'Frame up' for more + What's New
> DEM o'Graphics Potpourri, Part 1 - 3DA#39
> DEM o'Graphics Potpourri, Part 2 - 3DA#40
> Quick 3D Terrains for Illustrators - 3DA#39
> Run, Don't Walk - 3DA#42 full text
'Frame up' for more + What's New |
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DEM o'Graphics - 3DA#37 > DEM o'Graphics Potpourri, Part 1 - 3DA#39 > DEM o'Graphics Potpourri, Part 2 - 3DA#40 > Quick 3D Terrains for Illustrators - 3DA#39 > Run, Don't Walk - 3DA#42 full text |
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Editor's note: This info supplements a feature article in 3DA#37. You may find this page useful by itself, but it will be considerably more useful if you have read the hard copy about how to get the new higher-res USGS 30m DEMs in SDTS .ddf format into LW3D correctly (and impressively) via Bryce 4, MicroDEM+, and Photoshop. The steps should be similar for many other applications, too. Getting DEMs into Bryce 4 For those of us who like to fly before we walk, a word of caution is in order. Bryce 4's manual (pages 434-435) doesn't shed much light on importing DEM objects into the program. The process is outlined near the end of the Readme.txt file in the Bryce4 directory. Digital Elevation Model File Formats Keeping Digital Elevation Model (DEM) file formats straight can be daunting and confusing. DEMs come in different flavors and a DEM may not be a .dem, but a .dt0. To confuse the issue further, as in the case of the .dem file designator, all .dems are not equal or compatible. For example: the USGS ASCII .dem, MicroDEM+'s .dem, and the GTOPO30 .dem are not the same. The simple way to avoid confusion is to focus on the DEM object's source--its type and its resolution, not its file designator. Think of models as being USGS SDTS 30-meter, USGS ASCII 30- and 90-meter, USGS GTOPO30 1-kilometer, and NIMA DTED Level 0 1-kilometer. For those interested in more, a FAQ is usually available on the data's download site, or the Help Desk at the GIS Data Depot. Global Data We know digital elevation models (DEMs) for the United States are available and free, but what about DEMs for the rest of the world? Are they out there? Yes, free 30 arc second data at approximately one kilometer resolution is available for most parts of the world from the USGS and DOD's National Imagery and Mapping Agency (NIMA). As with the USGS's SDTS .ddf and ASCII .dem 30-meter elevation models, the file formats for the USGS's GTOPO30 and NIMA's DTED Level 0 models are different, and few, other than specialized programs, read their files. MicroDEM+ (MD) can. MD will decompress and read the available .dt0 NIMA DTED Level 0 format, as well as read already decompressed .dem USGS GTOPO30 files. Global Data Sets GTOPO30 breaks the world into 33 large data sets. For example, the set covering most of Western Europe is 16.6Mb .tar.giz-compressed and 96.6MB uncompressed, with the .dem file alone 56.3MB. Fortunately, around 28Mb can be deleted safely--only the .dem and its .hdr header file are required for the approach here. Trying to use even a 28Mb file can be a problem, but MD's Data Manipulator Import solves it by just extracting and saving an area of specific interest. You need know only the latitude and longitude of the N.W. and S.E. corners of your area of interest. After saving, the grayscale approach can then be applied to the DEM. ___ The DTED Level 0 (one-kilometer) data is the only resolution NIMA makes publicly available. Rather than the world being organized in a few large data sets, the NIMA data is organized in one-degree latitude by one-degree longitude tiles. Tile file size is small and MD's Data Manipulator Import and Merge will decompress and open individual tiles, or merge multiple tiles. After saving, the tiles can be converted to grayscale and used in Bryce 4 (B4), or as displacement maps. A nice feature is the download page for each NIMA tile contains a GIF reference map that can be saved by right clicking. |
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Opened in MD, the one-kilometer resolution terrain appears blocky in both data sets. MD will smooth this. ___ When you right-click on the image to select Gray scale in the Elevation Map Options window, also check the "Interpolate within grid" box. A caution: When the grayscale is loaded in B4, you need to keep in mind that you are dealing with large areas in terms of coverage and damping down in Blend is the order of the day, given the way B4 reads grayscales. (Note: This technique also applies to USGS ASCII 30- and 90-meter data sets when a small subset area is carved from a larger area. It will not work in the current version with SDTS Level 2 DEMs that are in feet, rather than meters. This will be corrected in the new version of MD+ due this Fall.) Non-square Grayscales Besides the more accurate Crop to Square approach outlined in the 3DA#37 article, there is a way by which non-square DEM grayscales from MD can be used in B4 and exported. To use it, load a rectangular DEM grayscale into B4, edit as usual in Terrain Editor, and then stretch the square to its correct dimensional proportions. To determine the proportions, load into MD the DEM from which the grayscale was made and go to Modify/Map area/Full DEM to ensure you have a full DEM on screen, then to Info to find the Pixels on screen. Do your arithmetic and make the appropriate size change in B4's Object Attributes panel. Some distortion can not be avoided, and may be more or less of a problem depending on the magnitude of the change and the DEM's resolution. As before, the .lwo object accompanying the .lws export exports as a square, while the .dxf and .obj export as rectangles. (Note: This method can be used to determine the correct dimensions for stretching the improperly proportioned 90-meter DEM objects when loaded in B4.) ___ The accompanying image illustrates a LightWave use of this variation: Four NIMA DTED tiles of areas of Kosovo and its environs were opened, merged and converted to grayscale in MD, with the same grayscale being used to create each of the objects in the image. The model on the left was created as a reference model in World Construction Set (WCS), with elevation exaggeration applied. The center and right models are from the same object created in B4 and exported to LightWave. The center model was sized in LightWave from eight meters square to 160 kilometers square. The right model represents an exercise to see if stretching the resized square model in the Z-axis to the correct rectangular dimension would introduce unacceptable distortion. You be the judge. ___ While suffering from distance inaccuracies, the square model provides a good representation of the area very much like those found frequently in newspapers. The stretched model, on the other hand, more accurately represents the area as it is on the Earth's surface. Over simplified, the difference is the distance of the chord of an arc on the Earth's surface and the distance along that arc. ___ The texture of the left object is based on four joined NIMA tile reference maps. The center object's texture was cropped from a 1997 CIA map of Montenegro-Serbia. It is not entirely accurate, particularly in its northernmost areas, as it was cropped from a Lambert Conic Projection. The texture of the right object is a combination of both the NIMA and the CIA maps. | |
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More Info Where to find 'dem data sets and other stuff: US Data USGS SDTS 30-meter & ASCII 90-meter data: edcwww.cr.usgs.gov/doc/edchome/ndcdb/ndcdb.html Global Data USGS GTOPO30 1-kilometer: edcwww.cr.usgs.gov/landdaac/gtopo30/gtopo30.html NIMA DTED Level 0 1-kilometer: 164.214.2.59/geospatial/products/DTED/dted.html U.S. State Resource Sites Many states have geographic information sites with useful data. A good place to start to see if your state has such a site is The Federal Geographic Data Committee list at www.fgdc.gov/stakeholders/partnerships/partrost.html. GIS and Map References (a start) Library of the University of California, Riverside, Infomine: infomine.ucr.edu/search/mapssearch.phtml National Geographic Society: www.nationalgeographic.com/maps/csites/index.html Perry-Castaneda Library Map Collection: www.lib.utexas.edu/Libs/PCL/Map_collection/Map_collection.html Odden's Bookmarks: kartoserver.frw.ruu.nl/html/staff/oddens/oddens.htm GIS Data Depot: www.gisdatadepot.com Freeware Programs Mentioned Bureau of Land Management sdts2dem: ftp.blm.gov/pub/gis/sdts/ MicroDEM+: www.usna.edu/Users/oceano/pguth/website/microdemdown.htm Commercial Programs Mentioned These are mentioned on this page and/or in the 3DA#37 article. Bryce 4: www.corel.com LightWave 3D: www.newtek.com Photoshop: www.adobe.com PolyTrans: www.okino.com Relief: www.haja.com World Builder: www.animatek.com World Construction Set: www.3dnature.com | ||
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