GPS Set-up - UTM or DDMMSS

**p38arover** · 12th July 2005, 07:35 AM

Note: Late edit to correct an error - I didn't have a map with me when I wrote this and I hadn't looked at one in a long time. Thanks Bushie.

I haven't bought RMP's book yet (soon) but I'd like to discuss coordinate location display on a GPS.

As you are aware we went looking for One Iota who was stuck in a forest. OI was able to give us his location in degrees, minutes, seconds - we'll use the abbrev DDMMSS.

I have my GPS set to UTM (Universal Transverse Mercator) which shows location in Eastings asd Northings - i.e., the number of metres east and/or north of a specified reference point (you don't have to worry to much about that as the GPS knows the reference - see http://www.dmap.co.uk/utmworld.htm).

I like UTM as topographic charts are calibrated in UTM and the grids are at easily used spacings, e.g. 250m, 1000m, etc. The black and white bars along the side of the maps are based on DDMMSS - on the map I'm looking at the moment they are, as Bushie notes, at 1 minute intervals.

It's easy to find your position on a map using Eastings and Northings. Not only that, street directories also have Eastings and Northings printed on the edges of the maps. You can work out a position quite quickly and accurately from a map. See http://www.maptools.com/UsingUTM/quickUTM.html

If you use DDMMSS, the topo maps have only very broadly spaced coordinates and they don't have grid lines. Working out a position is much messier. If I can scan some bits of maps, I'll add them to illustrate this post. Another point is that the map grids are symmetrical in both the horizontal and vertical (the cells formed by the grids are square). If you check, you'll find that the length of the minute markings on the horizontal and vertical grids are not the same - so you can't make up a simple scale for sub-dividing the minutes of latitude/longtitude into smaller units, e.g. seconds or 1/10th of a minute. I'm not sure but I assume that the closer you go to the Poles (North and/or South), the worse this becomes. I don't have any maps which are sufficiently separated to check this hypothesis.

In the retrieval situation mentioned above, I switched my GPS to DDMMSS but while I could deduce in which direction IO was located, I didn't have a map of the area so I didn't know how far away he was. If he'd given me his position in Eastings and Northings, I could have quickly worked out (to the metre) the distance and the bearing from where we were.

Oh, in my previous job with Telstra, I was Manager, Marine Operations and used hydrographic charts quite a bit in conjuction with dealing with both commercial fishermen and the large submarine communications cable repair ships. There, one uses DDMMSS (or decimal minutes rather than seconds).

Ron

**Bushie** · 12th July 2005, 12:45 PM

Land based navigation - just about everyone uses UTM. The grid lines are designed for use with UTM.

The black/white bars along the sides of the maps are 1 minute intervals lat/long.

My thoughts are that the GPS should be set to the same system, that the map/chart is, that you are using.

Bushie

**p38arover** · 12th July 2005, 03:01 PM

Originally posted by Bushie+--><div class='quotetop'>QUOTE(Bushie)</div><div class='quotemain'>Land based navigation - just about everyone uses UTM. The grid lines are designed for use with UTM.[/b]

Dunno, I've not met many people in 4WDing who have set their GPS to UTM. For some reason, the 4WD mags give positions in DDMMSS - as do the Boiling Billy publications, as does John & Jan Tait's Gregory's 4WD Weekends book. I agree that, generally, the grid lines are for use with UTM. The Gregory's book has the grids in DDMM. I've noticed that HEMA use UTM and DDMM depending on nthe map. HEMA's map of Victoria uses DDMM, their map of the High Country is UTM.

Originally posted by Bushie@
The black/white bars along the sides of the maps are 1 minute intervals lat/long.

Silly me. I thought that when I wrote the above than thought, no, I'm wrong. I didn't have a map to check. I've now amended the para above.

<!--QuoteBegin-Bushie
My thoughts are that the GPS should be set to the same system, that the map/chart is, that you are using.[/quote]

Yes. Agreed - see my comment above.

Ron

**rmp** · 12th July 2005, 06:02 PM

OK, here's the story.

There is no *right* answer. Both lat/long (which can be represented in three different ways) and UTM are as accurate as each other.

As has been said, use whatever the map is calibrated in.

There's a VERY simple reason most guidebooks use lat/long. It's because the authors just use the GPS receiver with the defaults set. In most cases, that's lat/long with the WGS84 datum. The fact that those coordinates and datum are often inappropriate doesn't seem to matter, they just read off the "GPS coordinates" and write them down.

Overlander magazine now gives a datum with their coordinates, and vary between UTM and lat/long depending on the story. For forest driving you'd have say a 1:25 or 1:50 topographic and use UTM. In the Outback, lat/long would be more the go. Both coordinates should ALWAYS be quoted with datums.

Plotting a position with lat/long is much harder than UTM, as Ron says. This is because UTM gridlines are equally spaced. Lat/long are not.

I could go on here but that is exactly how I ended up writing the book, one explanation led to another to another!!!!!!!!!

**p38arover** · 12th July 2005, 07:42 PM

Originally posted by rmp
I could go on here but that is exactly how I ended up writing the book, one explanation led to another to another!!!!!!!!!

And that's why I'm buying the book.

Robert, are you coming up to the LR Expo on the 24th July? If so, I'd appreciate it if you bring a copy with you. I can pre-pay.

I dunno if the LROC might worry if you had some that hadn't been ordered - but I can't see why they'd complain if you brought one that I've already either ordered and/or paid for.

Ron

**rmp** · 12th July 2005, 07:50 PM

No, won't be at the LR Expo unfortunately, will be playing with Nissans in the snow that day! I am not aware of anyone from Melbourne that is going, although I'd like to there are not enough days in the month for me!!!

Almost all maps shops stock the book, quite a few places in Sydney including GPS Oz in Mona Vale, places in the CBD....it gets around, always impressed with where it ends up!

**p38arover** · 12th July 2005, 08:00 PM

No worries, I'll order it from your website.

Ron

**barney** · 12th July 2005, 08:39 PM

sadly, i have been roped into running a "GPS for dummies" trip by flash for the lroc in october, so i've got to brush up on this stuff.
ron, if your mate gave you his pos in ddmmss and you changed your gps datum to ddmmss and entered the position, you then would have been able to change the datum again to UTM and it would've converted his position thus giving you the result you wanted, i.e. the distance in meters from of his position.

**rmp** · 12th July 2005, 08:46 PM

Originally posted by barney
sadly, i have been roped into running a "GPS for dummies" trip by flash for the lroc in october, so i've got to brush up on this stuff.
ron, if your mate gave you his pos in ddmmss and you changed your gps datum to ddmmss and entered the position, you then would have been able to change the datum again to UTM and it would've converted his position thus giving you the result you wanted, i.e. the distance in meters from of his position.

Coordinate formats are not the same as datums!!!

To match positions two people need to be on the same datum. They can use different coordinate systems if they are capable of working out the translations, which is possible if we're talking DMS to say DDM, but not from any lat/long to UTM.

**barney** · 12th July 2005, 09:03 PM

UTM zones of the world. it's just like 1 big uniform grid pattern based on the premise that the surface of the earth is a cylinder rather than a sphere.

you'll notice that the antarctic is rather long and skinny

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