Re: GPS Accuracy
- From: bill horne <redydog@xxxxxxx>
- Date: Wed, 25 Mar 2009 23:54:42 -0400
Andy Asberry wrote:
On Wed, 25 Mar 2009 10:47:36 -0400, Matt Colie <Matt@xxxxxxxxxx>
wrote:
Andy,OK, let's see if I understand you. If I list a stone as X N and Y W
There are several issues here that have not been touched, and so, it is about time I dove in here.
If others are going to use the information that you are compiling, those persons will undoubtedly have different GPS units and this will compound the problems that will affect the usefulness of your data.
What I will suggest that you do is set several obvious features in the cemetery as reference marks. Use the difference from those references to locate specific graves. How you do that is your choice.
There are four separate issues in this discussion:
Accuracy, Resolution, Repeatability short term, Repeatability long term
The actual accuracy is model dependent. As the manufactures improved the units, one of the things that they all did very well was make the processor (computer and program) more capable of managing a better algorithm (the method used to resolve location from the satellite signal). Then came the differential beacon that transmits local correction data to an auxiliary receiver that the GPS can use to correct the satellite data. After that came WAAS which is similar but as the corrections are transmitted by the orbiting satellites, no auxiliary receiver is required. Summary, most newer units are better than most older units.
Resolution is the minimum offset that the unit will reveal. It has nothing to do with accuracy. It has nothing to do with anything but the fineness of the units displayed values. Summary, if the unit says you are less than a foot from the target, it may have the target location considerably wrong and not care.
Short term repeatability is the capability to indicate the location over an hour or so (definition varies). This has as much to do with the signals coming in as the unit itself. Even this will vary if you watch it for a little while. Summary, if you set point and walk away, it may tell you where you started for a few minutes.
Long term repeatablity is the capability to indicate the location after a significant period of time (definition varies). This has as more to do with the algorithm and if the unit is WAAS enabled. Expect this to vary in the few meters range no matter what you do. Summary, when you come back to your reference point after a day or more, don't expect the unit to be dead on.
This system was conceived as a means to guide ships at sea and high flying aircraft. I have used it for both and enjoyed the improvement over its predecessors. The current uses were not even imagined as the hand held unit was a more than decade in the future and not deemed technologically feasible at that time.
but I'm off by 30 feet. Then the next visitor uses those coordinates
and his unit is also off 30 feet, he could actually be 60 feet off? Or
dead on?
If I get a surveyor to mark several locations around the cemetery and
check my unit against his, can I use that difference as a correction
value? Maybe recalibrate every hour? Would that make my measurements
more accurate? How long must I hover over a location for the unit to
average?
The oldest grave is from 1963. Many of the stones are marble which is
soft like sandstone. A LOT of these are simply set on concrete bases.
Evidently at that time there was only one monument company in the area
because there are a bunch of like bases.
The crudest stones are from 1918 (flu epidemic) and during the
depression. Many are local sandstones with information hand carved
with a knife or sharp tool. Then just stood on edge.
It is these that are easily moved that are likely to be misplaced.
The bottom line is that no consumer GPS is going to get you close enough to confirm that John's headstone is on Mary's grave, Fred's is on John's, and Mary's is on Fred's if those graves are close to each other.
If I were you, I'd go out there with a compass and a tape, find a few immovable objects in the cemetery, and see how consistent and close I could be with ranges and bearings.
If it looks that would work, then I'd GPS the immovable objects, and use a transit for the bearings to give a fighting chance to those who come looking for graves while bearing a GPS, compass, and tape.
--
bill
Theory don't mean squat if it don't work.
.
- References:
- GPS Accuracy
- From: Andy Asberry
- Re: GPS Accuracy
- From: Matt Colie
- Re: GPS Accuracy
- From: Andy Asberry
- GPS Accuracy
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