UTM - does it need a datum?

[+Delta-S]

... or is UTM always on its own datum???

 

I got a fancy new map from my local state park today and was pleased to see UTM grids on the map.

 

Read all of the legend bits and nowhere did it say what map datum they were using. Got to thinking that maybe UTM is always the same and datum is just part of it.

 

Comments?

 

The Bigg Family "Delta-S"

[+Cachetrotters]

Someone more expert than I will give you the technical brief, but generally UTM is UTM. Just need to make sure you're in the same/correct Zone. When you enter the numbers into your GPSr, make sure you enter all of them for the Northing and Easting, with Easting being the first set of numbers, I believe. Don't play with it as much as I should.

 

don

edit grid to zone

 

[This message was edited by Cachetrotters on May 10, 2003 at 06:36 PM.]

[+Cachetrotters]

Well, I performed a little experiment by changing the datum while observing the UTM display for my location. There was a difference in Northing of as much as 200 meters, less in the Easting. After reading some on-line material, I find that this is not addressed specifically.

Any takers?

 

don

[dsandbro]

Yes, you need to check the datum. The difference between NAD27 and WGS84 will vary slightly depending on where you are, but 200 meters sounds about right. For all intents and purposes assume WGS84 and NAD83 to be the same. The difference is safely ignored for recreational geocaching (out in decimal point land of a meter).

 

The fine print boilerplate on a USGS quad lists the datum used for that map. Recent editions often show a small cross tick in one corner showing the location in another datum.

 

The state park map should list the datum somewhere (if they used good cartography and used the standard protocols).

 

========================================

"The time has come" the Walrus said "to speak of many things; of shoes and ships and sealing wax, of cabbages and Kings".

[Tahosa and Sons]

Different datums will put the waypoint in a different place. The distance may not be a real factor, but where the waypoint winds up topographically speaking could be a real problem.

 

Tahosa - Dweller of Mountain Tops.

[+Cachetrotters]

So, what we're saying here is that UTM is overlayed on each datum, and does not have it's own, or is not a "datum" in and of itself, with its own geoid, etc.?

 

don

[+jonboy]

Every map needs a datum, whether one is using UTM or Lat & Long. It will be either WGS84 or NAD27. If you can determine the UTM coordinates for a easily identifiable location (such as a benchmark) on the map you have, and compare those coordinates for that point on a gridded map you have that has a specified Datum, you can see if they match up or not. Most UGGS topographic maps will be using NAD27, unless you have one of the new 1:25,000 Metric maps for that area. If the grid lines are not shown, you can pencil them in using the black UTM hash marks on the edge of the map and a long ruler. In my area (metro NY/NJ), the difference between the two datum wil be about 225 meters, and most of the change will be on the Northing (the second set of coordinates). You can obtain a NAD27 topo online at www.topozone.com .

[Kerry.]

Lat/Long and UTM coordinates must be referenced to a datum. UTM is basically a projection of the Lat/long vales, which in turn are a function of cartesian (XYZ) values.

 

The same coordinates used with different datums will be different physical points or the same physical point will have different coordinates relative to the respective datum.

 

Also with UTM one also requires the zone otherwise the coords are basically useless.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go