Knowledge Base - Technical Articles


Technical Article   HowTo:  Apply a geographic (datum) transformation in ArcMap

Article ID: 17430
Software:  ArcGIS - ArcEditor 9.2, 9.3, 9.3.1, 10 ArcGIS - ArcInfo 9.2, 9.3, 9.3.1, 10 ArcGIS - ArcView 9.2, 9.3, 9.3.1, 10
Platforms: N/A

Summary

ArcMap automatically projects data layers to the Data Frame's coordinate system. This is called 'project-on-the-fly'. However, if the Geographic Coordinate Systems (GCS) are not the same, a geographic (datum) transformation must be applied manually to the ArcMap Data Frame. This is in addition to the operation performed with project-on-the-fly, which only temporarily converts coordinate values from the projection definition of the data to the coordinate system applied to the ArcMap Data Frame.

The geographic (datum) transformation cannot be automatically applied because many transformations may exist for a given area. Geographic transformations use methods with different accuracies, or are defined for particular areas. This document discusses how to manually apply a geographic transformation to the ArcMap Data Frame.

Procedure

  1. Open the Data Frame properties dialog.
  2. Click the Coordinate System tab.
  3. Click the Transformations button on the right side of the dialog. The Geographic Coordinate Systems Transformations dialog box appears.



    [O-Image] Transformations dialog

    The 'Convert from' section lists the geographic coordinate systems of the layers. The 'Into' listbox displays the Geographic Coordinate System of the ArcMap Data Frame.
     Do not change the value in the 'Into' box in this dialog.

    The 'Using' dropdown lists any predefined transformation between the two systems.

     Don't worry if the transformation name goes in the opposite direction. ArcMap applies the transformation in either direction.

  4. Select one of the listed transformations, or click New to define a custom transformation.
     Only one transformation can be selected in this dialog.

  5. To determine the correct geographic transformation to use for the area of interest, click the link in the Related Information section below to Knowledge Base article 21327, entitled "Select the correct geographic (datum) transformation when projecting between datums".

Related Information


Created: 3/22/2001
Last Modified: 3/2/2011

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Comments

By Anonymous - 04/07/2003 2:18 PM

Rating:

By Anonymous - 03/17/2003 2:35 AM

Where does a new datum transformation get stored? After having defined a new coordinate system, will the datum transformation I've entered into one ArcGIS project be available in others each time I use that coord system?

By Anonymous - 01/28/2003 11:11 AM

How do you know which one of the transformations to use?