FAQ: Projection Basics: What the GIS professional needs to know
Projection Basics: What the GIS professional needs to know
The following concepts are fundamental to understanding the use of map projections in ArcGIS. Please note though that the topic of projections is extremely broad, and this article can do no more than touch on a few important topics.
1. Coordinate systems, also known as map projections, are arbitrary designations for spatial data. Their purpose is to provide a common basis for communication about a particular place or area on the earth's surface. The most critical issue in dealing with map projections is knowing what the projection is and having the correct coordinate system information associated with a dataset.
2. When the first map projections were devised, it was assumed, incorrectly, that the earth was flat. Later the assumption was revised, and the earth was assumed to be a perfect sphere. In the 18th century, people began to realize that the earth was not perfectly round. This was the beginning of the concept of the cartographic spheroid.
3. To more accurately represent locations on the earth's surface, map makers studied the shape of the earth (geodesy) and created the concept of the spheroid. Then geographic coordinate systems (GCS) were devised, which include a datum, units of measure, and a prime meridian. A datum links a spheroid to a particular portion of the earth's surface. Recent datums are designed to fit the entire earth's surface well.
4. The most commonly used datums in North America are:
• NAD 1927 (North American Datum 1927) using the Clarke 1866 spheroid • NAD 1983 (North American Datum 1983) using the GRS 1980 spheroid • WGS 1984 (World Geodetic Survey 1984) using the WGS 1984 spheroid
Newer spheroids are developed from satellite measurements and are more accurate than those developed by Clarke in 1866. The terms 'geographic coordinate system' and 'datum' are used interchangeably, but as noted above, a GCS includes a datum, spheroid, units of measure and a prime meridian.
5. The coordinates for data change depending on the datum and spheroid on which those coordinates are based, even if they are using the same map projection and parameters.
For example, the geographic coordinates below are for a single point located within the city of Bellingham, Washington, using 3 different datums:
6. A principle of good data management is to obtain the projection parameters from the data source providing the data. Do not make an educated guess about the projection of data, because an inaccurate GIS database will be the result. The necessary parameters are the following:
• Projection • Units of measure • ZONE (for UTM) • FIPS zone (for State Plane) • Datum
Other parameters may be required, depending on the projection. For example, Albers and Lambert projections require the following parameters:
• 1st standard parallel, in degrees, minutes and seconds (DMS) • 2nd standard parallel (DMS) • Central meridian (DMS) • Latitude of projections origin (DMS) • False easting and units of measure • False northing and units of measure • X-shift and units of measure • Y-shift and units of measure
7. Projections can be defined for data using the following options: