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Overview of Maps


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, threatened resources).
  • 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.



Key Points When Working With Maps

When working with maps there are some key points to remember, especially when using the map in conjunction with a compass and GPS receiver.

Datum

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.

Geographic North

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, which is discussed in Chapter 4, Using a Compass and Clinometer.

Maps Are Not Perfect

For a map to be considered reliable and accurate, a point or symbol marked on a map must be in proper relation to known landmarks or positions located on the ground. In 1947, the “United States National Map Accuracy Standards” were established as the standards of accuracy for published maps and are currently in effect. The standards require a stringent percent of accuracy within centimeters of both location and elevation points tested. However, even with these standards, maps are not absolutely accurate because:

  • Maps represent a curved and uneven surface that is drawn on a flat piece of paper, which results in a distorted picture.
  • There is a margin of error (human error and inadequate survey procedures) in surveys that were used to create maps. Also, there are factual matters (errors such as names, symbols of features, and the classifications of roads or woodlands); sometimes the information is wrong and names and features change.
  • On incidents, if a map has been photocopied, it most likely is not to scale. It is important to watch out for this and learn how to make adjustments.

Maps Can Be Outdated

Maps are outdated from the day they are made, including USGS topographic maps (for example, new roads may not be on a map). When working on an incident try to obtain the most up-to-date map. USGS topographic maps have the revision date in the margin.

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