GPS Datum...??
 

Here's a question - I've got a couple of Garmin GPS units, a small GPSIII and a StreetPilot III. I'm off to Morocco soon and will take both, the SP3 for the trip down and the GPSIII for the waypoint guided routes.

I've finally figured out that the datum used in the GPS needs to be set for the units to work in concert with my OS Maps (I was struggling with building offsets into my co-ordinates!) but it made me wonder what datum would be needed to use in N. Africa.

Any thoughts??

Cheers

Greg

The same as your maps I guess. Originally all GPS positions are on the WGS84.

Greg:

In general, you won't need to worry too much about using the correct datum for automotive navigation. The differences that exist between WGS84 (what the GPS constellation and your GPSR uses by default) and datums used on any paper map that has been printed in the last 20 years will not be big enough to cause you grief if your objective is simply to navigate on roadways or well established trails.

You might encounter differences of, for example, 100 feet or so between WGS84 and a datum used on a map printed in the last 20 years. It is very unlikely you will encounter a larger difference unless you are using a very, very old map - for example, something printed before 1970 or so. This is because most map publishers have already made the appropriate adjustment to the lat/long lines that appear on the map to bring them into fairly close conformity to WGS84.

Mariners need to pay attention to datums, because in many cases, maps that are printed today are still based on datums developed 150 years ago, and the lat/long lines on these nautical maps have NOT been corrected.

The two biggest reasons you don't have to worry much about datums are as follows:

1) Road maps are generally very large scale maps (e.g. 1:300,000 or more), which means that the tip of a very sharp pencil will have a greater error than the map datum will. Nautical maps are often 1:20,000 or less - at these very detailed scales, you will notice the datum difference.

2) If you are riding along a road, and see that because of a datum difference, there is a huge cliff where the map suggests there is a bridge, you can rely on your common sense to not lead you off the cliff. This is more difficult for mariners, who cannot see a big rock that is 5 feet below the waterline.

Basically - forget about datums, unless you have a very, very detailed map, such as an old map of a city center or something like that. In such a case, look in the margin of the map to see if there is a note there indicating the datum used. If there is no note, then the lat/long overlay on the map has probably already been (roughly) corrected to WGS84 by the company that prints the map.

Michael

Michael - thanks for your post..

I take the point about datums in general navigation being accurate enough to be used in conjunction with a little common sense - my question was brought about by a 700 yard discrepancy in the (European) datum that my GPSIII loaded as default and the British National Grid. When I changed the datum the differential disappeared. Now, here's a weird one - the SPIII and GPSIII still show different coordinates even when side by side! I can't see how that change the SPIII datum so it might be something to do with that. I'm planning to call Garmin in the morning - will post anything interesting.

ps - I just sold my '96 ST(PP) to pay for the Moroccan jaunt - still miss the old girl!

Michael - I've just seen you're in Trana... do you know Spock??

I was there last August for the family vacation - fantastic country, did a 2 week loop from Tobermory through Manitoulin, Huntsville, Niagara, Sarnia ... If I last that long I plan to have my 50th birthday lunch up the CN Tower!

How much difference is there in the readings? If each GPS unit quotes an accuracy of 50ft then there could easily be upto 100 ft difference between the two.

spoke to Garmin today ...
 

and figured it out!

The SPIII doesn;t have a user definable datum - easy to sort that one out then!

The GPSIII however has about 20 to choose from! The Garmin guy said to select the "Lat/Long" datum, but there wasn't one, so I selected 'user' instead and hey-presto... coincident coordinates.

My problem came with trying to use the SPIII (remember - no user definable datum!) when my MapSource datum was 'European 1979' but 'position' was defined with British National Grid co-ordinates. This is where the 700m offset shows itself because there IS a differential between BNG coordinates (converted) and absolute lat/long coordinates!! No bloody wonder I didn't know where I was going!

Solution: Just use the GPSIII for my domestic waypoint guided offroading, making sure that it and the MapSource datum are both set to BNG.

I'll leave the SPIII in the car...


Greg

Yes, though it has been a few years (maybe 3 or 4) since I saw him in person, or spoke to him. I spend most of my time in Europe now, in fact, when I read your comment, I thought your reference to 'Trana' meant Transylvania, which is where I was up until a few days ago.

Michael

And, FYI, Garmin has just released a software update for the StreetPilot 27xx and 28xx series GPSRs that provides about 20 different datums and several different position formats to choose from. This was a direct response to requests made by touring moto riders.

If you have a Garmin GPSR, click on this link to download and install the new 'WebUpdater' program from Garmin. This is a little application that runs on your PC to automatically retrieve and install software updates for your Garmin GPSR. It greatly simplifies the process of keeping the unit operating software (not the maps) up to date. Everything associated with it is free.

Michael

Small world innit?
 

Ha ha ... what a coincidence - he and Rob Brodie showed me the delights of a Wendy's Triple Cheese Combo while I was there - what an experience!! Check the My-MC.com website and see what he's upto!

Back to the GPS - I'm not so up-to-date to have a 2xxx series box - sadly the brick of an SPIII remains single datummed (?)

Cheers

Greg