A map is a navigational aid that represents a specific area, such as part of the earth’s surface. Conventional symbols are used to identify objects and features on a map. Maps are critical communication tools for incident planning and operations, and are used for a variety of purposes, for example:
- To assist with navigation.
- To determine the location of a specific point or area (e.g., water sources)
- To calculate distance.
- To determine size of an area.
- To determine terrain and vegetative cover.
- To determine routes of travel.
- To determine names of streets, rivers, mountains, and other features.
- To visualize a specific area.
Most maps are made based on a datum (horizontal and vertical), which is the origin or reference point from which all points on a map are measured. Several different datums have been used to develop maps; however, commonly used datum includes: North American Datum of 1927 (NAD27), North American Datum of 1983 (NAD83), and the World Geodetic System of 1984 (WGS84). The datum is important for Geographic Information Systems (GIS) and GPS applications to ensure consistency of map data. When using a GPS receiver the datum must be set to match the horizontal datum on the map. If the datum does not match there will be errors when plotting data on a map.
Maps are usually based on the geographic North Pole (geographic north or true north). This is important to remember because a compass is based on magnetic north, which is different than geographic north. Magnetic north changes over time, while geographic north does not change over time. When using a compass and map together an adjustment has to be made to the compass to account for this difference. The difference is referred to as the magnetic declination.
Topographic both the horizontal and vertical (relief) positions of features. The datum used for most currently available USGS 7.5 minute series topographical maps is the NAD27. A new datum, NAD83, is now being used and others are being developed.
Two types of topographic maps include:
1 – Contour maps – Contour maps are the most common way to show the shape and elevation of the land. A contour is an imaginary line, where all points on the line are at the same elevation (above or below a specific reference elevation, usually sea level). Contour lines reveal the location of slopes, depressions, ridges, cliffs, and other topographical features.
2 – Shaded-relief maps – Shaded-relief maps use a shadow effect color to simulate the terrain. Different color shades are used to accentuate the shape of the physical features. The darker the shading the steeper the slope.
Map Legend and Map Symbols
Most maps have a legend that is used to interpret symbols on the map such as what color line delineates a road or land ownership boundary, or what symbol represents a building, stream, or heliport. The symbols used vary with every map, depending upon the purpose of the map. The legend may also include the map scale and other important information.
**Information adapted from the National Interagency
Incident Management System Basic Land Navigation
Manual, PMS 475 dated June 2007.