High on the Mountain Top
Join Date: Jan 2004
Location: North Vancouver, BC, Canada.
Interest: Mountain biking, hiking, nature photography, astronomy, music...
A four metre difference is really nothing to be concerned about. In fact, it's quite low and tells me that your GPS is doing a fine job.
GPS units can measure elevation in two different ways. Method one involves calculating the elevation based on the satellite positions. This method can result in a 10 metre error or more, so it's no surprise that the elevation reading was -4.
The second, and good old fashion method involves an internal barometer which calculates the elevation from atmospheric pressure.
Both methods have their advantages. The elevation calculated in method one is not subject to change due to a change in the weather, but it will tend to bounce around by as much as 15 to 20 metres within a few mnutes. The elevation calculated in method two will not tend to bounce around much, but it can change significantly in the same place over time due to weather changes, by many tens of metres. Better GPS units use satellites to automatically calibrate an internal barometre, based on an average elevation value.
Another thing to consider is that "sea level" does not actually refer to the water level. I know, that sounds strange. Sea level is a theoretical number derived from a calculated shape of the earth, and is fixed for a given datum. Think of it this way - the water level changes continuosly due to tides, but the elevation above sea level for any point on the earth does not change. So, you could be standing right at the water line at low tide, and your elevation would be below sea level. Make sense? In some minds it does.