The method used to portray a part of the spherical Earth on a flat surface, whether a paper map or a computer screen, is called a map projection. No flat map can rival a globe in truly representing the surface of the entire Earth, so every flat map misrepresents the surface of the Earth in some way. A flat map can show one or more--but never all--of the following:
- True directions
- True distances
- True areas
- True shapes
Different projections have different uses. Some projections are used for navigation, while other projections show better representations of the true relative sizes of continents.
For example, the basic Mercator projection yields the only map on which a straight line drawn anywhere within its bounds shows a true direction, but distances and areas on Mercator projection maps are grossly distorted near the map's polar regions. On an equidistant map projection, distances are true only along particular lines, such as those radiating from a single point selected as the center of the projection. Shapes are more or less distorted on every equal-area map.
The scale of a map on any projection is often crucial to the map's usefulness for a given purpose. For example, the extreme distortion that is present at high latitudes on a small-scale Mercator map of the World disappears almost completely on a properly oriented Transverse Mercator map of a small area in the same high latitudes. A large-scale (1:24,000) 7.5-minute USGS topographic map based on the Transverse Mercator projection is nearly correct in every respect.
The USGS Map Projections poster summarizes and compares eighteen common map projections and their uses.
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How do I find, download, or order topographic maps?
The U.S. Geological Survey (USGS) has been the primary civilian mapping agency of the United States since 1879. USGS topographic maps have been published at many scales, but 1:24,000 (also referred to as a 7.5-minute quadrangle) has been the standard topographic map scale since 1947. For Alaska the standard scale is 1:25,000. All dates and all scales of our topographic maps can be freely...
What is the State Plane Coordinate System? Can GPS provide coordinates in these values?
The State Plane Coordinate System (SPCS), which is only used in the United States, is a plane coordinate system (north-south and east-west lines are perpendicular) in which each individual state has between one to six zones, depending on the state's size and shape. This coordinate system’s high level of accuracy is achieved through the use of relatively small zones. The State Plane Coordinate...
How are UTM coordinates measured on USGS topographic maps?
The UTM (Universal Transverse Mercator) coordinate system divides the world into sixty north-south zones, each 6 degrees of longitude wide. UTM zones are numbered consecutively beginning with Zone 1, which includes the westernmost point of Alaska, and progress eastward to Zone 19, which includes Maine. If UTM ticks are shown on a USGS topographic map, the zone is indicated in the credit legend in...
What do the different north arrows on a USGS topographic map mean?
A diagram at the bottom of most USGS topographic maps shows three north arrows--true north, grid north, and magnetic north--and the angles between them. Some maps, especially very old maps, do not have this diagram. True north, also called geodetic north or geographic north, is the direction of the line of longitude that bisects the quadrangle. All longitude lines converge to points at the north...
What is the projection, horizontal datum, vertical datum, and resolution for a USGS digital elevation model (DEM)?
Projection: 3DEP DEMS have different projections/coordinate systems depending on the product: 1/3-, 1-, and 2-arc-second (also the discontinued 1/9-arc-second) DEMs are not projected. They are all in geographic coordinates (latitude/longitude). 5-meter DEMs (Alaska only) are Alaska Albers Equal Area. 1-meter DEMs are in Universal Transverse Mercator (UTM). Original Product Resolution (OPR) DEMs –...
What are the projection, horizontal and vertical datum, format, and distributed tile extent of 3D Elevation Program (3DEP) contours?
Contours are not projected, but are provided in geographic coordinates (latitude and longitude) in units of decimal degrees, and horizontally referenced to the North American Datum of 1983. Contour elevation values are vertically referenced to the North American Vertical Datum of 1988. Contours are staged in Esri File Geodatabase 10.1 or Shapefile format, and distributed in 1x1 degree tiles.
Related Content
- FAQ
How do I find, download, or order topographic maps?
The U.S. Geological Survey (USGS) has been the primary civilian mapping agency of the United States since 1879. USGS topographic maps have been published at many scales, but 1:24,000 (also referred to as a 7.5-minute quadrangle) has been the standard topographic map scale since 1947. For Alaska the standard scale is 1:25,000. All dates and all scales of our topographic maps can be freely...
What is the State Plane Coordinate System? Can GPS provide coordinates in these values?
The State Plane Coordinate System (SPCS), which is only used in the United States, is a plane coordinate system (north-south and east-west lines are perpendicular) in which each individual state has between one to six zones, depending on the state's size and shape. This coordinate system’s high level of accuracy is achieved through the use of relatively small zones. The State Plane Coordinate...
How are UTM coordinates measured on USGS topographic maps?
The UTM (Universal Transverse Mercator) coordinate system divides the world into sixty north-south zones, each 6 degrees of longitude wide. UTM zones are numbered consecutively beginning with Zone 1, which includes the westernmost point of Alaska, and progress eastward to Zone 19, which includes Maine. If UTM ticks are shown on a USGS topographic map, the zone is indicated in the credit legend in...
What do the different north arrows on a USGS topographic map mean?
A diagram at the bottom of most USGS topographic maps shows three north arrows--true north, grid north, and magnetic north--and the angles between them. Some maps, especially very old maps, do not have this diagram. True north, also called geodetic north or geographic north, is the direction of the line of longitude that bisects the quadrangle. All longitude lines converge to points at the north...
What is the projection, horizontal datum, vertical datum, and resolution for a USGS digital elevation model (DEM)?
Projection: 3DEP DEMS have different projections/coordinate systems depending on the product: 1/3-, 1-, and 2-arc-second (also the discontinued 1/9-arc-second) DEMs are not projected. They are all in geographic coordinates (latitude/longitude). 5-meter DEMs (Alaska only) are Alaska Albers Equal Area. 1-meter DEMs are in Universal Transverse Mercator (UTM). Original Product Resolution (OPR) DEMs –...
What are the projection, horizontal and vertical datum, format, and distributed tile extent of 3D Elevation Program (3DEP) contours?
Contours are not projected, but are provided in geographic coordinates (latitude and longitude) in units of decimal degrees, and horizontally referenced to the North American Datum of 1983. Contour elevation values are vertically referenced to the North American Vertical Datum of 1988. Contours are staged in Esri File Geodatabase 10.1 or Shapefile format, and distributed in 1x1 degree tiles.
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