Merging Surface Data With Cave Data
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Once you are satisfied with surface data, you can link the data to a cave. The first step is to locate the entrance or connecting point between the cave and the surface terrain. Be sure the "Cusor Option" is set to "Link Point". When this mode is enabled, you use the mouse to select the link point. You just click on the location in the terrain that corresponds to the entrance or linking station. When you click on the map, the geographic position and elevation of the cursor will be displayed at the bottom of the screen. With these three parameters and a topographic map, it is usually pretty easy to zero in on the exact location of the entrance.
Note: some DEMs have inaccurate longitude/latitude or UTM data associated with them. Thus, you should not rely too heavily on this information. The most reliable way to locate a cave feature is visually, looking for terrain feature that match the map. It helps set the contour interval to roughly the same spacing the topographic map you are using.
Setting Parameters. After you have selected a connecting point, you can should set the "Parameters" associated with the surface data. To do this, click on the "Actions/Parameters" tab at the top of the window.
Linking Station. First you need to enter the name of connecting station. This is the station label name for the station that comes the closest to the surface. Usually, it is the entrance station. If for some reason, the connecting station is below the surface, you will need to set a "Vertical offset". The vertical offset is the vertical distance between the connecting station and the surface terrain.
Feature Name. Next, you need to give a "Feature" name to the terrain model. This name is used by the viewer to help you identify, select, hide or color the surface terrain in your cave map.
Surface Model. Finally, you need to decide if you want the terrain modeled as a grid or contour lines. Contours are more useful for small terrain models or where the cave will be viewed from above. Grids are better for large areas of terrain and are best viewed from profile or an oblique view. You also have the option of display put both contours and grid on at the same time.
Grid Spacing. Grid Spacing defines the distance between each point in the grid. Normally, you do not need to set the "Grid Spacing" since it is specified by the DEM file. If however, you are entering data manually, you must set the spacing based on the way you have sampled the data.
Merging With A Plot File. When all the parameters have been set, you can merge the surface data with the cave. To do this, click on the "Merge" button at the top of the screen. The program will display a dialog box asking for the name of a COMPASS plot file (.PLT). If you don't have the plot you need, you can run the COMPASS Project Manager and generate the file.
Once you have selected a plot file, the program will display a dialog box asking for the name of a file to put the merged data in. Normally, merged data files have the extension "PLF" to indicate that it is a plot file with surface features merged into it.
Viewing Terrain Models. Once the program has finished merging the data, you can view the surface terrain and the cave, by running the Viewer and opening the new PLF file.
Note: Terrain models are best viewed with the Viewer in "Profile Mode" with the image rotated slightly and the Pitch set slightly negative. You should also enable the “Color-By-Depth” mode.