Doe's anyone have a good way to judge accuracy on a GPS

[+Bebop]

Back in the late 90's before the satellites were change, I was using a Lowrance Eagle that had six chanel and had a programmable offset for the latitude and longitude. I would use my digital calipers to measure a benchmark on a USGS topo map and get the latitude and longitude. Then I would set the GPS map datum to mach the map datum on the topo map. Then I would go out and put my GPS on that benchmark and let it average for 30 minutes or so, then I would set the offset in the GPS so my GPS was more accurate to that benchmark. I would get 25' to 50' accuracy. But I doun't know if that was any good for judgeing the accuracy of a GPS.

[+Red90]

Yes, just go to a benchmark. With WAAS and a clear view of the sky <5' is reasonable.

[+blindleader]

I would use my digital calipers to measure a benchmark on a USGS topo map and get the latitude and longitude. Then I would set the GPS map datum to mach the map datum on the topo map. But I doun't know if that was any good for judgeing the accuracy of a GPS.

 

No it wasn't any good for measuring accuracy.

 

Your own hand held GPS unit (modern) has the means to estimate the accuracy of its own readings. Newer Garmin units generally display a +/- number that is a 95% confidence circle of uncertainty. I don't know what newer Magellan units display but older Meridians displayed EPE which was essentially the same thing as the Garmin +/- but with an unpublished and unknown confidence percentage. Under certain circumstances, these estimates can be way off, but generally they're OK.

 

Technical rant follows.

Your old method isn't useful for the following reasons:

First: When SA was in operation, you needed about two hours of averaging to cancel out the induced errors.

Second: Scaling a symbol off a paper map is no way to get the correct coordinates of a bench mark. To do that you need to go to the NGS data pages, make sure the coordinates given for it are GPS data and not scaled from a map.

Third: Once you have the real GPS coordinates of a benchmark and you take your GPS receiver to it, all you'll be measuring is the accuracy of the specific readings you are taking at that spot at that time. These measurements mean very little a half hour later a mile away, and they mean nothing at all two hours later in the woods down the road. This is all because the measuring system consists mainly of a about thirty components screaming through the sky at tens of thousands of miles per hour and therefore is constantly changing their geometry. Also hills, trees and buildings sometimes determine whether of not you're getting signals from the best satellites.

[itchytweed]

I would suggest that you put the GPSr on a tripod or something of that nature to minimize ground reflections.

 

Where I live, I am surrounded by four benchmarks. One is 1/2 mile from me. A project that I have in mind is to rent some surveying equipment and apply what I learned in college aeons ago in my surveying class and extend the BM to my place and put in a marker to reflect that. Then the marker can be used for calibration checks.

 

Before doing the readings, I would let the unit run for about three hours. This will warm up the electronics and stabilize the internal oscillator better. This will allow for a more reasonable and accurate reading.

 

Good Luck!!!

[Grasscatcher]

When you go to the NGS pages or to the Benchmark info on GC.com, be sure to look for coordinates that are "Adjusted" not "Scaled".

 

"Adjusted" coordinates are "as" accurate, or possibly more accurate, than your consumer grade GPSr is capable of.

"Scaled" coordinates are ones that have been scaled from paper maps, and can be significantly in error.

 

Locate the data for a nearby BM and verify that it's horizontal coordinates are "adjusted" and use it to see how close your GPSr "agrees". The same principal applies for elevation data. (adjusted/scaled)

[itchytweed]

When you go to the NGS pages or to the Benchmark info on GC.com, be sure to look for coordinates that are "Adjusted" not "Scaled".

 

"Adjusted" coordinates are "as" accurate, or possibly more accurate, than your consumer grade GPSr is capable of.

"Scaled" coordinates are ones that have been scaled from paper maps, and can be significantly in error.

 

Locate the data for a nearby BM and verify that it's horizontal coordinates are "adjusted" and use it to see how close your GPSr "agrees". The same principal applies for elevation data. (adjusted/scaled)

 

These are not NGS BM's. They were laid by the Regional Planning Commission and are measured (hopefully as they measure to +/- 0.005 ft) as they are used as references for real estate control and planning. The issue that I have with them is that they used Easting/Northing in survey feet measured to 0.01 foot. So I had to convert them to Lat/Lon using NGS software.

 

The real twist is that these are every 1/2 mile in square grid and as luck has it, all the monuments lie in the center of very busy intersections.

 

The whole process will be challenging to do solo and probably the time that I will have available to do the work will be in the winter and on a day that it is snowing heavily :-) All I need to do is rent a total station and two retroreflectors and I can do the measurements in one shot turning one common corner that is at about 90 degrees. Plus, I can also fill in the blanks on my property deed as well. The description is lot/block and has never been really surveyed.

 

Such Fun!!!