Projection Basics: What the GIS professional needs to know

Answer

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 a perfect sphere. 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:

DATUM		X-Coordinate               Y-Coordinate 
NAD_1927	-122.466903686523	   48.7440490722656 
NAD_1983	-122.46818353793	   48.7438798543649 
WGS_1984	-122.46818353793	   48.7438798534299 

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:

ArcInfo Coverage:

ArcInfo Workstation - All versions
Use the PROJECTDEFINE command to define the projection parameters for coverages, grids, and tins.

ArcGIS 9.x, 10.x - ArcInfo only
ArcToolBox > Coverage Tools > Data Management > Projections > Define Projection tool

Shapefile:

ArcGIS 8.x - ArcInfo, ArcEditor and ArcView
ArcToolBox > Data Management Tools > Projections > Define Projection Wizard (shapefiles, geodatabase)

ArcGIS 9.x, 10.x - ArcInfo, ArcEditor and ArcView
ArcToolBox > Data Management Tools > Projections and Transformations > Define Projection tool

Geodatabase Feature Dataset/Feature Class:

ArcGIS 8.x - ArcInfo, ArcEditor and ArcView
ArcToolBox > Data Management Tools > Projections > Define Projection Wizard (shapefiles, geodatabase)

ArcGIS 9.x, 10.x - ArcInfo, ArcEditor and ArcView
ArcToolBox > Data Management Tools > Projections and Transformations > Define Projection tool

8. If the data has a projection definition, but the projection does not match the typical projection used by an organization, re-project the data.

ArcInfo Coverage:

ArcInfo Workstation - All versions
Use the PROJECT command to project coverages and grids to new coordinate systems.

ArcGIS 8.x - ArcInfo only
ArcToolBox > Data Management Tools > Projections > Projection Wizard (coverages, grids)

ArcGIS 9.x, 10.x - ArcInfo only
ArcToolBox > Coverage Tools > Data Management > Projections > Project tool.

Warning:
Project must be followed by a build. This applies to both the workstation command, and the project tools in ArcToolbox.

Shapefile:

ArcGIS 8.x - ArcInfo, ArcEditor and ArcView
ArcToolBox > Data Management Tools > Projections > Project Wizard (shapefiles, geodatabase)

ArcGIS 9.x, 10.x - ArcInfo, ArcEditor and ArcView
ArcToolBox > Data Management Tools > Projections and Transformations > Feature > Project OR Batch Project

Geodatabase Feature Dataset/Feature Class:

ArcGIS 8.x - ArcInfo, ArcEditor and ArcView
ArcToolBox > Data Management Tools > Projections > Project Wizard (shapefiles, geodatabase)

ArcGIS 9.x, 10.x - ArcInfo, ArcEditor and ArcView
ArcToolBox > Data Management Tools > Projections and Transformations > Feature > Project OR Batch Project