GPS accuracy - altitude

Submitted: Friday, Jan 07, 2005 at 20:59
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What accuracy do users typically get from their GPS in regard to Altitude????

I got a cheapie GPS for X-Mas and it consistantly reads around 70m higher than actual sea level. Checked the manual and it claims accuracy +/- 100m (which I thought was a very large error range). Also noticed that the reading fluctuates up and down a few metres constantly.

Put it beside 2 very expensive ones this afternoon, the cheapie read between 87-95m, a Navman read 81-85m and a Garmin read -12m (yes below sea level).

Lat & Long readings were within a whisker on all 3 so that wasn't an issue.
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Reply By: David Au - Friday, Jan 07, 2005 at 21:44

Friday, Jan 07, 2005 at 21:44
+/- 50 metres is more the average, but yes, 100 metres is the accepted.
I find my eMap is generally within 30 metres, my Holux is more often spot on or within 10 metres. Altitude does benefit from averaging if your GPS has an averaging features. The altitude I have found does not calculate as fast as the Easting and Northing.
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Follow Up By: Col_C - Wednesday, Jan 12, 2005 at 22:51

Wednesday, Jan 12, 2005 at 22:51
There are several reasons why you can be getting differences in height readings.
To compare two units you must be sure both units are on the same map datum and coordinate system. (I use WSG84 & UTM). The systems vary on the mathematical model they use for the shape of the earth.

Height measurements are least accurate with GPS as the unit is using the satellites closer to the horizon. They are more likely to be obscured by trees, buildings etc. If you look at the screen which indicates the satellites which are being used, You can come to understand how much weight you can put on a measurement.

If you wish to get a better height determination, hold the unit steady and try to move so the satellites shown on outer ring of the satellite screen lock in. In the short time I have had my M.Meridain I have found the heights to be better than 3 metres if there is a good satellite fix.
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Follow Up By: snailbate - Friday, Jan 14, 2005 at 19:06

Friday, Jan 14, 2005 at 19:06
Mad Dog
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Reply By: Willem - Friday, Jan 07, 2005 at 21:49

Friday, Jan 07, 2005 at 21:49
I have had up to an 11 metre variation at sea level. Away from SL it is pretty accurate
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Reply By: 80scruiser - Friday, Jan 07, 2005 at 21:55

Friday, Jan 07, 2005 at 21:55
My magellan is reasonably accurate but is slow at reading the alt as well.
Position is fast which is the main thing.
I am the only one in our club with one so I can't compair.
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Reply By: Member - JohnR (Vic) - Friday, Jan 07, 2005 at 22:39

Friday, Jan 07, 2005 at 22:39
My Meriplat seems to be pretty right to within 3-4 most of the time but does get up to 10-12 metres out before settling down. My garage seems to vary only slightly from the 83 metres. If I am trying to judge one point against another I always try to do it within a short period in the same session and maximising satellites recieved. It was remarkable how accurate it seemed to be round the Lake Eyre South marker all along the road near by watching the fluctuationson the GPS as we travelled the road there.

The ute is frequently by the sea and gets very close there too according to levels above sea level.
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Follow Up By: geocacher (djcache) - Monday, Jan 10, 2005 at 17:40

Monday, Jan 10, 2005 at 17:40
Platinums have a barometer in them.

I can't remember if it is just for the weather station, or whether it is used to correct for altitude measurements. Someone else will know.

I also vaguely remember reading somewhere that altitude variation is greater than positional variation by a factor of fixed value and it has something to do with the fact that the earth isn't round - well not exactly round, and on position of satellites particularly if they are all low on the horizon. I'd look it up but I just loaned the GPS books to Dad so he can bone up about his new toy.

Luckily geocaches only need lat/lon and not altitude accuracy. So at the end of the day I don't really care, but I hope this helps a little.

Dave
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Follow Up By: Member - JohnR (Vic) - Thursday, Jan 13, 2005 at 16:59

Thursday, Jan 13, 2005 at 16:59
Dave, I have an upgrade of the firmware which removes the barometer function alltogether.
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Follow Up By: geocacher (djcache) - Thursday, Jan 13, 2005 at 19:52

Thursday, Jan 13, 2005 at 19:52
Hi JohnR,

Why would you want to do that??

I use it when I'm away. Infrequently but I do use it. Interesting to see what it's doing when the cod stop biting.

Dave
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Follow Up By: Member - JohnR (Vic) - Thursday, Jan 13, 2005 at 22:52

Thursday, Jan 13, 2005 at 22:52
Dave,

I had to upgrade a year ago to the later firmware so the GPS would recognise the memory card I got for it. It didn't even see a 256 Meg SD card let alone format it. I see there is another upgrade available now but I have firmware vers 5.12.

I tend to look at the weather 10 days out, then 5 then my observations rather than use a barometer anyway. I do like to know duration of events and so the BOM and international sites are valuable for that...
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Follow Up By: geocacher (djcache) - Thursday, Jan 13, 2005 at 22:56

Thursday, Jan 13, 2005 at 22:56
That's odd. I also have 5.12 and the barometer works fine. What about your electronic compass?

It was Platinum Firmware I assume.

Dave
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Follow Up By: Member - JohnR (Vic) - Thursday, Jan 13, 2005 at 23:04

Thursday, Jan 13, 2005 at 23:04
It was noted with the firmware as I downloaded that some features were lost - i.e. the barometer. Yes, I still have a compass, and it was Platinum firmware.

Dave that was a quick response
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Follow Up By: geocacher (djcache) - Thursday, Jan 13, 2005 at 23:09

Thursday, Jan 13, 2005 at 23:09
From the Magellan Meridian Frequently Asked Questions file on the Magellan Yahoo group. Try a net search for Magellan Meridian FAQ. It's got loads of useful info in it.

Paste:
26) WAAS - What it is, do I need it, and can I turn it off?
WAAS stands for Wide Area Augmentation System. This system is still experimental. The Meridian can pick up special WAAS satellites (shown by a “W” on the satellite screen) which transmit various correction factors. There are several ground stations in the US which pick up the GP S signals and calculate these correction factors. If you can pick up a WAAS signal, are relatively close to a ground station, and are in the open (no overhead obstructions), then WAAS may improve accuracy. Sometimes WAAS has been known to decrease accuracy.

1. To turn off WAAS, with the Meridian turned off, hold down GOTO and NAV and tap the PWR button.
2. In a couple of seconds you should see a box pop up with "00" inside of it. At this point, release GOTO and NAV.
3. Use the direction pad to change the "00" to a "03" and then press ENTER.
4. A few boxes should pop up.
5. Press ENTER to turn a "YES" to a "NO" (all boxes will change simultaneously).
6. You may need to use the three-finger salute (simultaneously GOTO, ESC, and ENTER) to get the unit to turn off.
7. Then, when you turn it back on the WAAS satellites should not be visible in the satellite screen.
Note that the boot screen will still say "WAAS."

Hope this helps.

Dave
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Follow Up By: Member - JohnR (Vic) - Thursday, Jan 13, 2005 at 23:14

Thursday, Jan 13, 2005 at 23:14
Thanks I will try it when I get it home again, son has it in Queensland at the moment.

I had a look at the geocaching site a year or so back but never got into it. You could start a bit of a navigation trial with some of the guys and gals one weekend Dave.

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Reply By: Mike Harding - Saturday, Jan 08, 2005 at 07:37

Saturday, Jan 08, 2005 at 07:37
Basic altitude error range for GPS is +/- 23m.

This page tells all.

http://gpsinformation.net/main/altitude.htm

btw if you are trying to compare readings from two (or more) GPS receivers _do not_ place the receivers immediately next to one-another - the internal signals produced by each device may interfere with the other and cause errors. Instead place them (say) 1m apart.

Mike Harding
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Follow Up By: Dennis (Mackay) - Saturday, Jan 08, 2005 at 12:23

Saturday, Jan 08, 2005 at 12:23
Yep, figured on that one so we had them apart by about a metre +.
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Reply By: Bonz (Vic) - Saturday, Jan 08, 2005 at 09:02

Saturday, Jan 08, 2005 at 09:02
My GPS states the vertical error is about twice the horizontal error, the difficulty with a GPS with no altitude sensors (whatever they are) is that the reference point for the altitude is an arbitrary line drawn around the globe, its not sealevel. So any altitude readings are made from this arbitrary line.
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Reply By: Patrol22 (Queanbeyan - Saturday, Jan 08, 2005 at 09:12

Saturday, Jan 08, 2005 at 09:12
All depends on the type of altimeter in the GPS. My Garmin GPS 76 has the normal run of the mill electronic altimeter which is prone to the errors listed above. Tere are GPS fitted with a combination GPS and Barometric altimeter which are very accurate.
My GPS is like Willem's with the error at sea level more marked. Came over the range the other day and the sign post was 1110 metres - my GPS read 1113 metres.
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Follow Up By: Willem - Saturday, Jan 08, 2005 at 09:35

Saturday, Jan 08, 2005 at 09:35
I have a Magellan Map330 and it shows my house at 538m asl constantly

I was down at the Coorong recently and the error showed at base sea level as 11 metres. Next mornign it was 1 metre after being stationary for a while. Maybe the earth bounces along in space without us being aware of it :o)
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Follow Up By: Bonz (Vic) - Saturday, Jan 08, 2005 at 14:51

Saturday, Jan 08, 2005 at 14:51
Its the effect of the TIDES Willem, at the Coorong they have king tides and you MUST have been there then , that the only explanation for the variation of such a precision device.
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Follow Up By: Mad Dog (Victoria) - Sunday, Jan 09, 2005 at 15:45

Sunday, Jan 09, 2005 at 15:45
Could be tides but it may also be weed. The more weed the higher ya get :)
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Reply By: Pluto - Saturday, Jan 08, 2005 at 09:40

Saturday, Jan 08, 2005 at 09:40
You won't get reasonable errors in altitude unless the GPS is fitted with a barometric altimeter.

I have an Etrex Summit that routinely gives less than 10m vertical error, but the self calibration of the altimeter takes about 30 minutes before you can trust it. This can be clearly seen on the elevation profile screen.

You should also be aware that the datum used will also affect the elevation reading. For example: WGS84 & GDA use the same origin, making lat & long readings the same. However their elevation offsets differ, giving significantly different altitude readings. These height datums will probably vary from the one used on the map you are using as a refference.

There is more info about Datums here.
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Reply By: ianmc - Saturday, Jan 08, 2005 at 12:37

Saturday, Jan 08, 2005 at 12:37
Maybe this is why the US blows up so many non-targetted targets in Iraq etc???
Also have wondered how the fatter diameter of the earth at equator versus the poles affects these toys?
Have read that if the sattellites are near the horizon rather than overhead the error is much greater!
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Reply By: snailbate - Sunday, Jan 09, 2005 at 14:54

Sunday, Jan 09, 2005 at 14:54
hi guys
While readig your Q and then reading the reply i looked at my Magellan Gold gps Receivers manual and read this i quote( HOW accurate will my GPS receiver be with WASS {Wide Area Augmentation System}) ???
ANSWER GIVEN IN THE MANUAL.
The FAA reports that expected accuracy can be improved to around 7 meters vertically and horizontally. Our own (Magellen) testing sugests that Magellan GPS recievers will typically experience improvements to around 3 meters . you can expect to see this leval of accuracy for 95% of the time that you are recieving WASS signals .
This is on page 76 of the User Manual Meridian series of GPS Receivers
i sugest that you go to Magellan site there are two one in Australia and one in usa 2001 edition page no may change with updates
in addition Thales Navigation is the owner of Magellan and Meridian
or look for WASS on the search engines if you want to see more
so accuracy should be around 3 meters vertically and horisontal when using WASS

SEE YOU ON THE WALLBY
SNAILBAIT


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Follow Up By: Mad Dog (Victoria) - Sunday, Jan 09, 2005 at 15:18

Sunday, Jan 09, 2005 at 15:18
hi ya Snaily

sure we can receive WAAS birds in oz but we have no ground correction stations. The signal you are receiving is meant for another part of the planet and will degrade your accuracy so turn WAAS off.
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Follow Up By: Dennis (Mackay) - Sunday, Jan 09, 2005 at 17:05

Sunday, Jan 09, 2005 at 17:05
But what do you actually get?

With your toes just getting wet at the beach type thing, or standing at a known (surveyed) height.

What a manual says you get and what you actually get can sometimes be miles apart.
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Follow Up By: geocacher (djcache) - Monday, Jan 10, 2005 at 18:00

Monday, Jan 10, 2005 at 18:00
I was appalled to see the equipment special of one of our national 4wd magazines article on GPS recently recommend that people look for WAAS compatible GPS units over similar models without WAAS recently. This just highlighted that the expert writing the article had no idea what they were on about.

WAAS is a system in the US that relies on over 20 Wide Area Ground Reference stations and the WAAS (Wide Area Augmentation System) satellites to provide correction particularly for ionospheric effects on satellite signals.

Australia is not serviced by ground reference stations and as such WAAS is fairly useless, tests at times have even shown decreased accuracy on WAAS equiped GPSr's and non WAAS units.

If you have the ability to do so on your GPSr turn WAAS off. Your accuracy probably won't be any worse and in some cases may be a bit better.

Instructions on how to turn WAAS off using the hidden menu on Magellan GPSr's can be found on Brian at GPSOz's site along with other explanations of what WAAS is and why we don't need to worry about it here.

Dave
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Follow Up By: Mad Dog (Victoria) - Wednesday, Jan 12, 2005 at 22:57

Wednesday, Jan 12, 2005 at 22:57
Can't say I've taken any notice Dennis. I'll nick down to the water tomorrow and try the 3 machines I have.
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Follow Up By: snailbate - Thursday, Jan 13, 2005 at 16:53

Thursday, Jan 13, 2005 at 16:53
hi Mad Dog
every time you go to the water the reading will be diferent in every one of your three machines there is the different ionsphere the polution in the air , the clouds that why the manufactures say 95% 2d RMS
It is not the machines fault
also i posted a sorry below
When i purchased my GOLD machine the Australian edition did not say that WASS was not in Australia
but also the USA turned off the SA and this made all the machines more acurate
hope that clears things up
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Follow Up By: Toyota Sucks - Thursday, Jan 13, 2005 at 16:59

Thursday, Jan 13, 2005 at 16:59
no worries snail bait, haven't nicked down to the water yet but should do so later this evening when it cools off a bit. I enjoy a stroll by the water at the end of the day.
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Follow Up By: Member - JohnR (Vic) - Thursday, Jan 13, 2005 at 17:10

Thursday, Jan 13, 2005 at 17:10
Dave,

we actually do have some ground stations but you have to pay a licence fee to receive them and they are no where near as accurate at the WAAS system of satellites in the US, being fewer of them. I imagine farmers using GPS systems to guide cropping gear would need to use them anyway, the mining companies do and seem to also do their own reference stations too.

When you have a licenced and funtioning receiver you will trig from the satellite and the ground station with your GPS all done within. I understand there is one in the SE of South Australia at Mount Schank, south of Mt Gambier.

My Meriplat seems to regularly pick a WAAS satellite somewhere. I have not looked for the menu to turn it off.
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Follow Up By: Mad Dog (Victoria) - Thursday, Jan 13, 2005 at 17:42

Thursday, Jan 13, 2005 at 17:42
Thanks Snaily all good info.

John, any idea of the cost or a link to further info about the ground station
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Follow Up By: geocacher (djcache) - Thursday, Jan 13, 2005 at 20:20

Thursday, Jan 13, 2005 at 20:20
My understanding is that it takes a network of 27 ground stations to cover the USA. A similar network here would be required to cover Australia, which is why we don't have it. Gov't won't spend the cash.

From the information that I have been given none of the ground stations active in Australia would cover eastern Vic anyway. One of the websites mentions a range of 300ish NM from groundstation but if they can cover the US with 25 then it must be a little more.

I think there's one on the east coast around Newcastle or Jervis Bay and one over near Perth somewhere according to the maps on one US web site.

For all intents and purposes in Aus you are better off with it turned off. Those who have invested time in testing claim an improved accuracy with it turned off here.

Dave
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Follow Up By: Member - JohnR (Vic) - Thursday, Jan 13, 2005 at 23:00

Thursday, Jan 13, 2005 at 23:00
Ray,

I don't know how much it costs but is in the 4 digit levels I think depending on what your useage. I may be wrong.

Dave I had heard of one south of Perth too. I know mine shows different tracks occasionally to the road I am following, out in the paddock and the like. You will have seen from the card comment above I have Streets and Tracks loaded for my mapping if I am not connecting the laptop.

I have looked for the menu to turn off the WAAS and have not seen it yet. I would be interested to see it.

I didn't know there were the requirements to have land stations in the US to get the accuracy, I thought the WAAS satellites did it all in that part of the northern hemisphere.
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Follow Up By: Mad Dog (Victoria) - Friday, Jan 14, 2005 at 12:42

Friday, Jan 14, 2005 at 12:42
ok, went down to the waters edge this morn and set 3 machines up side by side. Lovely clear morn with no visible pollution, fog or smog etc.

Magellan 2000xl...quite a consistent reading of 0 feet with very infrequent jumps to 8 feet

Magellan GPS320...Mostly reading 6 feet, ocassionaly 0 feet with jumps to 8 feet

Garmin 12xl....Mostly on 11 feet but had a few jumps to 28 feet

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Reply By: snailbate - Tuesday, Jan 11, 2005 at 20:24

Tuesday, Jan 11, 2005 at 20:24
hi guys
sorry i was talking about something the Australian Gov wont do AND THAT IS WAAS
Talking about accuracy magellan meridian series claim this to be with out WAAS is to be Horizantil Accuracy to be 7 meters 95% 2D RMS
VERTICAL ACCURACY TO BE 10 METERES RMS
VELOCITY 0.1 KNOTS RMS

When the USA Turned off the SA this is when gps became moore accuracte
sorry for the conflicting info
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Follow Up By: snailbate - Friday, Jan 14, 2005 at 19:01

Friday, Jan 14, 2005 at 19:01
hi all
go look at your gps brand specifiction and see what they say
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