UTM Conversion Question - Zones

[Growley]

Here's a question for those of you who are experienced in converting Lat/Lon to UTM and back;

I am currently using my GPSr at work with ArcMap GIS. I have converted the GIS program to read UTM as well as my GPSr. Now, when I find a waypoint with my GPSr it lists the UTM coordinates and Zone 18S. The coordinates match the GIS coordinates however, the GIS lists the coordinates as being in Zone 18N. I also have a converter which matches all the coordinates however it also lists them as being in Zone 18S. Why are the Easting and Northing coordinates matching on all 3 readouts but the zone is different? May I have some help here please.

[Kerry.]

Your 18S and/or 18N, if the N/S is trying to indicate the hemi-sphere then maybe check the latitude sign of the software as some software packages allow the latitude sign to be ignored and maybe there a toggle in there somewhere which overrides signs and simply assmumes (by default/user setting) N or S.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+gnbrotz]

Although you can use the zone letter to determine the hemisphere, it's not quite as simple as N/S. UTM zones are lettered C through X, with C starting at the south pole. The equator runs along the top of Zone M, and the bottom of Zone N. A nice representation of the UTM zones can be found here.

 

As for the original question, I'm not sure why it's happening. Is there any way to manually change it in the GIS program? UTM coordinates cannot be duplicated in vertically related zones (because the Northing is measured from the equator), but they can be duplicated in horizontally related zones (because the Easting is measured from an artificial reference line drawn parallel and 500,000 meters to the west of the zone's central meridian). Thats where the zone letter provides differentiation.

 

Greg

N 39 54.705'

W 77 33.137'

[Growley]

Kerry & Greg, Thanks for your responses. I went to the site supplied by Greg and I am right in the middle of UTM Zone 18S. What is interesting about the GIS program is that it defaults to State Plane coordinates however, an ESRI Instructor gave me the instructions to switch to UTM. When I do, the Easting and Northing coordinates match however, There is not even a Zone 18S listed with the program! Only 18N. Now that I have two programs displaying Zone 18N and the GPSr displaying 18S I guess I will just blow off the zone reading (Since I know its faulty, or something) and concentrate on the coordinates. Again, thanks for your input.

Dave.

[Kerry.]

Maybe someone can clarify why there's any need for Zone letters for pure UTM as opposed to "Grid References", which are a little bit of the zone number, zone letter, and only a part of the full UTM coordinates.

 

Ok Grid references are different but UTM is really controlled by the zone number alone as UTM can not be negative and hence has false origins in the southern hemisphere as the northing is not directly measured from the equator but a point 10,000,000m south (and 500,000m west of the CM.

 

UTm might be nice for some applications but one thing is for sure at least Lat/Long is basically unique (no confusion )

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go