HELP!! Clark66->WGS84 ???#%@*!

[Michael Shapiro]

It isn't quite so simple. UTM is a projection and  is  valid  for
any  spheroid.  Many of the older and recent map data is based on
the North American Datum of 1927 (NAD27) which uses  the  clark66
spheroid.   This  is  for the latitude and longitude coordinates,
which  are  then  projected  to  UTM (or Albers  or  Lamberts  or
some other projection).

The switch to NAD83 isn't as simple as converting UTM coordinates
back  to  lat-long using the clark66 spheroid, then from lat-long
to UTM using WGS84.

The "datum" not only specifies the spheroid, but the  orientation
of  the  spheroid  with respect to the earth itself. The NAD27 is
not an earth centered datum while NAD83 is (i.e., for  NAD27  the
center  of the clark66 spheroid isn't at the center of the earth,
while the center of the WGS84 spheroid is at earth center).  This
means  that  the   lat-long  coordinates  change when you go from
NAD27 to NAD83.  There is no  program   in   GRASS   to   perform
this  part  of  the transformation.

Recent quad sheets have approximate datum shifts for the  corners
of  the  map and perhaps can be used to devise a linear shift for
the entire map.

|
|Date sent:  25-AUG-1992 21:39:15 
|Thanks Fred:
|
|But this brings up the question what spheriod are the UTM data referenced
|too?? or isn't it referenced to a spheroid at all ?! (I'll look into it).
|
|>If I understand your question it is quite easy to get the results you want
|>the site data from the clark66 data set should be routed through m.ll2u
|>with the spheroid set to clark66 while the WGS84 stuff would be routed through
|>with the spheroid set to wgs84.  Both will come out in the correct UTM zone
|>with the differences in the spheroids corrected. 
|
|Thanks again,
|
|Gerry Hatcher                   
|
|Ocean Mapping Development Center|      gerry at omdc.gso.uri.edu {Internet}
|University of Rhode Island      |            cbeam::gerry {span}
|voice (401)-792-6940            |            fax (401)-792-6849
|
|
|



Michael