Wgs84 Coordinates & Utm

[+Z_Statman]

Why is it that these two are, in most cases that I notice, so different. I when to a site today, 'stood' right on top the cache based on WGS84 then when to the UTM location and the difference was 20-30+ feet. My GPSr is set to use WGS84.

[+gallahad]

Check out this site. I think you'll find your answer (and lots of others) in the library they offer.

[Tahosa and Sons]

If the UTM's are set to WGS 84 the location should be the same. I've done it with almost all of my back country caches and it does not matter wheater its (° ' ") or whatever format I use.

[+Alan2]

WGS84 is a Datum, UTM is a position. The two are different things.

 

Changing Datums only matters when you're using the coordinates from one GPS and entering to another or when comparing to a map. If they don't match then you will be off the difference between the Datums. Then the location will "shift".

 

The 20-30 feet you are seeing is just normal accuracy changes due to satellite reception and the accuracy of the GPS. You can prove that to yourself. After entering the coordinates in your GPS, change the Datum. The waypoint will be in the same place. Only the coordinates wil read differently.

 

Converesely. write down the coordinates, switch Datum, and enter the position again. You'll have two waypoints in different locations.

 

Alan

Edited June 14, 2004 by Alan2

[+writer]

If the datum remains the same, I wonder if the accuracy of the degree coordinate measurements might not actually differ from the accuracy of the UTM coordinate measurements. Did you check both against the *actual* location of the cache to see which seemed closer?

[+Alan2]

They will vary depending on the coversion formula. If you go to NADCOM they explain how they use impirical data tables from thousands of actual measurements areound the country to determine the corret formula. There is no one calculation. The shape changes depending which part of the country you are in. NADCOM supposedly has the nearest conversions. (I'm tlaking about the difference between NAD27 and WGS84; from USGS Topo maps to WGS84. The accuracy then moves even more becuase UTM is a conversion to a flat map with it's inherent distortions. But I don't know if tghe difference is large enpough to matter since the acuracy of our GPS's are 20 feet or so anyway.

[+Z_Statman]

OK, understand

 

Thanks much

 

Topic can be closed

[Kerry.]

NADCOM as Alan mentioned is a conversion between "different" datums and this is quite different to the conversion from lat/Long to UTM in the same datum.

 

Lat/Long to UTM is a mathematical process and there is really only one possible answer (as such).

 

.... I when to a site today, 'stood' right on top the cache based on WGS84 then when to the UTM location and the difference was 20-30+ feet.

 

Something in that sounds a little incomplete? So where did the UTM coordinates come from? as if they were simply conversions of Lat/Long (as stored/input in the GPS) then the differences would have effectively been the same.

 

This 20-30 feet difference is compared to what?

 

Cheers, Kerry.

[+GEO*Trailblazer 1]

I have noticed that no one has mentioned that LONG-LAT goes and starts from East to West.

UTM starts West and goes East.

So yes there will be some difference because of the curvature of the Earth and the Point Started from.(IP) Initial Point of Data sets.

 

Again accuracy(precision)

In dealing with abstract quantities,we have become accustomed to thinking largely in terms of exact values.

 

At the start,the student of surveying ought to appreciate that he is dealing with physical measurements which are correct only within certian limits,owing to errors that can not be completely eliminated.

 

The degree of precision of a given measurement depends upon the methods and instruments employed and upon other circumstances involved.

 

ERROR,PRECISON AND ACCURACY

Edited June 16, 2004 by GEO*Trailblazer 1

[Kerry.]

I have noticed that no one has mentioned that LONG-LAT goes and starts from East to West.

 

That's because it doesn't matter and just in case your not aware what occurs in the U.S isn't the norm worldwide, you things things are different elsewhere.

 

.... UTM starts West and goes East....

all part of false origins and to avoid negative coordinates.

 

So yes there will be some difference because of the curvature of the Earth and the Point Started from.(IP) Initial Point of Data sets

 

There will be absolutely no difference based on mathematical conversions as in the same datum the values that define a single datum simply have to be the same be it Lat/Long or UTM or ECEF XYZ

 

Cheers, Kerry

[+GEO*Trailblazer 1]

From the Link

 

6. Beware of False Precision and False Accuracy!

GIS users are not always aware of the difficult problems caused by error, inaccuracy, and imprecision. They often fall prey to False Precision and False Accuracy, that is they report their findings to a level of precision or accuracy that is impossible to achieve with their source materials. If locations on a GIS coverage are only measured within a hundred feet of their true position, it makes no sense to report predicted locations in a solution to a tenth of foot. That is, just because computers can store numeric figures down many decimal places does not mean that all those decimal places are "significant." It is important for GIS solutions to be reported honestly and only to the level of accuracy and precision they can support.

This means in practice that GIS solutions are often best reported as ranges or ranking, or presented within statistical confidence intervals. These issues are addressed in the module, Managing Error.

[Kerry.]

Until this question This 20-30 feet difference is compared to what? is answered then Precision, Accuracy with respect to the same datum with regards is irrelevant.

 

The question is 20-30 feet difference is compared to what? as if one is referring to cache type coordinates then there is no way that any physical conversion from lat/Long to UTM will differ by 20-30 feet.

 

Cheers, Kerry.

[+Alan2]

If you compare NADCOM conversion of Datums to Garmin's Mapsource or JEEEP, you'll get different results, some relatively substantial. NADCOM (in the US at least) is suppose to be the best because of the 100,000's of measurements taken. The problem, if I'm correct, is that NADCOM uses its data base which is quite substantial while Garmin probably uses a set formula for all calculations due to the memory limitations. (Kerry?)

 

ALso, the little corrective notes to adjust for NAD27 to NAD83 or WGS84 at the bottom of 24,000 USGS quad maps are wrong based on the results NADCOM has come up with which are more recent.

Edited June 17, 2004 by Alan2

[Kerry.]

Alan, yes with NADCOM that is based on a distrotion model along the lines of NTv2 and as with any transformations (as opposed to a conversion) there will in many cases be different results depending on the underlying method of the trasnformation. If one looks at say Garmin's implementation there is something like 11 different options/selections from WGS84 to NAD27 depending on which part of the country one is situated.

 

However converting WGS84 Lat/Long to WGS84 UTM then there is only and can only be one result and any minor issues with CPU floating point limitations (affecting precision) would not account for 20-30 feet.

 

I think we need to know just where these UTM coordinates came from as if the way I read the original post one was using a waypoint in Lat/Long and basically sounded like the fix was perfect (which it simply can't be anyway) then changing to UTM it said it was 20-3- feet different.

 

Now if the UTM coordinates were acquired separately and all things being equal then the lat/Long and the UTM positions simply aren't the same physical point on the ground.

 

Cheers, Kerry.