# Determining a map's projection

To find information about the projection used to create a map, look at its legend. The legend of a map may list a projection by name and give its parameters, such as Lambert conformal conic with standard parallels at 34° 02' N and 35° 28' N and origin at 118° W, 33° 30' N. Or it may list a coordinate system and zone number, such as California Coordinate System zone 5 or State Plane Coordinate System zone 3376.

It is also important, especially for large-scale maps, to know the spheroid used. The U.S. standard was Clarke 1866 (for NAD27), but the standard now is GRS80 (for NAD83). The spheroid is sometimes inherent to a coordinate system, such as Clarke 1866 for older State Plane maps or GRS80 for newer ones.

The following is an example of a map's projection information for a USGS 7.5-minute topographic map:

• Mapped, edited, and published by the Geologic Survey Control by USGS and USC&GS Topography by plane table surveys 1942. Revised 1955 polyconic projection. 1927 North American Datum 10,000-foot grid based on Rhode Island coordinate system 1000-meter universal transverse Mercator grid tz

Along the margins of most maps, you will find one or more sets of coordinates that reference locations on the earth's surface. On a USGS 7.5-minute topographic series map, three types of coordinates are provided: the projection of the map, universal transverse Mercator meters, and latitude-longitude degrees. The example below displays three different systems of coordinate and projection information along the graticule of a USGS 7.5-minute quadrangle.

 USGS 7.5 topo clip

For small-scale maps that contain multiple coordinate systems, there is an additional consideration. As the following graphic shows, all map projections distort the earth in different ways; therefore, lines of equal coordinate value may have different curvatures.

 Geographic vs transverse

For this reason, it is best to use graticular intersections on the map. In contrast to drawing straight lines across a map to connect coordinates on either side, graticule lines may be represented by curved lines. This is less of a concern for large-scale maps. It is not a problem for equatorial aspects of cylindrical projections, such as Mercator.

## Related Topics

Published 6/8/2010