Accuracy of Vista

[Guest john hewitt]

I had my gps on my dashboard yesterday, and was viewing my progression on the highway, when I came upon a bridge over Lake Cumerland. It showed me I had crossed the lake just as I was getting to it. (The bridge is about 1/4 mile long.) The accuracy showed it was accurate to within 9 and 15 feet. Is the map not accurate, the unit in need of calibration (I did not know you could recalibrate it), or is this acceptable? I guess I would understand if it had a bit of "lag time", but this was putting me ahead of where I actually was.

[Guest Mike_Teague]

The maps are not accurate... You will notice that _a lot_ with a GPS.. Mapmaking is still not the science it would seem!

 

If anything, the vista would be MORE accurate than a typical GPS because it can recieve the WAAS (If I recall correctly)

[Guest trilobites]

At best, most of the highway maps we purchase and those bases used in GPS units are at a 1:100,000 scale. This is because standard USGS digital linework that covers the USA are the 1:100,000 scale DLG (digital line graphs). I don't work with the 1:100,000 scale maps that often but I know the standard error on the 7-1/2 topo maps (1:24000 scale) is 1/50th of an inch or about 40 feet. Assuming this error rate on the 1:100,000 scale maps, your looking at an error of at least 50 meters. If the base map data is some of the 1:500,000 scale stuff that I see in a lot of GPS maps then your map error can be as much as 250 meters (~800 ft). Also, your GPS probably updates its location every 2 seconds or so, thus at 60 mph you move about 176 ft in 2 seconds (about 54 meters) so expect to be at least 104 meters off when driving down the highway.

[Guest trilobites]

Sorry guys, just found the mapping standards for the USGS 1:100,000 scale data. The standard error on thos maps is about 85 meters or about 280 ft. So at 60 mph your looking at 80+54 134 meters or at least 440 ft or about 0.1 mile.

[Guest johnny42544]

I racked myself going to town and coming home. My route on the GPS was almost superimposed on itself! Guess it is a lot more accurate than the maps!