Mine shows 192ft whilst at sea level - actually on the rocks and sand

any ideas on how I can reset it? 

You might want to read this.

GPS Altitude Readout > How Accurate? (rev. 2/10/01)

New GPS buyers are frequently concerned about the accuracy (or lack of it) of the altitude readout on their newly purchased GPS. Many suspect their equipment may even be defective when they see the altitude readout at a fixed point vary by many hundreds of feet. This is NORMAL.

With most low cost GPS receivers, the horizontal error (without SA now that it is off) is specified to be within about +/- 15 meters (50 feet) 95% of the time. Most users find this is a conservative specification and that their modern GPS receivers routinely perform better than this worst case specification. But.. Users should expect that SOMETIMES they may see the error approach the specification limits. AND.. 5% of the time, the error may be "any value" from zero to whatever". Note: Unless you have a CLEAR AND UNOBSTRUCTED view of the sky (on your dash or looking out of an airplane window with no externally mounted antenna, or similar obstructed view does not count!) you can count on your error excursions to be much greater than the above numbers. Your GPS <depends> on this clear and unobstructed view or it cannot make accurate range measurements to the satellites.

Generally, Altitude error is specified to be 1.5 x Horizontal error specification. This means that the user of standard consumer GPS receivers should consider +/-23meters (75ft) with a DOP of 1 for 95% confidence. Altitude error is always considerably worse than the horizontal (position error). Much of this is a matter of geometry. If we (simplistically) consider just four satellites, the "optimum" configuration for best overall accuracy is having the four SVs at 40 to 55 degrees above the horizon and one (for instance) in each general direction N, E, W, and S. (Note: You will get a very BAD DOP if the SVs are at the exact same elevation. Luckily, this is a rare occurrence.) See: DOP demonstration site by Norris Weimer> How SV geometry affects GPS accuracy(Java Required).. The similar "best" arrangement for vertical position is with one SV overhead and the others at the horizon and 120 degrees in azimuth apart. Obviously, this arrangement is very poor from a signal standpoint. As a result, of this geometry the calculated solution for altitude is not as accurate as it is for horizontal position. Almost any calibrated altimeter will be more stable at reading altitude than a GPS.

GPS altitude measures the users' distance from the center of the SVs orbits. These measurements are referenced to geodetic altitude or ellipsoidal altitude in some GPS equipment. Garmin and most equipment manufacturers utilize a mathematical model in the GPS software which roughly approximates the geodetic model of the earth and reference altitude to this model. As with any model, there will be errors as the earth is not a simple mathematical shape to represent. What this means is that if you are walking on the seashore, and see your altitude as -15 meters, you should not be concerned. First, the geodetic model of the earth can have much more than this amount of error at any specific point and Second, you have the GPS error itself to add in. As a result of this combined error, I am not surprised to be at the seashore and see -40 meter errors in some spots.

DGPS operation (where available) will dramatically improve the performance of even low cost GPS receivers. Horizontal accuracy of +/- 5 meters and altitude accuracy of +/- 10 meters (relative to the WGS-84 geode) with suitable DGPS receivers and low cost GPS receivers such as the Garmin GPS-12XL can be expected.

In any case, it is extremely unwise to overly depend on the altitude readout of a GPS. Those who use GPS altitude to aid in landing their small plane should have their insurance policies paid up at all times.

Interesting reply nicedayforit - good to read and know. Thanks.

I have found GPS is broadly accurate and within some degree reflects the accuracy of the horizontal position (ie the better the accuracy the GPS is claiming, the nearer the altitude reading the GPS gives for height as well). I tend to treat it as a gauge of height rather than an absolute reading.

pcheaven - where do you see the altitude on the sat nav in a MY2013? I can't recall seeing it.

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select the nav screen, then press the hard NAV button and select off road, then on screen bottom left, where you normally select split nav screen is a further option to select compass etc, its there 

Cool, thanks pcheaven. One to have a play with! 2016 BMW X3 30d SE in red
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pcheaven wrote:

select the nav screen, then press the hard NAV button and select off road, then on screen bottom left, where you normally select split nav screen is a further option to select compass etc, its there

I had been wondering how that worked.
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Had a look at this on the sat nav today. Interestingly, the height is listed as 534 feet, according to an OS map we are approx. 360 feet. Also the longitude is listed as Exxx but we are in fact Wxxx!!

Click image to enlarge

In clouds... Your higher than sea level
In water..... Your lower than sea level
Fairly dry.... Just about right....

Job done.

T Terran



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Winslet wrote:

Had a look at this on the sat nav today. Interestingly, the height is listed as 534 feet, according to an OS map we are approx. 360 feet. Also the longitude is listed as Exxx but we are in fact Wxxx!!

Don't doubt the altimeter is wrong but the Latitude/Longitude is correct in its presentation.

In Latitude/Longitude co-ordinates there are only 'Northings' and 'Eastings', either side of Greenwich Meridian is denoted by a + or a - but always with an 'E'

Whether yours is right or not I haven't a clue

My height is shown as 180 feet yet I live only 3 metres above sea level, plus we don't have a hill that high in East Yorkshire.

I once set manchester airport parking co-ordinates in to the the sat nav and it directed me to the irish sea.
my recent portugal hotel gave 3 differentstyle coordinates for the same place

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As far as I can find, if the coordinates are reported degrees/minutes/seconds, then the longitude is reported as E or W of Greenwich. If reporting as decimal degrees, then longitude is reported as - for west and + for east of Greenwich.

The sat nav reports in D/M/S (sorry, I covered up quite a bit of the actual values in my photo for obvious reasons) and has reported my home position as E of Greenwich, we are W. So it is incorrect as far as I can tell.

Edit: Just had a look at an app called GPS Test and it reports the altitude at about the same height as the car sat nav. When I tick the option in settings on the app to geoid correct the altitude, it gives a figure much closer to the OS map value.

Therefore, possibly the car sat nav altitude is not geoid corrected and that explains the difference. Only a suggestion, I need to read up more. 2016 BMW X3 30d SE in red
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