GPS Lat Lon

I think GPS won't do much better than 10-20'.

I think that depends entirely on the GPS. Maybe our everyday Garmin or whatever is not that accurate - I don't think they have even 5 digits - but there are more accurate GPS units and there are ones that carry the digits out much further - like 6, 7 , 8 I think - which can get pretty darn accurate. Many things have to be in line - and the receiver you use has to be picking up enough satellites to get that degree of accuracy. Sometimes we see only 4 satellites on our display but we have gotten up to 11 and I would guess getting more could be possible. Though Google Earth is not a GPS it seems they can get 6 digits. But since we are RVers we do not need that degree of accuracy so we are not going to spend what it might cost to get a device that is that accurate. Below gps.gov claims they can get under 10' accuracy 95% of the time with a high quality unit - and even down to centimeters and millimeters. I wonder what one of those costs. The fact that we are pretty darn close most of the time tells me we can get as close as we will ever need. A GPS used for survey equipment can run over $5,000.


Official U.S. government information about the Global Positioning System (GPS) and related topics

http://www.gps.gov/systems/gps/performance/accuracy/


The actual accuracy users attain depends on factors outside the government's control, including atmospheric effects, sky blockage, and receiver quality. Real-world data from the FAA show their high-quality GPS SPS receivers attaining better than 2.168 meter horizontal accuracy, 95% of the time.

Higher accuracy is attainable by using GPS in combination with augmentation systems. These enable real-time positioning to within a few centimeters, and post-mission measurements at the millimeter level.