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

[Gerry Creager n5jxs]

Curt, WE7U wrote:
> 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

I'll look at this soon.  I've downloaded it.  No internet in the Motel 6 
I'm in, in OKC.  No better hotels available in town, either...

> 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.

I'll try to render a translation

>>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.

Accurate statement.  And at times, when they're readjusted at the same 
time, they reconverge.

>>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.

How should I better explain it?

>>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.

"If I don't understand it, you shouldn't either!"

>>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.

Never ascribe to malice that which can adequately be explained by stupidity!
-- 
Gerry Creager -- gerry.creager at tamu.edu
Texas Mesonet -- AATLT, Texas A&M University	
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