Knowledge Base - Technical Articles
HowTo: Identify an unknown projected coordinate system using ArcMap
| Article ID: | 24893 |
|---|---|
| Software: | 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 8.0.1, 8.0.2, 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 |
Summary
Instructions provided will help identify the projected coordinate system for datasets that do not have coordinate system information in the metadata or a projection definition, using the project-on-the-fly capability in ArcMap.
Coordinate system (projection) information should be obtained from the data source or provider, but are not always available, such as with legacy data. The technique described below helps to identify the correct projected coordinate system.
The term 'coordinate system' can refer to data in a Geographic Coordinate System (GCS) with coordinates in decimal degrees; or a Projected Coordinate System (PCS) with linear units in meters or feet. The term projection or PRJ, is an older term that is also used, but it is not as precise.
Coordinate system (projection) information should be obtained from the data source or provider, but are not always available, such as with legacy data. The technique described below helps to identify the correct projected coordinate system.
The term 'coordinate system' can refer to data in a Geographic Coordinate System (GCS) with coordinates in decimal degrees; or a Projected Coordinate System (PCS) with linear units in meters or feet. The term projection or PRJ, is an older term that is also used, but it is not as precise.Procedure
Before beginning, read "Projection Basics: What the GIS professional needs to know." This article includes the parameters required for various coordinate systems, as well as useful information about working with coordinate systems in ArcMap. -show me-
| Question |
Projection Basics: What the GIS professional needs to know |
| Answer |
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 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 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 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) 7. Projections can be defined for data using the following options: For special instructions for the ArcInfo coverage access Knowledge Base article 27108. -show me- - 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- |
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- Start ArcMap with a new, empty map, and add the data with the unknown coordinate system to the map. This data must not be associated with a PRJ file or coordinate system definition.
- Right-click the layer name in the Table of Contents, select Properties > Source tab, and examine the extent of the data. Click on the 'show me' for an example.
-show me-
If the coordinates are in decimal degrees, such as between longitude -180 and +180, and latitude -90 and +90, identify the Geographic Coordinate System (datum) used for the data. Prior to version 9.2, ArcMap assigns GCS_Assumed_Geographic_1 to the data, by default. This places the data on the NAD_1927 datum, which is incorrect in most cases.-show me- Question Is GCS_Assumed_Geographic_1 a coordinate system?Answer No, CGS_ASSUMED_GEOGRAPHIC_1 is not a real coordinate system. If the spatial reference for a data layer displays this name in ArcGIS Desktop, it means that there is no projection file associated with the data. To overlay the data correctly in ArcMap, take steps to define the coordinate system.
Some data in later versions may have a projection actually defined with this default coordinate system. This definition is incorrect in all cases, and data with this definition does not align correctly with other data in ArcMap.
The 'GCS_ASSUMED_GEOGRAPHIC_1' spatial reference definition was created for earlier versions of ArcGIS to permit ArcMap to 'guess' at the coordinate system for data, which has coordinates in decimal degrees. This causes ArcMap to make the same assumptions about the spatial reference for data that exist in ArcInfo Workstation and ArcView 3.x.
To identify and define the projection for the data, see the link in the Related Information section below for more information in 'Projection Basics: What the GIS professional needs to know' and 'What geographic goordinate system or datum should be used for my data?'.- 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 comm... - Shapefiles defined as GCS_Assumed_Geographic_1 display as Unknown and the definitions are gone
Shapefiles defined as GCS_Assumed_Geographic_1 display as Unknown coordinate system and do not project on the fly. - What geographic coordinate system or datum should be used for my data?
To determine which Geographic Coordinate System and datum to use for data, select one of the 'Geographic Coordinate Systems by Area of Use' links in the Related Information section below. The related documents lists Geographic Coordinate Syste...
In ArcGIS Desktop at version 9.2 and later versions, no default coordinate systems are assigned to any data.-show me- Description Shapefiles defined as GCS_Assumed_Geographic_1 display as Unknown coordinate system and do not project on the fly.Cause GCS_Assumed_Geographic_1 is a default geographic coordinate system (GCS) assigned to certain data sources which was used for data in ArcGIS versions 8.x, 9.0 and 9.1.
It was used only for shapefiles that had coordinates in decimal degrees (DD): the data extents were between -180 and +180 for X coordinates, and -90 and +90 for Y coordinates. These data sources will now use Unknown for the coordinate system. ArcMap will not be able to project them on the fly to match other layers in the map. Data that used to align correctly in ArcGIS version 9.1 or earlier will no longer line up in version 9.2.
This default coordinate system definition was removed from ArcGIS version 9.2 for the following reasons:
1) The default coordinate system identified the datum as D_North_American_1927. In most cases, this datum was not correct for the data, and can result in alignment errors of the data when displayed in the ArcMap data frame.
2) The datum D_North_American_1927 is defined only for the North American continent, and cannot be used for areas outside North America.
3) GCS_Assumed_Geographic_1 is not a valid coordinate system. ArcMap does not have any geographic (datum) transformations that convert from this coordinate system to any valid GCS.
4) Data having this default coordinate system cannot be projected to a different coordinate system in ArcToolbox.
5) When ArcGIS 8.0 was first released, many shapefiles did not have coordinate system information. GCS_Assumed_Geographic_1 was added to ArcGIS to allow shapefiles that did not have a defined coordinate system to be projected on the fly in ArcMap.Solution or Workaround Determine the correct geographic coordinate system for the data to assign a valid coordinate system definition using the Define Projection tool.
The Define Projection tool is located in ArcToolBox > Data Management Tools > Projections and Transformations. The Define Projection tool can be used to define coordinate systems for both vector and raster datasets.- The most reliable method to find out the coordinate system for the data is to contact the data provider for this information.
If the data provider is unable to provide this information, go to Step 2. - Data obtained from a Global Positioning System (GPS) is most frequently collected on the WGS 1984, and will be defined as GCS_WGS_1984 in ArcGIS Desktop. Find this definition in the Geographic Coordinate Systems > World folder.
- Data with coordinates in decimal degrees within the United States is most frequently on the North American Datum 1983 (NAD 1983), although it may be on the North American Datum 1927 (NAD 1927).
Because the coordinates for data within the U.S. on the NAD 1927 datum will be south-southwest of coordinates for the same point on the NAD 1983 datum by approximately 200 to 450 feet, test the geographic coordinate system to determine which is correct for the data.
-show me- - If the data still does not align correctly, contact ESRI Technical Support at http://support.esri.com or call (888) 377-4575 and create a new support incident if you are a maintenance user or paid support subscribers within the United States. If you are an international customer, please contact your local ESRI distributor.
- 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 comm... - What geographic coordinate system or datum should be used for my data?
To determine which Geographic Coordinate System and datum to use for data, select one of the 'Geographic Coordinate Systems by Area of Use' links in the Related Information section below. The related documents lists Geographic Coordinate Syste...
If the data is in the United States and shows an extent in which the coordinates to the left of the decimal are 6, 7 or 8 digits, the data is probably projected to the State Plane or UTM coordinate systems. - Projection Basics: What the GIS professional needs to know
- If the unknown data lies within the United States, add comparison data indicated below to ArcMap:
ArcGIS 8.x
Navigate to the folder <drive_letter>\arcgis\arcexe8x\Reference System, and add the file USSTPLN83.SHP to ArcMap.
ArcGIS 9.x
Navigate to <drive_letter>\Program Files\ArcGIS\Reference Systems, and add the file USSTPLN83.SHP to ArcMap.
ArcGIS 10.x
Navigate to <drive_letter>\Program Files\ArcGIS\Desktop 10.0\Reference Systems, and add the file USSTPLN83.SHP to ArcMap. If the computer has a 64-bit operating system the install folder will be 'Program Files(x86)'. The original data and the newly added shapefile are not displayed in the same location.
Click this link to view an image of USSTPLN83.SHP.-show me- - A. Click View > Data Frame Properties > Coordinate System tab.
B. In the lower window labeled 'Select a coordinate system', click Predefined > Projected > State Plane.
C. Select and assign the coordinate system for the State Plane FIPS zone in which the data should be located. Select the relevant projection files from the folders as listed below. Note that additional folders have been added at later versions of ArcGIS Desktop:
ArcGIS 8.x
NAD 1927
NAD 1983 with no descriptor, units are in meters
NAD 1983 (feet)
NAD 1983 HARN
ArcGIS 9.0; 9.1
NAD 1927
NAD 1983 with no descriptor, units are in meters
NAD 1983 (feet)
NAD 1983 (Intl Feet)
NAD 1983 HARN
NAD 1983 HARN (Feet, Intl and US)
ArcGIS 9.2, 9.3, 9.3.1
NAD 1927
NAD 1983 with no descriptor, units are in meters
NAD 1983 (feet)
NAD 1983 (Intl Feet)
NAD 1983 HARN (Feet, Intl and US)
NAD 1983 HARN
ArcGIS 10.0
NAD 1927 (US Feet)
NAD 1983 (CORS96)(Intl Feet)
NAD 1983 (CORS96)(Meters)
NAD 1983 (CORS96)(US Feet)
NAD 1983 (Intl Feet)
NAD 1983 (Meters)
NAD 1983 (US Feet)
NAR 1983 HARN (Intl Feet)
NAD 1983 HARN (Meters)
NAD 1983 HARN (US Feet)
NAD 1983 NSRS2007 (Intl Feet)
NAD 1983 NSRS2007 (Meters)
NAD 1983 NSRS2007 (USFeet)
Other GCS The U. S. Survey Foot (foot_us) is defined as exactly 1200/3937 of a meter, or 0.3048006096012192....
The International Foot (foot) is defined as exactly 0.3048 of a meter.
D. After selecting a projection file, click Apply, and click OK.
E. Repeat assigning a State Plane projection file from the available folders until the USSTPLN83.SHP file snaps into place and the data with the unknown coordinate system appears in the correct area in the proper state.
F. Verify the correct place by zooming into the layer and using the Identify Tool on the state where the data is drawn.
ArcGIS 9.x
Additional coordinate systems are available for the following states: Alaska, California, Georgia, Idaho, Michigan, Minnesota, Oregon, Texas and Wisconsin. State-wide projections are added for Florida and Mississippi at 9.3. These projected coordinate systems can be found in Projected Coordinate Systems > County Systems, National Grids, or State Systems folders. If the data is located in one of these states, the specific projections for the state or county should also be considered and tested.
When the coordinate system is assigned to the Data Frame, a warning box may display:
ArcMap 8.x, 9.0, 9.1 Warning
If the above warning appears for 8.x; 9.0; 9.1: Click View > Data Frame Properties > Coordinate System tab > Transformations button.
ArcMap version 9.2/9.3/9.3.1/10.x Geographic Coordinate Systems Warning
If the above warning appears, click 'Transformations...'
All Versions: In the 'Using...' box, verify that the appropriate transformation method was applied. To access the list of supported datum transformation methods and the areas for which each should be used, refer to "Select the correct datum transformation method when projecting between datums" in Related Information, below.
H. If the data lines up correctly using one of the State Plane options, go to step 6. If not, continue to step 5. - If the data does not line up after testing the State Plane options:
A. Open the UTM folder.
B. Perform the same steps as described in step 4, A-F. Select the correct UTM zone for the unknown data from the following datums; they are the most commonly used in the United States:
NAD 1927
NAD 1983
WGS 1984 Coordinates in UTM meters on the NAD 1983 datum and coordinates for the same point on the WGS 1984 datum in the continental United States are within a meter from each other.
Data in the UTM coordinate system, on the NAD 1983 datum, is approximately 200 meters north of the same data on the NAD 1927 datum. There may be a slight shift either east or west between data on these two datums but an approximate 200-meter difference in the northing is diagnostic.
The 200-meter difference is comparatively slight; therefore, it is essential that precise comparison data be used to determine whether the correct datum is NAD 1927 or NAD 1983 for data in the UTM projection.
- When the correct coordinate system is found:
A. Write down the path and name.
B. Refer to the instructions in Knowledge Base article 23025 for the steps to define the coordinate system for your data.-show me- Question Projection Basics: What the GIS professional needs to knowAnswer 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, 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.
-show me-
- 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 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:SHAPE DISTANCE AREA DIRECTION
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...
C. When the coordinate system is identified and defined, the data lines up in ArcMap with other data added to the ArcMap session, provided that the correct datum transformation was specified. - Where can more information be found about coordinate systems, map projections, and datums?
- If the above steps do not line up the data in ArcMap, the data may be in a custom coordinate system. Custom projection files can be created in ArcMap to align data in an unknown coordinate system.
Additional information is provided in the book "Lining Up Data in ArcGIS: A Guide to Map Projections" published by Esri Press. For further information and ordering, click the link in the Related Information section below.
For further assistance, contact Esri Support Services at 888-377-4575 or at support.esri.com.
Related Information
- 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 comm... - 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... - Is GCS_Assumed_Geographic_1 a coordinate system?
No, CGS_ASSUMED_GEOGRAPHIC_1 is not a real coordinate system. If the spatial reference for a data layer displays this name in ArcGIS Desktop, it means that there is no projection file associated with the data. To overlay the data correctly in ArcM... - Lining Up Data in ArcGIS: A Guide to Map Projections
Created: 5/7/2003
Last Modified: 5/20/2011
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Comments
By Anonymous - 03/12/2007 12:05 PM
Other - See details below.
This article was extremely helpful to me in solving a long-standing problem. Thanks for posting it!
Rating:
By Anonymous - 06/25/2004 9:07 AM
Step 3 says to navigate to a webpage that does not exist and add a file that I was unable to find. I cannot tell from reading the document whether or not this file is needed. I was also cinfused by the information about datum transformation methods. I not sure where to look for help in lining up the data I am using.
By Anonymous - 08/07/2007 6:52 AM
Other - See details below.
The article only refers to a solution if your image is of the USA. It would be extrememly helpful if some tips for the rest of the world were included!....even if it was to say this is extremely difficult it would be preferrable to reading through the entire article anticipating some information on the rest of the world.
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