
Greetings from Idaho !

I have some concerns about explaining datum conversions.  I have had 
someone who has gotten six different answers from six different people 
(of course, I think my answer was a correct one!), but I think that is 
more a result of asking the right questions in the right way and giving 
the right answer in an equally right way.  I`ll pose the question to you 
folks who I have come to rely upon for sound cartographic answers as well 
as GRASS answers.

By the way, the question arose when the person was attempting to transform
NAD27 soil data on a 7.5 minute quad base to NAD83.

Given:  The Alpha quad, in NAD27, of soils data, has a NE corner of 
       -114 00 00 longitude and 44 00 00 latitude.  The corresponding 
       UTM easting and northing values are 547100 and 4341400.  (These 
       values are made up just for discussion purposes.)

Given:  A transformation of the entire file is made to the NAD83 datum.

Given:  All coordinates in that file have changed due to the
                transformation.

Question:  What are the longitude and latitude of the NE corner now?
           Is it still -114 00 00 and 44 00 00?
           Have the easting and northing of the NE corner changed?
           Would they now be 547032 and 4341479 (or whatever the
                actual shift for that quad is)?

I tried to tell this person that latitude longitude are surface 
expressions of angular measurements and will not change the same 
way we think of coordinates systems such as UTM changing.  For 
example, in the geographic coordinate system, 90 degrees will still 
be straight from the center of the earth to the north pole and 0 
degrees will be perpendicular to that, and datum shifts will not 
change that.  Thus, the NE corner would still have the same spherical 
(lat/long) coordinate values under both datums.  What does change is 
the location on the earth's surface of the NE corner of that quad in 
relation to a plane coordinate system such as a UTM.

If all the quads in an area are converted individually, then there will
be no gaps in the data.

How does this fit with your interpretation of datum shifts?  Maybe we
are mixing apples and oranges here in talking about point coordinate
changes and how we graphically represent them on a map.  Of course the
plane coordinates for a quad's corner change, but we will still publish
that map on a 7.5 minute base and have evenly divided corner locations
in 7.5 minute increments.

Thanks in advance for shedding some light on how I might explain this
business better.  Even if you agree with the way I am presenting this,
let me know so I can get some feedback!

David Hoover, Manager
Cartography and GIS Section
USDA - Soil Conservation Service
Idaho State Office (Boise)
internet!attmail!a16dhoover
208 334 1525


