GPS Altitude error

[WhiteD3]

Hi guys.

My Win CE powered GPS has decided to show the altitude incorrectly by reading ~ 20m low. This occurs in both Oziexplorer and the street nav program so it's a problem with the device itself or Win CE.

Any ideas?

[RR P38]

Does your GPS use a barometric sensor or is it estimating your altitude via the map?
With barometric pressure being subject to atmospheric conditions it is normal for it to change and be in error.

[PhilipA]

I saw a response on this by a car manufacturer saying that unlike some misconceptions, the world is not round.
It has pits and bumps and GPSs measurement is the mean distance from the satellites which may be inaccurate.
Regards Philip A

[WhiteD3]

Does your GPS use a barometric sensor or is it estimating your altitude via the map?
With barometric pressure being subject to atmospheric conditions it is normal for it to change and be in error.

Being an el cheapo Chinese GPS I doubt it has any sort of sensor. I always assumed altitude was calculated from GPS coordinates and it's been spot on rain, hail or shine up until yesterday.

[vnx205]

In addition to what PhilipA said, I just found and article that said that vertical error is about 1.5 times horizontal error.

If horizontal error can be up to 15 metres, then vertical could be out by up to 23.

Another article said this:
http://users.erols.com/dlwilson/gpsvert.htm
A vertical error specification something like 95% within 20 meters with VDOP of perhaps 2.0 is likely.

[theresanothersteve]

Basic GPS needs 4 satellites to establish an accurate fix, 3 used for establishing X, Y, and Z coordinates and one for a time signal.

If there are only 3 satellites available the unit uses 2 for coordinates and the last for the time signal, the unit guesses the elevation based on last known figure.

This is OK if you are at sea, but dangerous if you are in hilly terrain, you can be 50 metres lower than before and the estimated position can be way out.

If the error is intermittent, or you know you are in a poor reception area, check the number of satellites being accessed. (And, if the GPS is using 4 and they're close together that can cause inaccuracies.)

HTH

[Xtreme]

Basic GPS needs 4 satellites to establish an accurate fix, 3 used for establishing X, Y, and Z coordinates and one for a time signal.

If there are only 3 satellites available the unit uses 2 for coordinates and the last for the time signal, the unit guesses the elevation based on last known figure.

This is OK if you are at sea, but dangerous if you are in hilly terrain, you can be 50 metres lower than before and the estimated position can be way out.

If the error is intermittent, or you know you are in a poor reception area, check the number of satellites being accessed. (And, if the GPS is using 4 and they're close together that can cause inaccuracies.)

HTH

x2^^

The very acute angle that the signals from the satellites are received also adds to the elevation inaccuracies referred to by vnx205 in a previous post.

I'm not sure about the GPS that you are experiencing problems with but with the Garmin Map60csx model there is a facility to correct the elevation reading if you are at a known elevation - sea level or a trig station for example. Might be worth serching through the menu's on your GPS.

[101RRS]

When I use my GPS inside the house I get errors in height and position (a few hundred meters as the strongest GPS signals are coming through the windows picking up low satellites on the horizon - I can see the spread on my display - go outside and it fixes itself - so were you shielded from some satellites or is it constantly wrong.

GPS determine their position in 3 dimensional space and as such do not need external references - the height datum is and averaged sea level datum. Heights listed on maps are not as accurate as GPS height.

Garry

[JDNSW]

As stated above. GPS determines the X, Y, Z coordinates from a minimum of four satellites. If there are not enough satellites, or the signal quality is not good enough, or the relative angle between the position surfaces is not large enough, most (all?) GPS receivers will revert to 2D operation where the altitude is assumed from the best available information, perhaps map, perhaps last calculated altitude - as stated, this works well at sea, less well on land, especially if hilly, or in the air.

In addition to these inaccuracies in the "fix" you need to bear in mind that the calculated altitude is relative to a mathematically expressed spheroid, which is a close approximation to the geoid, the rather irregular shape of the earth's equipotential surface. GPS uses a spheroid that fits the geoid fairly well, averaged over the whole world. Australian maps use one that fits better than this over the Australian landmass (and there are dozens of others), so that there may be an error if your GPS is using the wrong spheroid. I am not an expert on current GPS receivers, but I would expect that it is easy to specify which spheroid to use, and it should be the same one as is used by your map data.

The actual error you see is likely to be a combination of all of the above!

John

[woody]

As stated above. GPS determines the X, Y, Z coordinates from a minimum of four satellites. If there are not enough satellites, or the signal quality is not good enough, or the relative angle between the position surfaces is not large enough, most (all?) GPS receivers will revert to 2D operation where the altitude is assumed from the best available information, perhaps map, perhaps last calculated altitude - as stated, this works well at sea, less well on land, especially if hilly, or in the air.

In addition to these inaccuracies in the "fix" you need to bear in mind that the calculated altitude is relative to a mathematically expressed spheroid, which is a close approximation to the geoid, the rather irregular shape of the earth's equipotential surface. GPS uses a spheroid that fits the geoid fairly well, averaged over the whole world. Australian maps use one that fits better than this over the Australian landmass (and there are dozens of others), so that there may be an error if your GPS is using the wrong spheroid. I am not an expert on current GPS receivers, but I would expect that it is easy to specify which spheroid to use, and it should be the same one as is used by your map data.

The actual error you see is likely to be a combination of all of the above!

John

Think of the Earth as an Egg, as such it is more round in some places and more flat in others. We should be using the Geodetic datum thats more suited to our part of the globe, most GPS receivers have their default settings as WGA84 but GDA2000 or GDA (as most units show it ) is more accurate in our neck of the woods.

Anyone playing with topographic maps should check the map datum and change the datum in their GPS accordingly to improve accuracy.

woody

[Quarks]

As above, altitude accuracy is usually 1.5 to 2 times worse than horizontal accuracy - unless you have locked on to 3 or 4 satellites near the horizon, in which case it's likely to be as good.
If you do as woody suggests, and use the Aussie datum, then your horizontal position would be better, but for accurate vertical position reporting you need to get your gps to give you height in terms of the Australian Height Datum (AHD71). That's what is used for the values printed on maps, and is based on mean sea level around the continent, rather than distance from the earth's centre of mass (which is the reference point for all gps positioning). Naturally, the different reference levels give differing heights.

:)