Incorrect GPS Altitiude readout.

russell2002 · Posted: Thu Oct 18, 2007 8:00 am Post subject: Incorrect GPS Altitiude readout.

Hi,

I have a GPS built into my Acer N35 handheld.

The altitude readout is incorrect. It states I am 290feet above sea level, when I can see the sea beside me.

Any ideas.

Thanks.

SimonCatlin · Posted: Thu Oct 18, 2007 9:25 am Post subject:

There are several reasons for this.

Probably the most important is that GPS in altitude terms is at best poor and at worst completely in-accurate. It only takes 3 sats to get a lat and log but to gain altitude it takes 4, which with todays receivers is not an issue, however, it is not totally correct even with 12 sats.

The other issue is what is sea level? In the UK it is set as the mean tide height of Newlyn Beach in Cornwall. This will be very different for example in Holland (under sea level according to the UK figues).

On my Flying (Garmin Pilot 111) GPS I can set a corrective figure for "sea level". Maybe your hardware (and possible software) has a similar feature?
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russell2002 · Posted: Thu Oct 18, 2007 10:23 am Post subject:

Hi,

So is it accurate relative to two different readings. I do not need to know my actual height, but my height relative to a different place a few miles away.

Thanks,

philpugh · Posted: Thu Oct 18, 2007 12:25 pm Post subject:

No - if you watch the altitude display without moving, it will vary by quite substantial amounts over a short time period. So relative differences are not precise either. Averaging over a long (days) time period will produce a more accurate result.

You can get a more accurate altitude reading using barometeric devices (I have a SUUNTO watch with altimeter). If you calibrate this, correctly, immediately before a walk then the altitude can be reasonably accurate for a few hours or more - depending on the weather conditions.

The variation in GPS altitude is caused by the poor satellite geometery for determing height. All the 'visible' sats are above the GPS unit. For a more accurate fix you would need to see all the sats surrounding the GPS. For 2D positioning the visible sats can do a decent job.

Also the GPS altitude is relative to the curved plane (or spheroid) defined by the datum being used (usually WGS84). This mathematical surface may be above or below the accepted 0 metres ('sea-level') for the country you are in. If you are in the UK - change between WGS 84 and OSGB - you should get a small change in height displayed due to the difference between the reference planes.
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russell2002 · Posted: Thu Oct 18, 2007 5:42 pm Post subject:

Thanks,

That makes sense now.

Skippy · Posted: Thu Oct 18, 2007 11:41 pm Post subject:

Garmin units report the correct height, and they do it reasonably accurately too....
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philpugh · Posted: Fri Oct 19, 2007 1:57 pm Post subject:

Skippy · Posted: Fri Oct 19, 2007 2:49 pm Post subject:

neil01 · Posted: Sat Oct 20, 2007 5:24 pm Post subject:

Put very simply. The GPS signal doesn't give the height above sea level - It can only give your distance from the satellite.

Software then deduces the height; and depending upon it's sophistication (as explained elsewhere) this may or may not need 'correcting' depending upon where on the earth you are. (Due to the Earth not being an exact sphere)

As usual the actual value reported is subject the the normal accuracies of the technology/hardware/software.

While everything being equal, more satellite fixes should give a more accurate value, the actual positioning of the satellites in the sky is a also a significant factor.

Without going into too much detail, the further apart the satellites are - the better, but satellites in a straight line (no matter how far apart) do not give the best results.

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