Knowledge Base - Technical Articles
FAQ: Projection Basics: What the GIS professional needs to know
| Article ID: | 23025 |
|---|---|
| Software: | ArcInfo Workstation 8.1, 8.1.2, 8.2, 8.3, 9.0, 9.1, 9.2, 9.3, 9.3.1, 10 ArcGIS - ArcEditor 8.1, 8.1.2, 8.2, 8.3, 9.0, 9.1, 9.2, 9.3, 9.3.1, 10 ArcGIS - ArcInfo 8.1, 8.1.2, 8.2, 8.3, 9.0, 9.1, 9.2, 9.3, 9.3.1, 10 ArcGIS - ArcView 8.1, 8.1.2, 8.2, 8.3, 9.0, 9.1, 9.2, 9.3, 9.3.1, 10 |
| Platforms: | N/A |
Question
Answer
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:
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 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 access Knowledge Base article 27108.
| Summary |
Instructions provided describe how to create and format projection files for coverages to be defined or re-projected using ArcToolbox tools.
The Define Projection and Project Tools for coverages are in the following locations: ArcGIS Desktop 9.x ArcToolBox > Coverage Tools > Projections ArcGIS Desktop 10.x ArcToolBox > Coverage Tools > Data Management > Projections A projection file must be created in Notepad, in the format 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 projection to be defined while the other allows the coverage 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.
|
- ARCINFO COVERAGE:
ArcInfo Workstation - All versions
Use PROJECTDEFINE command to define the projection parameters for coverages, grids, and tins. ArcGIS 8.x - ArcInfo only ArcToolBox > Data Management Tools > Projections > Define Projection Wizard (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, reproject 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. 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 DATASETS/FEATURE CLASSES:
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. |
Related Information
- 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 original European Petroleum Survey Group's (EPSG) coordinate system model. T... - 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 geoid is defined as the surface of the earth's gravity field, which approximates mean sea level. It is perpendicular to the direction of gravity pull. Since the mass of the Earth is not uniform at all points, the magnitude of grav... - 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}SHAPE DISTANCE AREA DIRECTION{/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 o...
Created: 8/6/2002
Last Modified: 4/10/2012
(11)
Comments
By Anonymous - 04/23/2010 5:06 AM
I have suggested related resources/links that can enhance this article. See below.
\"The terms "Geographic coordinate system" & "datum" are used interchangably\". If they are they should not be! A GCS is a particular type of coord sys, ie one that is based on angles. A datum is a component of any coordinate system whether that be a GCS or PCS and essentially defines the location of the origin point for the coord sys. A particular pet hate of mine is when a GIS professional (so called) states that they would like their data \"IN\" WGS84. WGS84 is a datum not a coord sys.
Rating:
By Anonymous - 01/09/2007 9:10 AM
Other - See details below.
I have tried using the Project tool to no avail...I have carefully compared Projections but have shapefiles that will not line up. Also, ArcMap sometimes projects on the fly and sometimes will not and I don't know why. Thanks! Dennis Smith Please respond!
By Anonymous - 12/11/2006 8:18 AM
The article has missing, misleading, or conflicting information.
The article states: "Coordinate systems, also known as map projections, are arbitrary designations for spatial data." Coordinate systems ARE NOT map projections. If you are attempting to help your customers work through these difficult concepts, then you should distinguish each from the other instead of using both interchangably. Geographic coordinate systems (NAD27, NAD83, etc) are reliable systems for recording and reporting locations on a 3-D earth model (e.g., Clarke1866, GRS80, WGS84
By Anonymous - 10/22/2004 10:38 AM
"When the first map projections were devised, it was assumed incorrectly that the earth flat." Typo needs to be fixed.
Rating:
By Anonymous - 03/29/2003 10:27 PM
This was very helpful in my field of GIS mapper but I am trying to find the course exercises to ArcGIS I.
Rating:
By Anonymous - 03/25/2003 11:42 AM
Rating:
By Anonymous - 03/25/2003 8:14 AM
It was very informative, but did not answer my question. I have .prj and .shp files among others which I'm trying to project into Arcview. The files were originally created in ArcCatalog and sent to me on a CD. I do not have access to ArcCatalog, so I'm trying to change the projection from Lambert to Michigan GeoRef. How do I do this in ArcView?
Rating:
By Anonymous - 03/25/2003 4:45 AM
Need information on vertical datums and conversions. I cannot find any information in esri stuff.
Rating:
By Anonymous - 03/24/2003 8:10 PM
Rating:
By Anonymous - 03/24/2003 7:21 AM
I know what projections are, I studied that in college. I need to know how to get data from different projections to overlay. Including when the coordinate systems are undefined. I have been wrestling with this problem for so long I have given up. Which happens a lot with ESRI products. I hope the next ArcWhatever update is a little more user friendly.
By Anonymous - 03/20/2003 2:00 PM
By Anonymous - 03/20/2003 7:02 AM
Rating:
By Anonymous - 03/18/2003 4:36 AM
Rating:
By Anonymous - 03/17/2003 9:12 AM
Rating:
By faisalfadhil - 06/01/2012 11:14 PM
Great article! It helped a lot!
While it may seem to be slightly inaccurate for some, I found it quite useful as a starting points. Maybe some more detailed information would make it even more useful.
Rating:
