Hi Ron,
Thanks for your reply. let me clarify my situation, I am NOT going to collect the information of the GPS location, instead, I am using GPS to "identify" the properties and assess the properties. so knowing where I am on the map is quite important and the more accurate the better
I reason I raise the concern was that I had a thought on GPS maps then I did a bit Google research on it. I can assume the GPS receiver is always giving me the right coordinates (lat/longs or other units in UTM projection). but the 2D map I am using to show the GPS dot is a result of particular projection, which is based on a datum and a projection method. by this point, maybe the question should really be: is the GPS and the map I am using are using the same datum? because I think, if the datum is the same, the actual lat/long in two systems (GPS and the map) for a location on the earth is the same, they should match.
in definition for WGS84 and Web Mercato Auxiliary Sphere, it seems the datum is the same, so maybe it should not be a question... if I am using another Geographical Coordinate System in my map, there may be a problem, the GPS location on the map can drift away (for example, I am standing in a property, but the GPS shows I am standing out of the property on the map)
I am curious what map projection do they use in the commercial GPS Unit in our cars.
Hopefully I am not making an obvious thing too complicated. I am not a surveyor, they should know these things better.