How accurate are GPS's

Submitted: Sunday, May 29, 2011 at 21:46
ThreadID: 86632 Views:3515 Replies:4 FollowUps:7
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We have just come back from Popell Corner in the Simpson Desert and while there we decided to check our GPS's against the reading at the corner marker.From memory it is supposed to be something like 26 deg 00 00 south and 138 deg 00 00 east but when checking our GPS's 2 of us were only 2 mts apart but unfortunatly we were 200 mts away from the corner marker while another friend was 100mts off in another direction while another was somewhere else but not near the corner marker.
I also rcently bought a GME PLB locating beaconwith a built in GPS and in the instructions it says it will give location to within 100 mts.
I have head people say they are accurate to within a few mts but I am doubting that..
Cheers Peter
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Reply By: The Explorer - Sunday, May 29, 2011 at 22:18

Sunday, May 29, 2011 at 22:18

Sounds like you have the datum on the GPS unit set to one different to that of the published coords of Poeppel's Corner. What datum have you set the GPS to? If the GPS datum is set different to the coordinates you are referring to then you are likely to have signifcant difference when comparing figures.

Handheld/in car GPS units are accurate to +/- 5 metres ....though often better than this...and sometimes a little bit worse. A 100m+ error (not assocaited with user error) would be very unusual under most circumstances.


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AnswerID: 455835

Follow Up By: Member - Tony V (NSW) - Sunday, May 29, 2011 at 22:50

Sunday, May 29, 2011 at 22:50

Explorer is spot on there is an excellent explanation on datum and the differences at Australian hydrographic web site
It explains charts and the differents in WGS84, AGD66 or AGD84 and GDA94 etc.

Quote from page

"So how does this affect me?

If you have two charts of the same area and they have been drawn on different datums, then the latitude and longitude for the same place is likely to be different on each chart. In Australia the difference between an AGD66 and a WGS charted position can be up to 200 metres (0.1 miles). This could be the difference between one side of a reef and the other!"

Good info page with good links.

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Reply By: Wokwon - Monday, May 30, 2011 at 07:56

Monday, May 30, 2011 at 07:56
If I get 8 sats I usually get an error of about 20 metres. 10 sats will get me 10-16 metres. I haven't ever noticed getting more than 10 sats.
AnswerID: 455851

Follow Up By: Andrew & Jen - Monday, May 30, 2011 at 10:43

Monday, May 30, 2011 at 10:43
Hullo Peter
Setting aside the possible error due to an incorrect datum, GPSs experience "drift". Assuming you have a modern and sensitive unit, if you put the it in one place and observe it for about 20 or 30 mins, the coords will drift around by up to 40m.
One of the reasons for this is the fact that the GPS is continually calculating it's position by triangulation from a number of satellites, some of which are low down on the horizon while others are higher. Your position on the earth (not altitude) is more accurate if the majority of the signals are coming from well spread out, low down satellites.
If you have a tracking option and put it on the most detailed scale, after a while a pattern will develop like a multi pointed star and the centre of the star is the most accurate indicator of position.
If you need to accurately record a position, this is about the only way to do (unless you have a differential unit)
FollowupID: 728802

Follow Up By: Steve63 - Monday, May 30, 2011 at 14:08

Monday, May 30, 2011 at 14:08
The majority of the GPS network are rapidly moving satalites. There is also a set of satallites in a geostationary orbit (I think they have numbers over 40). The more satellites your unit can pick up the better. If it can also calculate the differentials from the geosyncronous satallites you will have less variance as the error effect is known. A lot of units will tell you if they have a differental on a satallite (WAAS). The more evenly spread the satallites are the better you are. Where the angles between satallites are very small the errors are more significant between those two satallites. Without differentials your error should be less than 10m and with differentials your error should be less than 3m.

FollowupID: 728825

Reply By: Ozhumvee - Monday, May 30, 2011 at 09:06

Monday, May 30, 2011 at 09:06
Did you come in from Birdsville or the west from Dalhousie as I thought access from the east was still closed.
AnswerID: 455859

Follow Up By: Bravo Man - Monday, May 30, 2011 at 09:31

Monday, May 30, 2011 at 09:31
Thanks for all the replies,I can see there are differences I didn't know about cause I'm an old fart and halve silly.

Ozhumvee....we came in from Mt Dare and did the loop east via the rig road and west by the French line back to Mt dare.Was very nice
FollowupID: 728796

Follow Up By: Ozhumvee - Monday, May 30, 2011 at 13:03

Monday, May 30, 2011 at 13:03
No worries thought that must have been the way you went, bet the desert was looking pretty green and lush I expect.
Re the GPS error when we first bought a Garmin GPS back in the late 80's it was a single channel unit but could multiplex to look at a max of eight satellites but of course one after the other. The error in those days was huge as the Yanks had the "scramble" on to stop the enemy from being able to use the system, errors of a few hundred yards were good nothing like now where a metre is commonplace. It all changed when Desert Storm started in the early 90's as the yanks didn't have enough Military GPS that could unscramble the signals so were forced to issue all the grunts with civilian GPS' which in turn meant they had to turn the scramble off and suddenly our GPS' would be very accurate. Initially they would turn it on and off when a mission was on and after a while it just stayed off as it is now.
FollowupID: 728818

Follow Up By: muffin man - Monday, May 30, 2011 at 13:26

Monday, May 30, 2011 at 13:26
Hey Bravo Man,
What were the big salt lakes like out poepel way. Had they dried and could you go through them or did you have to take the detours.
FollowupID: 728820

Follow Up By: Bravo Man - Monday, May 30, 2011 at 15:37

Monday, May 30, 2011 at 15:37
Hi Muffin Man.
All the salt flats where dry but you could see tracks from earlier where they had been wet.

The only detour we had to do was the big one around Peopel Lake which had a fair bit of water.

The WAA track was blocked alltogether near the eastern end by a large lake.

Regards Peter
FollowupID: 728837

Reply By: Bazooka - Monday, May 30, 2011 at 14:46

Monday, May 30, 2011 at 14:46

If you were at the marker this will partly expalin why your GPS wasn't showing 26S, 138E.
Poeppel Corner - marker V confluence
The comments regarding GPS accuracy have been dealt with by others.

I'm not an expert in PLBs but it is quite likely that a few metres is correct (for terrestrial Australia), depending on where you are on the earth and how many satellites are available to your device.

It's possible that the instructions might be reprints of the old days of Selective Availability, which the USA turned off about a decade ago. This (SA) basically made 'standard GPS' less accurate than it could have been - on average +-100m. Or it could simply be that the company/instructions are being conservative with the accuracy.
AnswerID: 455894

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