How to test 7100 GPS accuracy?

[Phred]

I used my 7100 to mark several cemetery locations, but when I checked the stored decimal coordinates on Google Maps, they were way off (on the order of 100-500 ft).

I retested today using a fixed spot near a lamp post in a large parking lot away from the street, and with the 7100 registering 7 satellites and HDOP=1.2 (which I think means it should be fairly accurate), the coordinates in "GPS Status" still appeared about 100 ft off (when checked on Google Maps).

I double checked using my Garmin IQue 3600 (an old first generation GPS), which also registered about 7 satellites and 15ft accuracy. The coordinates it came up with showed spot on in Google Maps.

I repeated this for two locations in the lot, about 500 feet apart, and in both cases the Garmin and Google agreed, but not the 7100.

So am I missing something about how to use the 7100 in regards to the GPS coordinates? It seems like all of these (Garmin, Navigon and Google) are supposed to be using Navteq maps.

Any suggestions or comments welcomed.

[Tim]

All of the readings (the GPS's calculated position, the mapping database, etc) have a margin of error. When put together, those margins of error are combined.

GPS itself has an accuracy of about 10 meters. The road map database could be off by any amount--- you can sometimes find streets that are offset by a thousand feet or more from where they "should" be.

Thus the only real way to measure the accuracy of the GPS's calculated position is to compare it to a surveyed/known good set of coordinates. USGS benchmarks are good for this, BUT you need to be careful about what types of benchmarks to use-- you want a mark that is location adjusted and not one that is location scaled. The latter, unfortunately, is much more common.

[Phred]

I purposely took readings off the road in a large parking lot, expecting (or more hoping) that this would be more likely to give me "absolute" coordinates not skewed by road locations.

In the case of the Garmin/Google combination, it seemed to work, but I agree that's not saying much without a firmer reference benchmark.

The 7100 also has problems with the roads around my office (where I did this test), often trying to navigate to about a 1/2 block past my office and to the other side of the street. This happens regardless of whether I use the address or a saved GPS location as the destination.

OTOH, since there's no calibration for the 7100, I guess the point is moot ...

Thx.

[Tim]

There is a margin of error in everything-- even satellite imagery and aerial photography doesn't get lined up correctly fairly frequently.