This Tech Tip discusses "projected" vs. "geographic" DEMs and how they can be imported into RiverTools.
Virtually all DEMs in common use fall into one of two categories, based on their pixel geometry, which may be called "fixed-angle" or "fixed-length". "Geographic" or "fixed-angle" DEMs are characterized by pixels that span a fixed angle, measured in angular units like arcseconds or degrees. The bounding box of a fixed-angle DEM will be given in terms of latitudes and longitudes. Some publicly available elevation data sets that have the fixed-angle pixel geometry include SRTM(HGT), DTED, NED, MOLA, GTOPO30, ETOPO2, ETOPO5, GLOBE and USGS 1-Degree DEMs. It is important to understand that "geographic" is not a map projection, but instead refers to elevations relative to the surface of a 3-dimensional ellipsoid model, like WGS84. Each of the elevation data sets listed above has an ellipsoid model and datum associated with it. (See our Data Links page for synopses of these data sets.) DEMs with fixed-angle pixel geometry are easy to mosaic, since seams fall on lines of latitude and longitude. The actual length along the x dimension of the pixels varies with the cosine of the latitude. When RiverTools makes area, length and slope measurements with a "geographic" DEM, it takes this latitude-dependence into account and integrates on the surface of the specified ellipsoid model to obtain the most accurate results possible.
The other type of DEM has a "fixed-length" pixel geometry. These DEMs have been projected from the surface of a 3D ellipsoid model onto a plane via a map projection such that each pixel in the DEM now corresponds to a rectangle with a fixed length along its x and y dimensions. The most commonly used map projection for this type of DEM is UTM (Universal Transverse Mercator), although Albers Equal Area, Mercator and some others are sometimes used. Whenever a map projection is used, pixels must be distorted so that their new side lengths and areas will not be the same as what they were on the ellipsoid surface. The USGS 7.5-minute DEMs (in ASCII or SDTS format) use a UTM projection such that each pixel is 30 meters on a side. Some of these DEMs are also available with 10-meter pixels. These UTM-based DEMs are most useful for small regions, since the Earth's surface is approximately flat over short distances. The UTM system breaks the Earth into 60 zones, each of which looks something like a thin slice of an orange. Each of these "lense-shaped" portions of the Earth ellipsoid are then projected onto a rectangle. This is why it is difficult to mosaic DEMs that fall in different UTM zones, especially in regions far from the equator.
When you choose Import DEM from the File menu in RiverTools, you see a list of supported file formats. Most of these formats can store elevation data with either the fixed-angle or fixed-length pixel geometry. RiverTools will take care of the details for you transparently. Once you have imported the DEM, you can choose View DEM Info from the File menu to get a dialog that shows you the georeferencing information, including whether the DEM has the fixed-angle or fixed-length pixel geometry and the bounding box. The bounding box for a fixed-length DEM will typically be given in terms of large integers that represent measurements in meters from some origin. The UTM projection uses a noncentered origin that avoids negative numbers and allows UTM coordinates to increase toward the east and north. If you have imported a non-UTM DEM with fixed-length pixels, or a theoretical DEM that does not correspond to a real location on Earth, then you may need to provide artificial bounding box numbers as measurements in meters from the lower-left corner, that are consistent with the pixel size and number of rows and columns in the DEM.
If you are working with a UTM-based DEM and want to know the latitude and longitude of a particular pixel, you should first create an image, such as shaded relief, with the tools in the Display menu. Then, when the image has been displayed, choose Value Zoom from the Tools menu at the top of the image and click somewhere in the image. The values in the grid that was used to make the image will be displayed in a 5 x 5 neighborhood of the pixel you clicked on, and the UTM coordinates will be displayed toward the bottom. By choosing Change Coords from the Options menu of the Value Zoom dialog, you can have these pixel coordinates converted to a longitude and latitude. You can also type values into the Value Zoom dialog and then press Enter on your keyboard to make the crosshairs jump to the specified coordinates.
If you have any additional questions or comments, please contact Rivix Technical Support for assistance.
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