Viewing Datum Shifts
The datum shift between two datums, such as WGS84/NAD83 and NAD27, can be computed in two ways:
- From the Lat/long on the two datums, using a Molodensky transformation.
- 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. This
problem is not present in MGRS, because the different
datums use a different 100,000 m grid square designator
and cannot be easily confused, although the numerical
parts of the coordinates are very similar. Because of the way
projections work, this will be more complicated:
- UTM in datum 1 to lat long in datum 1
- Lat long in datum 1 to lat long in datum 2 (Molodensky
- Lat long in datum 2 to UTM in datum 2
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.
Plot the results and
them using standard options.
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.
Last revision 1/18/2018