The TRex Terrain Extractor was designed to expedite the elevation assignment process by automatically assigning elevations to the model features according to the elevation data stored within Digital Elevation Models.

Digital Elevation Models were chosen because of their wide availability and since a reasonable level of accuracy can be obtained by using this data type depending on the accuracy of the DEM/DTM.

The TRex Terrain Extractor can quickly and easily assign elevations to any or all of the nodes in the water distribution model. All that is required is a valid Digital Elevation Model. Data input for TRex consists of:

1. Specify the GIS layer that contains the DEM from which elevation data will be extracted.
2. Specify the measurement unit associated with the DEM (feet, meters, etc.).
3. Select the model features to which elevations should be applied; all model features or a selection set of features can be chosen.

TRex then interpolates an elevation value for each specific point occupied by a model feature. The final step of the wizard displays a list of all of the features to which an elevation was applied, along with the elevation values for those features. These elevation values can then be applied to a new physical properties alternative, or an existing one. In some cases, you might have more accurate information for some nodes (e.g., survey elevation from a pump station). In those cases, you should create the elevation data using DEM data and manually overwrite the more accurate data for those nodes.

The TRex Terrain Extractor simplifies the process of applying accurate elevation data to water distribution models. As was shown previously, accurate elevation data is vital when accurate pressure calculations and/or pressure-based controls are required for the water distribution model in question. All elevation data for even large distribution networks can be applied by completing a few steps.

In the US, DEM data is usually available in files corresponding to a single USGS 7.5 minute quadrangle map. If the model covers an area involving several maps, it is best to mosaic the maps into a single map using the appropriate GIS functions as opposed to applying TRex separately for each map.

When using TRex, it is necessary that the model and the DEM be in the same coordinate system. Usually the USGS DEMs are in the UTM (Universal Transverse Mercator) with North American Datum 1983 (NAD83) in meters, although some may use NAD27. Models are often constructed using a state plane coordinate system in feet. Either the model or DEM must be converted so that the two are in the same coordinate system for TRex to work. Similarly, the vertical datum for USGS is based on national Vertical Geodetic Datum of 1929. If the utility has used some other datum for vertical control, then these differences need to be reconciled.

The TRex Terrain Extractor can read the USGS DEM raster data in SDTS format. Raster profiles provide a flexible way to encode raster data. The SDTS standard contains small limited subsets called profiles. In a raster transfer, there should be one RSDF module, one LDEF module and one or more cell modules. Each record in the RSDF module denotes one raster object. Each raster object can have multiple layers. Each layer is encoded as one record in the LDEF module. The actual grid data is stored in the cell module which is referenced by the layer record. A typical USGS DEM data set contains one RSDF record, one LDEF record and one cell file.