WAAS GPS quick questions

Benjamin_baker

What is the highest accuracy one can get from GPS? I had seen some websites offering WAAS with 3 cm resolution, but others say 3m, so maybe that was a typo...?

Thanks,
Ben

Forrest

With consumer grade equipment, I believe +/- 3 meters the best accuracy. If you have a lot of money ($10K), then the GPS equipment that surveyors use is more accurate.

steve_b

3meters is about it....
If you get a DGPS ($$) you'd get finer accuracy....but that's too much money to know my MPG down to the n'teenth decimal place! [noparse];)[/noparse]

Hey Forrest, do the surveyors use a single DGPS to reference their main survey tool and then all their 'flags/reflectors' are referenced to it?

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Steve

"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."

Forrest

I don't know exactly how surveyors use the equipment. I just know about the existence of Differential GPS and that it differs from consumer grade equipment like my Garmin eTrex Legend.

John Couture

To all,

A friend of mine is in the mapping business and so I asked him.

1) Accuracy standard GPS.

• Varies widely depending on the vintage and quality of the GPS instrument.
• All instrument's native accuracy is dependent on the available satellite constellation, atmospheric conditions, etc., know as "ephemera."·
• Recreational grade may be 30 to 100 meters off in various directions. In other words, a particular unit may read 25 meters west of the real location on one day and 12 meters east the next due to the above variables. Repeatability is the main factor with these units.
• Mapping and Survey grade will do somewhat better (maybe 10 to 30 meters) without post-processing or real-time corrections. They really shine when you add in postprocessing or the real-time corrections. Then mapping grade are in the sub-foot range and survey grade are in the sub-centimeter range. FYI-we use a mapping grade GPS with an embedded Windows CE device running GIS software call ArcPAD. The units we buy cost around $8000, complete.

2) Accuracy of DGPS.
D(ifferential)GPS is, if setup and operated correctly, can easily get you to accuracies less than 1cm. Essentially you are running 2 GPS units, one set up over a known location, the other being the rover unit. Software then removes the effect of the ephemera, assuming that the effect is the same for both instruments. This is either done real-time or via postprocessing.

I really have no idea about the cost of DGPS capable units, but it would be more·since, at least for a classic DGPS rig you will require·2 units.
DGPS techniques can also be used using one rover unit and a publicly or privately available reference beacon.

3) Is there a difference between DGPS and the WAAS?

WAAS is a Wide Area correction service. The US is divided into 3 zones; east, central, and west. WAAS enabled units use this to apply a general correction which can improve their accuracy to the sub 10 meter level. Good enough usually for driving, flying (VFR)·or orienteering. Set up by the FAA.
Essentially this accuracy gets you close enough to see things or have a higher accuracy localized service take over.

DGPS uses "local" beacons to apply more area specific corrections. This will drive the accuracy 3 meters at worst to sub cm at best. Most probably will give you sub decimeter accuracy.·DGPS can use publicly available beacons from the Coast Guard, state or county services or private subscription services. A public network of beacons called CORS is probably the best source of such signals. Set up by the NGS (National Geodetic Survey). This is set up for uses that require higher accuracy than beeing able to see the object you're trying to locate.

Here's· at least one link you can mine for info:
http://www.edu-observatory.org/gps/

And of course, there's always the NGS for CORS:
http://www.ngs.noaa.gov/CORS/

And the FAA for WAAS:
http://gps.faa.gov/index.htm

Many thanks to Peter Moreau, Houston TX

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John J. Couture

San Diego Miramar College