Introducing Geographic Information Systems with ArcGIS: A Workbook Approach to Learning GIS, 3rd Edition

Comparing the DEM and the DRG

____ 49. Compare the elevations “printed” on the DRG with those values given by the DEM: Zoom to the bookmark River_Bend. Turn off the DOQ layer if it is on. Arrange the DRG and DEM layers so that the DEM is at ’the bottom and the DRG is just above it. Turn both layers on. The DRG image will obscure the DEM. Unhide the Identify window again and activate the tool if it is not already. Set up “Identify from layer” as COLE_DEM. Use the Identify tool and MapTips on the topographic quadrangle, pointing at contour lines where the text on the map shows elevations (e.g., 600, 700, 850, and 900 feet, zooming out as necessary) near the bend in the river. You visually see the elevations on the DRG, but the Identify results and MapTips come from the DEM. Note the agreement (or, sometimes, lack thereof) between what is printed on the topo map and the values of the underlying grid. Zoom out somewhat and try this exercise again. You may notice better agreement between the two datasets as you get further away from the steep slopes around the river. Auto-Hide the Identify Results Window.

____ 50. Zoom to the extent of the DRG. Locate Lexington Reservoir No. 4 in the north central part of the quad. Zoom in on it. By turning the DRG on and off, flip between viewing the DEM and the DRG. Determine, from the DEM, the elevation of the surface of the water in the reservoir. ___________ feet. According to the topo map, what is the elevation of the spillway? ________ feet. ...