Agency in 1-degree by 1-degree units which correspond to the east half or west half of USGS 1- by 2- degree topographic quadrangle maps series, for all the United States and its territories. 1-degree DEM data consist of a regular array of elevations referenced horizontally using the geographic (latitude/longitude) coordinate system of the World Geodetic System 1972 Datum. A few units are also available using the World Geodetic System 1984 Datum. Spacing of the elevations along and between each profile is 3 arc seconds with 1,201 elevations per profile. The only exception is DEM data in Alaska, where the spacing and number of elevations per profile varies depending on the latitudinal location of the DEM.