This LightWave image loader plug-in will read USGS SDTS files that contain a Raster element.
Load an image using any of the techniques available in LightWave, and when the file requester appears, choose any one of the files that was extracted from the .tar.gz archive. (Note: SDTS data will be comprised of numerous files, all named with the same first four characters, and all the files should have a .ddf extension.) The SDTS image loader will automatically get the data it needs from the appropriate files. (For information on where to get SDTS files, see the bottom of this document.)
Once Lightwave has loaded the data, a requester will appear displaying the minimum and maximum elevation found in the SDTS files. This information can be used to determine the best values for displaying the image in Lightwave's Image Editor.
If only one SDTS dataset is going to be used in a scene, the default values displayed in the Image's black point elevation and Image's white point elevation requester fields will be the correct settings for the best image display possible. In addition, if the SDTS dataset contains invalid height values (usually around the outside border of the quadrangle), the user is informed of this, and is allowed to choose the height value that will be used anytime an invalid elevation is encountered.
If you load multiple, independent datasets into Lightwave, the plugin will remember and display the previous black point and white point elevations in the user requester (why it does this is explained in the following section). However, since these datasets will be independent of each other, you can quickly change the values to the current file's Minimum and Maximum values by pressing the Reset button. This will allow the Image Editor to display each file in the best possible manner.
Now, if a user would like to load multiple, neighboring SDTS datasets in an attempt to tile them into a larger landscape, things are a little trickier. It is necessary for the SDTS image loader to transform the height data into a 0...1 range, so that it can be displayed correctly in LightWave's Image Editor. The user must ensure that the transformation is being done consistently for each SDTS dataset that is to be loaded. This transformation is controlled by using the black point and white point requestors and will probably require that several of the datasets will have to be read in a second time.
The correct black point and white point should be the minimum and maximum value found in all of the SDTS dataset that are going to be loaded. The SDTS loader will assist in keeping track of the overall minimum and maximum, because it will remember the min and max values from the previous SDTS datasets that were loaded.
So load the first SDTS dataset, and just accept the default values. Now load the second SDTS dataset, and if the minimum elevation in the new SDTS file is less than the current black point, then change the black point value to reflect the new minimum. If the maximum elevation is greater than the white point value, then change the white point to reflect the new maximum. Repeat this step again until all of the SDTS datasets have been loaded.
Now to insure that all SDTS dataset are transformed to 0...1 range consistently, use the Image Editor's Replace Image button to reload all of the previously loaded SDTS files again. The black point and white point values will be retained, so the user will now just need to accept the default values in those requestor fields, and all of the SDTS data will have been transformed consistently and correctly into the displayable 0...1 range necessary for display in the Image Editor.
If correct display of the images is not important, the most important thing to remember is to transform all the SDTS data into the image's display range consistently by using the same white point and black point values for every SDTS dataset. If you don't use the overall minimum and maximum, the data will still be transformed correctly and consistently, but the images will just not be displayed optimally in the Image Editor.
WARNING: If you are loading multiple SDTS files into a scene, make sure you write down the black point and white point settings that were used. It is not possible for an image loader to save these values in a scene file, so if the scene is saved and reloaded, the SDTS requestor will always appear asking for these new values.
SDTS files for the entire US are available for download from the web. Some data sets have values spaced every 30m apart, and others have height values spaced 10m apart. Here is a site that has a nice graphical interface for selecting the desired SDTS files:
Click on the "7.5-Minute" link on the left side of that page (next to the words "High Detail"), and they'll present you with a map that shows the covered areas. They make it look like you have to order the data files, but in fact the 10m and 30M SDTS DEMs are freely downloadable, you just have to go through the "order" process.
You can also download SDTS files at (there is both 30m and 10m data files at the geocomm site, but there is no way of telling which ones are the 10m higher resolution data without doing the download):
Click on the state of your choice, and then select the appropriate county. Choose the link titled "Digital Elevation Models (DEM) - 24K", and finally select the desired city or area and click on the green down arrow button just to the right of the word "Download:".
The files downloaded from these sites will be archived in a tar.gz format, commonly used on all Unix systems. However, there are programs available for Windows-based machines that will decompress these types of files. Winzip should be able to gunzip and untar them just fine.
Once the file has been gunzipped and untarred, you should end up with a number of .ddf files, all named with the same first four characters.
This loader was written by Marvin Landis.