Datum conversion explained

[Harvey Greenberg]

With regards to my question of last week:

After studying the problem in detail, being directed to read the
ARC/INFO docs more carefully, and sitting around with my
garmin, I think I have it figured out.

1) Yes, ARC/INFO gives correct results.  To convert the coordinates
of a point in Washington from NAD27 to NAD83, ARC/INFO
will correctly subtract ~95 meters in X and add ~198 meters in Y.

2) 48N122W NAD27 is not the same as 48N122W NAD83.
The different spheroids are responsible.

3) When people talk about quad corners in NAD27 and quad corners
in NAD83, they are referring to different points on the face of the
earth.

4) Those NAD83 ticks on your NAD27 paper topo sheet are of no
practical value.  They would be useful only if you had a NAD83 map
sheet of the same area, and wanted to overlay the maps so that you
could find a location of the first map and read its NAD83 coordinates
off the the graticule of the second map.  The ticks do not indicate the
change in coordinate values for a location on the map.

~harvey


--
hgreen at u.washington.edu  162 Johnson Hall  206-685-7981
http://duff.geology.washington.edu/

        Harvey Greenberg
        University of Washington
        Department of Earth and Space Sciences
        63 Johnson Hall (Box 351310)
        Seattle WA 98195-1310