Depending on your location, values may differ and jump from actual for as much as 100ft and this is the best that GPS can do without help of some other system like DGPS (http://en.wikipedia.org/wiki/Differential_GPS). Though none of the smartphones features DGPS support at the moment.
Let me provide few excerpts from his article here:
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.
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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.
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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.
Excerpts provided with permission from Joe Mehaffey, http://gpsinformation.net - a site covering every aspect of GPS you can think of.