# UTM conversion

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Isn't UTM the same no matter what datum? I thought UTM was a specific spot on the globe. Why should switching datum matter?

I used the UTM conversion from the website, since that's how my Magellan was already set up. Man, was I waaaaay off. That's why I'm still scratching my head. I didn't think it would matter. BTW, the datum in my GPS was NAD27 and I switched it to WGS84.

BTW, in order to calculate deviation in feet, do you have to have WAAS?

Thanks,

Eric

Ha, Ha. I've had more headaches at work because of data being in NAD27 not in NAD83(WGS84). There can be approx. 200m shift in coordinates because the difference in these datums. NAD83(WGS84) is geocentric. NAD27 is tangent to the earth at Meade's Ranch, Kansas.(I have the lat. long. somewhere, but it would involve digging through too many books!)There are also problems with UTM because the farther you go from the central meridian of the zone the more deviation there is. I can take digital mapping data that is in lat/long, project it to UTM in one zone, project it to another UTM zone, then back to lat/long and the data will not end up in the same spot. It may only be a metre, but mapping people get upset when their data "moves" on them!

So from a mapping standpoint, in Ontario especially (it spans 4 UTM zones), Geographic coordinates(NAD83), with many decimal points, are the way to go.

- Donna G.

[This message was edited by DonnaG on March 18, 2002 at 04:39 PM.]

I find that it helps if I don't try to understand why and just do what they tell me to.

This can be a really lengthy subject, but the short answer is: Think of UTM as a form of readout:

ddd.dddd, dd mm ss, dd mm.mmm, dd mm ss.sss, z e n...

Think of Datum as the size and shape of the ball. Since the 'ball' is a different size of NAD27 than for NAD83 (WGS84) (the latter being a lot more accurate). 1 degree, and fractions thereoff, have a different meaning.

Good Luck,

-jjf

With regard the ball bit if one was to find the centre of each respective ball then that's the difference (shift/direction etc) in the positions relative to respective datum.

Cheers, Kerry.

I never get lost everybody keeps telling me where to go

With regard the ball bit if one was to find the centre of each respective ball then that's the difference (shift/direction etc) in the positions relative to respective datum.

Cheers, Kerry.

I never get lost everybody keeps telling me where to go

I'm surprised by the UTM differences, but all the explanations helped me understand why there is a shift.

I especially liked the one about just following directions.

Thanks,

Eric

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