Recently, I get a Emtac Bluetooth GPS receiver, and I’ve been using it with both my laptop and smartphone. So far it’s been great, and I’m amazed at its horizontal resolution. This is my first GPS device, so the fact that such a device can show me not only the street I’m on, but which side of it I’m standing on is mind-blowing to me.
Vertically, however, it’s a bit odd. I know that altitude is trickier for GPS devices, and that it’s the direction more prone to error. And my position does wander in that direction a good bit when I have a shaky satellite fix. But more often than not, I get a good, stable altitude reading that only wanders a few meters.
The real issue with that altitude is that the stable reading it likes to settle on is consistently about 100 ft lower than what I see on topographic maps. My house, for example, is at an altitude of 940 feet. But my receiver likes to settle on number in the area of 840-850 feet. The same behavior ocurrs in a half-dozen other locations I’ve been able to compare.
Why would my readings be so consistently wrong, by the same amount, in the same direction? You’d think if it were going to have that kind of precision, it would be accurate, as well.