chantililace said:
If I am missing something big here, please let me know. Thanks
I think
the point (made by both The Spook and myself), is what you missed.
There is no
GPS Coordinate <--> Physical Address mapping.
If you select a navigation point by address, your GPS unit will approximate the location of that address, based on the
address range saved with the definition for that road segment. The algorithm used to determine what spot on the map to point at for a given address may very well differ between different GPS units (and, perhaps, moreso between different brands of GPS units!).
You've stated that your old Garmin unit would even tell you which side of the street a given address was on -- this is solely because they've programmed this into the address location algorithm in their units. It is not, however, correct (or, for that matter, without flaw). Garmin makes the
assumption that all even number addresses are on one side of the road, and all odd number addresses are on the opposite side of the road (as this is customary in the US; It is not, however, a hard and fast rule!). My old Garmin unit would consistently tell me that my eye doctor was on the east side of the road (along with all the other odd-numbered addresses), when in fact it is on the west side of the road! (2727 Capital Circle NE, Tallahassee, FL 32308) It's wrong, because the
assumption made by the programmers who created Garmin's algorithms is invalid. Odd number addresses don't
always show up on the eastern side of north/south running roads, nor does it follow that if one address on the left side of the road is even, that all addresses on that side of the road are also even. It may well be true
most of the time, but it's not true all of the time.
For this reason, you will not get exact address placement on a GPS map, at
any time. It will
always be an approximation, perhaps valid, perhaps not.
The only address information stored in the map definition for any given road segment is an address range. It is
not possible, on any GPS unit, to place a specific address accurately on a road given the limitations of the data available. A specific address may be calculated, however, and that calculation may be "close", "close enough", "spot on!", "way the heck off!" (or anywhere in between) in reference to the actual physical location of that specific address.
- Brian
(...who knows a thing or two about GIS...)