How To: Create a custom transformation between South American Datum 1969 and SIRGAS 2000
This article was contributed by Imagem Geosistemas E Comercio Ltda., Brazil
A geographic transformation, also known as a datum transformation, is a method to covert data between two geographic coordinate systems. A geographic transformation is necessary every time data from two different datums is projected.
From ArcCatalog or ArcMap, open ArcToolbox and navigate to ArcToolbox > Data Management Tools > Projections and Transformations and open the Create Custom Geographic Transformation tool.
In the Geographic Transformation Name field, specify the name of the geographic transformation.
Note: Geographic Transformation names must not contain spaces, and must be in the format "From_Input_GCS_To_Output_GCS". In this case, a transformation named SAD_1969_To_SIRGAS_2000_IGBE is suitable.
Warning: The transformation name must be unique, and must not duplicate the name of an existing geographic transformation.
In the Input Geographic Coordinate System field, select the data's current geographic coordinate system.
A. Click the button to the right of the Input Geographic Coordinate System field to invoke the Spatial References Properties dialog.
B. In the the Spatial References Properties dialog, click the Select button. C. In the Browse for Coordinate System window, double-click the Geographic Coordinate Systems folder. D. Double-click the South America folder. E. Select South American Datum 1969.prj and click Add. F. In the Spatial Reference Properties window, click OK.
In the Create Custom Geographic Transformation dialog box, select the geographic coordinate system that the data is to be transformed to.
A. Click the button to the right of the Output Geographic Coordinate System field to invoke the Spatial References Properties dialog box. B. In the Spatial References Properties dialog, click the Select button. C. In the Browse for Coordinate System window, open the Geographic Coordinate Systems folder. D. Double-click the South America folder. E. Select SIRGAS 2000.prj and click Add. F. In the Spatial Reference Properties window, click OK.
In the Create Custom Geographic Transformation dialog box, under Custom Geographic Transformation, select MOLODENSKY from the Method drop-down list.
In the Parameters field, populate the Value column with the following values. Be sure to include the minus (-) signs in the value if the parameter value is negative. The plus (+) sign is not entered in the value field.
• X Axis Translation (meters) = -67.35 • Y Axis Translation (meters) = +3.88 • Z Axis Translation (meters) = -38.22
Note: These parameters are provided by the Brazilian Institute of Geography and Statistics (IBGE).
Warning: These values for the Geographic Transformation parameters are only valid in one direction. In this example, the transformation parameter values provided transform the geographic coordinate system from SAD 1969 to SIRGAS 2000. These values are not valid when creating a geographic transformation in the opposite direction; from SIRGAS 2000 to SAD 1969. However, once the transformation has been created, ArcGIS Desktop can apply the transformation in either direction.
Close ArcToolbox, and then close either ArcCatalog or ArcMap. When the application is closed, the GTF file is written to the location of the user profile on the computer.
Note: This transformation, officially named SAD_1969_To_SIRGAS_2000_1, WKID 15485, has been added to ArcGIS Desktop at version 10. The transformation is used for Brazil - onshore and offshore, including Rocas, Fernando de Noronha archipelago, Trindade, Ihlas Martim Vaz and Sao Pedro e Sao Paulo.