What Is a Slope Map?

A slope map is a topographic map showing changes in elevation on a highly detailed level. Architects, landscape designers, and water control planners use a slope map to evaluate a particular site. Detailed data are required to generate one of these maps.

Many people are familiar with topographic maps, commonly called topo maps, provided by several national geological survey agencies. These diagrams plot lines through points of constant elevation: the closer the lines on the map, the steeper the landform. Most topo maps are constructed so that the vertical height between two adjacent lines is 20 to 50 feet (6 meters to 15 meters). This distance is not useful for those professions working on a single residential or commercial site. On a slope map, the isolines, or the lines having the same elevation, may indicate height differences of as little as six inches (15 cm).

Surveys of the site are taken using traditional equipment, including tape measures, levels, and an instrument that measures angles, called a theodolite, which is mounted on a tripod. Other equipment includes electronic distance measuring (EDM) and global positioning system (GPS) devices. The systems are sophisticated, but users still require significant experience to generate good data.

Once the data for a slope map are obtained, several software packages can create the maps, linking points of equal elevation. This task is easy when the slopes are steep, and the elevation is rapidly changing. In flat areas, the isolines are less obvious, sometimes requiring human choices or verifying the data with a site visit. A tick sheet, a card with varying isolines scaled to the map, is a handy tool for creating and verifying survey data.

One of the most valuable uses of a map is to predict the flow of water. Ground water can cause heaving and shifting of buildings and parking lots. Runoff from storms can also damage property. A slope map may also indicate the placement of utility lines and their depth.

An aspect-slope map overlays the elevation information with aspect information. The direction of view is shown on the map by shading the isolines. These maps are used to visualize the views of the future occupants of a building, the effectiveness of solar energy equipment, and the best locations for fruit or ornamental trees, among other uses.

The aspect-slope map can be further expanded by adding bioclimatic data. Color-coded fills are drawn on the map to indicate marshy, arid, windy, biologically active places, freeze zones, or other areas. Knowledge of these small zones helps maintain migratory bird flyways, healthy plants, and ice-free walkways and driveways.