I compared three instruments, a Garmin 60CSx GPS, a Garmin Oregon 400i and the VX-8. I looked at both GPS and barometric altitudes.
|Barometric altitude ||GPS altitude |
|Garmin 60CSx||1072 ||1069 |
|Oregon 400i ||1081 ||1079 |
|Yeasu VX-8 ||1092 ||1030 |
Measurements were taken in a single location (my living room) at about the same time, meaning very little change in the GPS constellation. The Garmin units show consistency and a difference of 9-10 feet. The VX-8 is in the ballpark but the GPS altitude is not as good. According the the topo map the altitude is 1055. Since I was not in the basement, maybe I need to add 8-10 feet or so and the map contour lines definitely predate the house...
My conclusion is not to try to land a plane using any of these three instruments only... Otherwise, gosh, it is good enough for me.
I also checked the GPS' precision in location and here the news are excellent! The table below shows the decimal part of the minutes in both longitude and latitude numbers, all in WGS 84 datum.
|Garmin Oregon 400i and 60CSx ||Yaesu VX-8 |
|,758 ||.765 |
|.878 ||.874 |
Now, this is only anecdotal data, a single measurement in the comfort of my living room, not taken in any extreme type of weather and conditions. But for me it is good enough.