by Downunder35m » Sun Oct 07, 2012 12:48 pm
One other factor is the area you are in.
For example high buildings and mountains can reflect the signal and with a lack of this info the map can go anywhere.
What makes a GPS chip "good" would be the support of the latest dat stream types as well as the ability to handle the maximum amount of 16 visible sats to gather position info.
Accuracy in other terms than the ral coordinates of your postion also depend highly on the following factors:
a) The way the position is being related to the map on the screen.
Example: Ozi Explorer shows a "moving map" with your GPS position always centered, routing programs like TT, Igo, Garmin and so on also can force your on the road.
That means if the actual GPS postion would be a few meters to the side of the road in your garden it will most likely show up as on the road.
b) The general quality of the mapping data.
If a map is not precisely calibrated the software must compensate for the error.
For a moving map you would be shown off the track, while normal programs would only show that you might be a meters from your position as the difference is compensted by the software.
c) For vector maps like the ones Garmin, TT and others use a little factor is also the interpolation of the real world data related to the zoom factor shown on the map.
Ok, I know, too complicated to undestand, so I give an example:
You are on a narrow and winding road. The map in low zoom (so you see more of the area) might only show the road as a straight line.
In high zoom you will see more of the curves and corners.
How much of the real world details is translated into the correct vector data again depends on the map maker, not the GPS or software.
One major drawcard for the newer GPS chips is the ability to accquire the signals very fast.
Some even store the data of current positions while in use to interpolate the positions for the new few hours (some up to a week if the US does not re-route the sats).
The real accuracy in terms of coordinates only differs in the last few digits after the dot ;)
An old GPS can be as accurate as the latest models, but won't be as fast in getting the first lock after long period without use.
Last hint: some modern cars use heat reflecting windscreens - they partially block the GPS signal, same for windscreens with a heating system intergrated. For both an external GPS receiver is recommended.
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