Reading Data From A Topo Map
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In some instances, you won’t be able to find DEM information for your caving area. Under these circumstances, you can still create terrain models by entering the data manually. One way to do this is by reading data from a Topographic Map. Here is a step-by-step list of procedures to generate surface models:
1. Select A Map. Locate a topographic map of the area you are working. Normally, you'll want to use 7.5 minute quadrangles to get sufficient detail.
2. Select A Grid Spacing. Grid spacing is the distance between points. The closer the spacing the more points there will be in the grid. Spacing of 100 to 500 feet is typical.
3. Layout A Grid. Using the scale bar on the topographic map and a ruler, lay out a grid of points on the map that covers the area you want to model. The points should be spaced the interval that you have chosen.
You can use a piece of Mylar, tracing paper or other clear media so you don't mark up the map. Although it is not essential, it is best to make sure that one point of your grid falls on the entrance of the cave or the connection point.
4. Read The Evelvations. Using the topographic contour lines, write down the elevation of each point on grid. If a grid point falls between topographic lines, you can estimate the elevation. For example, if a point fall 3/4 of the way between the 100- and 200-foot contour, the elevation would be 175 feet.
5. Enter The Data Into A Feature File. You can now Manually Enter the data.