To communicate specific qualities, quantities, and locations, graphic description is often required. For this, people use maps which have been prepared by surveying an area and drawing a representation of that area to a specific scale. Two types of maps that are often used by urban and regional planners are planimetric maps and topographic maps. Planimetric maps show the positions of features without showing the hills and valleys of the land. Topographic maps are a basic resource for physical planning and design. One can tell from the scale of the map that there is about a 25-foot change of elevation in a distance of 100 feet; this is generally considered to be a fairly steep slope. The form of the earth's surface is commonly indicated through the inclusion of contours on a map. The basic source for topographic maps in the United States is the United States Geological Survey, which is a division of the U. S. Department of the Interior.