GPS accuracy - altitude

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.

Mad Dog
You have not got a gold

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

Dave, I have an upgrade of the firmware which removes the barometer function alltogether.

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

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...

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

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

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

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.

Cheers

Yep, figured on that one so we had them apart by about a metre +.

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)

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.

Could be tides but it may also be weed. The more weed the higher ya get :)

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.

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.

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

Can't say I've taken any notice Dennis. I'll nick down to the water tomorrow and try the 3 machines I have.

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
snailbate

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.

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.

Thanks Snaily all good info.

John, any idea of the cost or a link to further info about the ground station

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

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.

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

hi all
go look at your gps brand specifiction and see what they say