Important Considerations When Working with Maps

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

Datum

Maps made based on a datum (horizontal and vertical).  Datum is the origin from which all points on a map are measured. Three primary datum have been used to develop maps in the United States.  These are:

• North American Datum of 1927 (NAD27)

• North American Datum of 1983 (NAD83)

• World Geodetic System of 1984 (WGS84).

Datum is a mathematical model that accounts for developing a map from a spheroid (the earth) to a flat map.

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.

Map datum information is found in the title block at the bottom left corner in the margin of a USGS map.  Other maps (e.g., US Forest Maps and commercial maps) seemingly provide datum in rather random locations. The hiker will have to scan the map to find this information.

Geographic North

Maps are laid out in relation to geographic North (also referred to as the north pole or true north.) This is important to remember because the traditional magnetic compass provides information based on magnetic north.  Magnetic north changes over time, while geographic north does not change. The difference is referred to as magnetic declination. When using a compass and map together the hiker must account for this difference.   Because of this I recommend that the navigator have an adjustable compass such as the Sylva Ranger.

For more on declination go here.

Maps Are Not Perfect

For a map to be considered reliable and accurate, map symbols 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 of 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.

• If a map has been photocopied, it most likely is not to scale.

• Maps editions are dated and not current.

Maps do not accurately represent the ground surface

Topographic maps provide an overview of the ground surface.   A USGS 7.5 minute map uses  a scale of 1:24,000.  This means that 1” of map distance equals 2000’ feet on the ground.  1square inch on the map is equal to 91 acres on the ground.

The symbolic information provided by colors provides only a general description on the ground.  For example, green coloration refers to vegetation such as forested areas.  But what color does not tell you what is the surface really like?  The map doesn’t tell the hiker if the area is full of brush, blown down trees or large area of loose rock.

Resolution

Though maps are not perfect there are things the hiker can do to lessen the issue with maps.  Here are some suggestions:

• Review trail guides of area to be visited.

• Visit with other hikers that have been to the same area earlier.

• Visit  with government agency officials  such as:

• Forest Service Backcountry Rangers
• Fish and Game law enforcement officials.
• Biologists
• Search and Rescue organizations  in the county to be visited
• National Park Rangers

The reference for this post is Basic Land Navigation produced by the National Wildfire Coordinating Group (NWCG.)  This is a straight forward document that covers the essential elements of land navigation: it is a good introduction and the source for this post. This is a free publication.  Down load it here:  http://www.nwcg.gov/sites/default/files/products/pms475.pdf.