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

[Wes Johnston]

About a year ago, I did a comparison between NAD27 and WGS84... Mind you, I'm
certianly not on a par with Gerry, but I did stay in a holiday inn express last
night!!...

I found that in my part of SC, that the shift from NAD27 to WGS84 was about
130'.

Now, two weeks ago, I had a discussion on the phone with our local GIS folks and
they told me all about state plane.  State plane is just like UTM, but uses feet
instead of meters.  Just for trivia, SC is one of 5 states that uses the
"international foot" which is .3048 meters instead of 0.3048006096 meters.  At
2million feet (the width of SC) it makes 4' difference.

In my search for details on international foot, I came upon
http://www.ngs.noaa.gov/TOOLS/Nadcon/Nadcon.html .  This shows the distortions
between NAD27 and NAD83.  I'm sure such a chart exists for NAD27 to WGS84.

Previously, I had believed the shift to be a pretty linear shift (if you are at
point X,Y, just add 130 feet north east), but if you are point x1,y1, the shift
is 125 feet, and so on....  But it's not.  So the point to all of this is that
you can't count on a simple "slide the map a little to the left" conversion
from one datum to another.

Now, UTM...  every document I've read about Military grid and UTM is quite clear
about disortions at the corners.  That's why UTM uses zones, and all of those
zones overlap.  You are supposed to use the zone who's center you are closest
to.

Now to answer the original question... the arguement they pose is like saying
you get less accurate measurements in meters than feet.  Units are Units are
Units... doesn't matter if you _display_ numbers in UTM or lat/lon, or feet or
meters.

WGS84 and NAD27 are models of the earth's spherical shape.  90degrees around a
circle will give a different measure of distance than 90 degrees around an
ellipse, even though they may have the same total circumference.

I think the guy that wrote that article pulled the old double switch-a-roo... if
you compare apples and apples (UTM and lat/lon both using WGS84), you'll find
they match very closely, until you get to the edges of the UTM zone.  If you
compare a given postion in UTM or lat/lon to WGS84 and NAD27, you'll find they
are both off by the same number of meters - in my case , 130 feet).

I think the fundamental confusion here is that most paper maps are in NAD27.  So
the author's only source of data for UTM was a paper map.  When he compared that
map to his GPS using WGS84, he switched to lat/lon at the same time... he
probably didn't know he could change his GPS to read UTM.

Further, I know I can change the eillipseoid model in my GPS independently of
the units that are displayed.

Wes
--



Quoting "Curt, WE7U" <archer at eskimo.com>:

>
> 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!"
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