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.

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.

5. The coordinates for data change depend 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 the city of Bellingham, Washington using 3 different datums:


6. A principal 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:

For special instructions for the ArcInfo coverage, at 9.x, access Knowledge Base article 27108 -show me-

Summary

To use the ArcToolBox > Coverage Tools > Projections > Define Projection, or the Project Tool, a projection file formatted is necessary as required by the ArcInfo Workstation PROJECTDEFINE or PROJECT commands.  The PROJECT command must be followed by BUILD for all feature classes in the coverage in order to restore coverage topology.

Procedure

Two different file formats are required. One enables the coverage projections to be defined while the other allows coverages to be projected to another coordinate system. Both files must be created using NotePad.  WordPad cannot be used. WordPad adds invisible formatting characters to the file, which the commands PROJECTDEFINE and PROJECT cannot read. The process fails. Below is a sample of a projection file read by the Define Projection Tool: Projection ALBERS Units METERS Datum NAD83 Parameters 29 30 0.000 /* 1st standard parallel 45 30 0.000 /* 2nd standard parallel -96 0 0.000 /* central meridian 23 0 0.000 /* latitude of projection's origin 0.00000 /* false easting (meters) 0.00000 /* false northing (meters) Below is a sample of a projection file read by the Project Tool: input projection geographic units dd datum nad83 parameters output projection albers units meters datum nad27 parameters 29 30 00 45 30 00 -96 00 00 23 00 00 0 0 end For further information, refer to the ArcInfo Workstation Online Help in the ArcDoc. Go to the Index tab, type in the name of the input or output projection required, and Display the results. The Help page displays the required projection parameters.  To open the ArcInfo Workstation Online Help, go to Start > Programs > ArcGIS > ArcInfo Workstation > ArcDOC. Alternatively, type the word "help", without quotes, at the ArcInfo command prompt.

- ARCINFO COVERAGE: -show me-
- SHAPEFILE: -show me-
- GEODATABASE FEATURE DATASET/FEATURE CLASS: -show me-

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

- ARCINFO COVERAGE: -show me-
- SHAPEFILE: -show me-
- GEODATABASE FEATURE DATASETS/FEATURE CLASSES: -show me-

• Where can more information be found about coordinate systems, map projections, and datums?
  The Web sites listed below provide information on coordinate systems, map projections, and datums. It is also recommended to search the Web for other useful sites.
• What is the Projection Engine?
  The Projection Engine is a developer's library that provides map projection and datum transformation support. The data model is derived from a modified version of the European Petroleum Survey Group's (EPSG) coordinate system model. The Projec...
• Select the correct geographic (datum) transformation when projecting between datums
  It is necessary to specify a geographic (datum) transformation when using the ArcToolBox Project Wizard (ArcGIS version 8.x) or Project Tool (ArcGIS version 9.x) to project shapefiles or geodatabase feature classes between different geographic coo...
• What do the terms geoid, ellipsoid, spheroid and datum mean, and how are they related?
  The {BOLD}geoid{/BOLD} is defined as the surface of the earth's gravity field, which is approximately the same as mean sea level. It is perpendicular to the direction of gravity pull. Since the mass of the Earth is not uniform at all points, and t...
• After installing ArcGIS - ArcInfo 9.x the Coverage Tools are missing from ArcToolBox
  After installing ArcGIS - ArcInfo 9.x, the Coverage Tools Toolbox is missing from ArcToolBox.
• Select a suitable map projection or coordinate system
  A map projection is the mathematical process of taking a view of a spherical globe and drawing that information on a flat piece of paper. Four properties apply to map projections: {PRE}DIRECTION DISTANCE AREA SHAPE{/PRE}To select a ma...
• Identify the spatial reference, projection, or coordinate system of data
  Instructions provided describe how to identify the spatial reference, projection, or coordinate system of data. GIS and Computer Aided Design (CAD) data are frequently received with no spatial reference information. Because ArcMap's 'project on th...