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

[Curt, WE7U]

On Fri, 4 Mar 2005, Wes Johnston wrote:

> 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'.

In my county it's between about 90 meters and 105 meters.  Varies.

Read this:

http://wvgis.wvu.edu/stateactivities/standardsandguidelines/coordinatesystems/datum_shifts_v2.pdf

Specifically the sections on UTM (fourth paragraph is of great
interest) and "Common User Problems Related To Datum Shift".

I think that one paragraph could just confuse the matter more if
they read that (in fact I need to get my head around what they are
saying too), but that other section is very helpful/clear.


> 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.

WGS84 and NAD83 are so close as to be the same for anything except
surveyors.  Even then most of them wouldn't care much.


> 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.

Yea, but have to get them to understand it in that light.


> 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).

Yep.  That's what I find.  I agree that the authors of the document
are misleading the reader into incorrect conclusions.


> 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.

Could be.  There's definitely something else going on.

--
Curt, WE7U.   APRS Client Comparisons: http://www.eskimo.com/~archer
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