NavList:

Tony:

You say your GPS was not giving consistent results in the fifth and sixth place after the decimal point.

Respectfully suggest you work out what that error amounts to.   If I'm doing it correctly, the sixth place (a millionth of a degree) is sixty millionths of a minute or sixty millionths of a nautical mile or sixty millionths of 6076 feet or 1852 meters.  

That's 11 centimeters or about four inches!

Okay, if error is in fifth decimal place, that's a bit over a meter or 40 inches.

GPS's stated horizontal accuracy is 15 meters "most of the time" so if you're seeing an error only in the fifth or sixth decimal place, you're getting a pretty accurate position!

Remember that among other things GPS does in calculating your position is estimating the speed of light through the atmosphere, which can vary based on everything from air pressure to atmospheric ionization.

PS - GPS's stated vertical (altitude) accuracy is ten times worse, so getting an altitude of six meters when your altitude was two meters is pretty darn good.  Oh, and GPS altitude is based on the WGS84 ellipsoid which might or might not match what you consider "sea level" at your particular location.

PPS - Strongly suggest you try Stan Klein's Celestial Tools program to double-check your sights.   It includes dip-short correction.