Datum Caution

All data sets (DEMs, satellite imagery, and scanned maps) are transformed to the local datum. The local datum appears at the bottom of the map window, as a 5 character code. This is the same code used to select datums. All coordinates displayed by the program, or that you enter, use this local datum. You set the local datum in the Maps tab of options form . This will take effect for all data sets loaded after you select it. You might need to close data sets loaded before you set the correct local datum and then reload them.

The data you use was digitized on a particular datum. The program should correctly identify and set this datum. For a DEM you can see this datum on the Analyze header menu choice, and if absolutely necessary you can change it on the data manipulation menu under the Edit, Header menu choice. You can only change the digitizing datum for a DEM stored in the program’s internal format. For a satellite image you can view (and change) the digitizing datum by looking at the XY file that goes with the DEM.

The datum can significantly vary coordinates, and you should take great care to understand about the effect of the datum. This can be particularly true for scanned maps, whose grid might not be on the expected grid. Users of CADRG and ADRG must take note that many of the maps show a local datum grid, and not WGS84. For example, many of the CONUS maps show a UTM grid and lat/long ticks on the NAD27 datum, displaced several 100 m from WGS84.

Datum shifts

Last revised 8/19/2006