Originally Posted by
BearSquirrel
No argument. But there is a big difference in how the different products are used. A handheld unit for field use has no navigation, so you wouldn't want to use it in your car. Likewise, a handheld GPS isn't optimized for bicycle use.
It appears that the Dakota provide for turn-by-turn navigation. There's no real technical reason that a handheld can't have turn-by-turn navigation.
Originally Posted by
BearSquirrel
Agreed, they don't work well at low speed. But speed has nothing to do with it. The issue is that GPS measures speed by measuring the difference in position. It does so at specific time intervals. So if you move in anything other than a straight line, the GPS will under-report your speed and distance travelled.
Speed is
part of it because slow speeds (moving in a straight line) means your position doesn't change very much. And there is also margin of error with measuring position too. That means one would probaly get unreliable speed measurements going around in tight circles even if your speed is high.
Originally Posted by
BearSquirrel
No you're dealing with a 3-dimensional structure. Even if you use spherical coordinates, it's still a 3-dimensional system. I don't think the military would have forgotten that they have aircraft when they designed the system.
No, GPS works
worse for altitude and it's an inherent issue (maybe, related to the number of satellites). The military isn't forgetting: There are other ways of determining altitude (more accurately).
Of course, the cyclist is also going to be
more sensitive to errors in altitude than he would be with errors in surface distance (a 500 ft climb is "significant" while a 500 surface distance traveled isn't).
http://www.gpsinformation.net/main/altitude.htm
Altitude error is always considerably worse than the horizontal (position error). Much of this is a matter of geometry. .... As a result, of this geometry the calculated solution for altitude is not as accurate as it is for horizontal position. Almost any calibrated altimeter will be more stable at reading altitude than a GPS. ... 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.
...
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.
Originally Posted by
BearSquirrel
The ground sits at a specific altitude. I'm pretty sure if the missle is flying too high, it will miss the target. It's a 3 dimensional system and the only reason it would not matter is if you dropped it from a standstill immediately above the target.
I'd guess cruise-missles have some sort of radar to keep them off the ground when flying. And, if you are at the target, you can have the missle steer into the ground. That is, nothing needs to be missed.