Thread: GPS Set-up - UTM or DDMMSS

Originally posted by barney
sorry rob, you got me there, it's been a while since i got into the thoery side of things but that is why i have to brush up.
as long as you're using the same map datum, you will (if your gps is capable) be able to skip between position formats and the gps will faithfully convert it. a position is a position and it is in the same spot no matter what format it is in. i have done it and often have to do it to translate info from other users.
for example, the geographical survey bus, between tibooburra and cameron corner is at 54J 0451969 6792245 which, if you change it to ddmmss is S 28.59'49.3" E 140.30'24.8"

Correct. You can also translate between datums too.

Originally posted by barney
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

UTM doesn't assume the earth is a cylinder, like all flat maps that image necessarily makes compromises in order to represent a sphere in two dimensions.

The UTM zones aren't actually rectangular, they are more a rectangle with convex sides. Their width varies but they are always six degrees wide (east-west).

UTM doesn't work well at the poles, so a variant called UPS is used instead.

Lat/long works everywhere very well which is why it's used by aviators and sailors. There is also no grief about swapping UTM zones.

Originally posted by barney
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.

Ahh, but that pre-supposes that I knew how to do enter his position. I don't.

Ron

ron,
that's the sort of thing that we want to familiarise our members with on the gpsfordummies day. a lot of people know their machine as a moving map and that's all. we would like users to know what all those little numbers on that other screen mean and how all that relates to a topographic map.
as you are probably aware, leading this trip with the club is not a matter of me professing to know everything about gps, as my posts will show i don't and need to do some cramming before october, but more of a case of no one else in the club putting their hand up for the job.
but hey, it's a day in the bush when it all boils down

This is an interesting discussion.

I spent a lot of time when I was a kid living in the country wandering and exploring the country side on foot without maps compass and without GPS. Finding your way around involved having an idea about your direction observing landmarks and having some idea about time and speed.

The initial directions given to Ace (Ace is about 45km away) that Saturday afternoon were:

I am beyond Oberon.

Take the Goulburn Road for about 3km then turn right on the road to a place beginning with R (Rockley). About 3km along the road you will see a homestead on the right called Essington Park and beyond that a pine forest on the right hand side. There are two entrances one with a gate and then the next without take that one. Follow the fence line. and I will hear you coming. I will see you before you see me.

It was only within the last 2km that the GPS coordinates were communicated.

Having driven there I had a map of my whereabouts in my head and was able to communicate this.

My view is regardless of all the techno aids some basic wayfinding skills need to be taught without reliance on any external aid.

Being lost is being not able to tell people where you are.

BTW my Magellan has a screen that enables two coordinate systems to be displayed and or Datum. It also has maps. All the tracks that I travelled that day were shown on the GPS. My track has been recorded. But I didn't refer to it as my primary means of getting around. I used my senses.

Originally posted by rmp+--><div class='quotetop'>QUOTE(rmp)</div><div class='quotemain'><!--QuoteBegin-barney
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.[/b][/quote]
And you need to remember that while most "Off the cuff" positions using DDMMSS will also be using WGD84, the basic default datum for most GPS units, but an imperfect model. Aust Topo maps generally use AGD66 (Australian Geographic Datum 1966) as the datum, with some later versions using AGD84. This will be printed in the "Fine Print" on the legend part of the map. Sounds confusing? Nah! By pushing as few as four or five buttons, you can convert these settings on your GPS. Have a practice with your unit and you'll find it's very easy.
Does different datums make such a difference? Well, yes and no. AGD66 and WGD84 differ on positions by an amount of 110metres NE to SW. Now if you were looking for a windmill in flat country, being 110m off won't be a problem. If you're faced with three gates out of a paddock to a particular track and these are 50m apart, well then you want all the accuracy you can get.
Try using your GPS in your driveway and change the settings between Agd66 and WGD84 (in either UTM or DDMMSS) and see how the position changes!

Ozi explorer converts all datum to and from WGS84 so no matter what your gps is in it will convert it.