[Xastir] UTM vs lat/long, GIS/GPS

[Curt, WE7U]

I'm having a discussion over on the sarcomm list and haven't found
any GIS-literate people to get some questions answered in order to
back up my points.  Since this is probably of interest here as well,
I'll see if Gerry Creager or other GIS-literate person might see it
here.

Start with this:

http://www.uaex.edu/Other_Areas/publications/HTML/FSA-1032.asp

In that document they say this:


  "How Much Error? For most receivers the default datum option for
  screen display probably will be WGS  84 or NAD  83. If the paper map
  datum is NAD  27, then NAD  27 should be selected from the GPS
  receivers option list. The position errors caused by choosing the
  wrong datum can be very serious for differentially corrected
  position data. How large the errors are depends on whether Lat/Long
  or UTM coordinates are being used for the GPS/map work. The errors
  may not be important for travel or recreation purposes, where the
  receivers screen-displayed maps are being used, or where precisely
  computed coordinates are not extremely critical.

   For lat/long, the position discrepancy will be less than 100 feet.
   For UTM coordinates, the discrepancy will be more than 650 feet."


I say that's BS.  Unfortunately some SAR guys on that list have
latched onto this as gospel and think that they get large errors
with UTM.

The fact that tick marks near the corners of the topo maps don't
line up with the lat/lon might tend to make them think they have
reinforcement for their view.

My points:

*) They're confusing datum-shift errors with UTM/Lat-Long.  They are
totally and entirely independent.  UTM does not give you ANY error
as compared with lat/long.

*) The above document is just plain wrong.  Wrong wrong wrong!

*) Some of the SAR guys are claiming errors of "two ridges over from
where they were supposed to be" or "up to 1/2 mile error when using
UTM".  They most likely had an incorrect datum for their paper or
electronics maps plus might have had lat/long format problems
compounding it.  You can be off by quite a bit if you think you're
given coordinates in one lat/long format but they're really in
another.

*) UTM gives you meter resolution in the numbers.  Nice.  Easy to
get to 10 meters or so plotting on a map without extra tools.

*) UTM gives you the ability to calculate distances in your head, A
squared plus B squared = C squared.


Can anyone shed some experienced light on this and help me get these
guys on the right track?

--
Curt, WE7U.   APRS Client Comparisons: http://www.eskimo.com/~archer
"Lotto:    A tax on people who are bad at math." -- unknown
"Windows:  Microsoft's tax on computer illiterates." -- WE7U
"The world DOES revolve around me:  I picked the coordinate system!"