负高度值

Question

I am getting negative altitude value when finding latitude , longitude and altitude. Can anyone help me to find the reason for it ?

Answer 1

Short of seeing the code which produces these values, there's not much I can offer beyond general knowledge.

GPS altitudes generally use a geodetic model for an idealised sea level (the zero altitude), basically mapping an ellipsoid onto a less-than-perfectly-shaped planet (which varies anyway with things such as lunar tidal forces). See WGS84 here for more details.

The normal error is about +/- 15m, and this only applies about 95% of the time. According to the specs I remember, the other 5% of the time can have an altitude of any value.

And, of course, if your GPS doesn't have an unobstructed pathway between it and the satellites, the error range is much greater.

These potential errors are no doubt why aircraft rely more on altimeters or ILS for their near-ground activities.

As one site puts it:

What this means is that if you are walking on the seashore, and see your altitude as -15 meters, you should not be concerned.

Of course, if you're having troubles breathing, you may want to look more closely at how wet you are :-)

Answer 2

Well, altitude in context of GPS coordinates represents your elevation according to the sea level. So I assume your current position is just below the sea level. Yeah, this is possible ;)

Answer 3

The GPS altitude is the altitude above the GPS WGS84 reference elipsoid (which is different from sea level and does not take hills into account!). A negative altutude means (if it not due to a bad signal) that you are below the reference elipsoid.

Here is more information on how to get more accurate altitude: https://stackoverflow.com/a/9432382/1127492