Viewing Datum Shifts

Projection and datum shift calculations.  The datum shift between two datums, such as WGS84/NAD83 and NAD27, can be computed in two ways:

  1. From the Lat/long on the two datums, using a Molodensky transformation.
  2. From the UTM coordinates on the two datums. This tends to be greater in magnitude, because the two datums likely use a different ellipsoid which can lead to large differences in the location of the point on the ground with the same UTM false easting and northing.  Because of the way projections work, this will be more complicated:

The datum shift option on the Cartography menu choice will display the shift between the two datums for both Lat/long and UTM coordinates. You will have the choice of labeling the shifts, using either the Lat/Long, UTM coordinates, or both.  The shift is computed by using the inversion projection equations to get the lat/long and UTM coordinates of the point on the map where you clicked.  Then the datum shift is performed on both sets of coordinates into the secondary datum, and distances between the two sets of Lat/Long and UTM coordinates.   

  You can Plot the results and Label them using standard options.  
Grid: You can also view the datum shift by plotting the UTM grid or lat/long graticule for both datums on any map.  You can pick the second grid, and the colors to use.

Additional references

Last revision 9/7/2018