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Posted: 10/27/2006 8:40:49 AM EST
I don't know much about GPS, but I have an application that needs to have accuracy plots of 1M or less.

All the handhelds that I can find have accuracy of 5-7 meters, and with WAAS (?) can get down to 3+ meters (still 10 feet).

The other GPS units I have found are for surveying and have very exact accuracy, but they cost $5,000 and up.

Is the unit I'm looking for (<1 meter/3foot accuracy) exist at a reasonable cost?
Link Posted: 10/27/2006 8:47:07 AM EST

Originally Posted By xanadu:
I don't know much about GPS, but I have an application that needs to have accuracy plots of 1M or less.

All the handhelds that I can find have accuracy of 5-7 meters, and with WAAS (?) can get down to 3+ meters (still 10 feet).

The other GPS units I have found are for surveying and have very exact accuracy, but they cost $5,000 and up.

Is the unit I'm looking for (<1 meter/3foot accuracy) exist at a reasonable cost?


Yes and no. For measuring speed of a vehicle, GPS is very accurate. Why? Because it measures relative positional changes. But measuring absolute position takes time.and as such, most units are best within 10 feet at the most. Unless you get into the surveying equipment, as you have seen.

BUT if you want to measure things like survey markers, you can average the measurements over time as long as the unit is NOT MOVED. And most of the units with an external antenna can get within a meter over an hour's worth of measurement.

I will have to defer to my brother who uses a USB based GPS unit to measure coordinates for his geology research. Part of his doctorate...yes, he is a smart fella.
Link Posted: 10/27/2006 8:48:14 AM EST
Link Posted: 10/27/2006 8:51:36 AM EST
I have never had anything approaching that kind of accuracy with any of my handheld GPS units. Plus or minus 40 meters is about as good as it got.

To get that kind of accuracy on the aircraft I worked on, I had to load the crypto code into it. That was the only way to get access to the GPS "P" code.
Link Posted: 10/27/2006 9:00:16 AM EST
ahem...

here you go .. subfoot handheld.. a little expensive...



www.trimble.com/geoxh.shtml
Link Posted: 10/27/2006 9:04:32 AM EST

Originally Posted By xanadu:
I don't know much about GPS, but I have an application that needs to have accuracy plots of 1M or less.

All the handhelds that I can find have accuracy of 5-7 meters, and with WAAS (?) can get down to 3+ meters (still 10 feet).

The other GPS units I have found are for surveying and have very exact accuracy, but they cost $5,000 and up.

Is the unit I'm looking for (<1 meter/3foot accuracy) exist at a reasonable cost?


you might be able to rent or lease.. otherwise see above for about the cheapest (around 6000)...
Link Posted: 10/27/2006 9:06:12 AM EST
My GPS Map 60C gets about 7 FEET on the highway when I am going 80 MPH actual. That is with the built-in quad helix antenna. And I have it fired up now INSIDE and still get under 15 feet all day long.

On top of a mountain, it got 5 feet.
Link Posted: 10/27/2006 9:19:22 AM EST

Originally Posted By GySgtD:
I have never had anything approaching that kind of accuracy with any of my handheld GPS units. Plus or minus 40 meters is about as good as it got.

To get that kind of accuracy on the aircraft I worked on, I had to load the crypto code into it. That was the only way to get access to the GPS "P" code.


That used to be true when we intentionally degraded the "civvie" C/A code, but since Clinton turned that crap off we don't even bother with the P codes anymore. The only advantage of .mil spec GPS these days is anti-spoofing and anti-jamming ability, but accuracy is the same for everybody now.
Link Posted: 10/27/2006 9:20:35 AM EST

Originally Posted By meltdown:
isn't 10-12 ft is as close as the .gov allows ?


It used to be 10 Meters, but we stopped introducing random error into the data stream so now <10 feet is attainable.
Link Posted: 10/27/2006 10:05:06 AM EST

Originally Posted By xanadu:
I don't know much about GPS, but I have an application that needs to have accuracy plots of 1M or less.

All the handhelds that I can find have accuracy of 5-7 meters, and with WAAS (?) can get down to 3+ meters (still 10 feet).

The other GPS units I have found are for surveying and have very exact accuracy, but they cost $5,000 and up.

Is the unit I'm looking for (<1 meter/3foot accuracy) exist at a reasonable cost?


I was part of a research project way back in college and we used Garmin (I think) that were Differential GPS ready- they take a radio signal to correct the error and we had an accuracy of 1 cm. I was trained on it but this was 10 years ago and I don't remember too much, but you can probably google differential GPS and found out more. I believe that my lab (at a university) paid a subscription to a local radio station to get the signal (but then again maybe it was free), anyway, look up GIS- I forget what the heck it stands for, but it is creating very accurate maps.

We did it back when there was selective availability but I think that the DGPS gets around that. Or maybe we got around it b/c we were working for the army. Sorry, the specifics are fuzzy.
Link Posted: 10/27/2006 10:06:56 AM EST

Originally Posted By st0newall:
ahem...

here you go .. subfoot handheld.. a little expensive...

www.trimble.com/graphics/geoxh.jpg

www.trimble.com/geoxh.shtml


cool
Link Posted: 10/27/2006 10:33:38 AM EST
Thanks all for the replies, The Trimble is $$$!

I'll search for infor on the DGPS - anyone else have any ideas?- I'm open to them.
Link Posted: 10/27/2006 10:41:01 AM EST

Originally Posted By xanadu:
Thanks all for the replies, The Trimble is $$$!

I'll search for infor on the DGPS - anyone else have any ideas?- I'm open to them.


My advice would bo to go with a quality consumer grade Garmin, ($300 range) that can interface with an external antenna, then buy a very good quality antenna for it. Other than that you are not going to find .3 meter resolution at anywhere near consumer prices. Best to buddy up with a surveyor to use their equipment if its a one time need.
Link Posted: 10/27/2006 11:11:31 AM EST
[Last Edit: 10/27/2006 11:14:49 AM EST by Keith_J]
I just went outside with my GPS Map 60C. 9 differential sat locks at full power. 5 foot error. With a good external antenna, that might go to 1 foot but not moving.

And best accuracy depends on the weather, the variable position of the constellation of satellites and if you have unobstructed view of the horizon. The sats directly overhead do very little to horizontal accuracy.



Link Posted: 10/29/2006 5:27:17 PM EST
Btt for more ideas
Link Posted: 10/29/2006 5:51:25 PM EST
I've done a fair bit of mapping with my Garmin 60CS and the accuracy is only OK. Definitely not submeter. It's an excellent little unit for what it is, but it's not meant to be a DGPS.

The Trimble is what you want. Even the GeoXT isn't really survey grade, but it is capable of submeter accuracy with postprocessing. But it's spendy. I had a Thales MobileMapper that was in between the Trimble and the Garmin in terms of both accuracy and price. I'm not convinced it's worth what you pay for it, but the data can at least be postprocessed, and in theory it will be submeter (but accuracy is still less than the Trimble GeoXT). I sold the Thales as I needed the money and wasn't that impressed with the unit, but it may be an option worth looking at.
Link Posted: 10/29/2006 6:45:05 PM EST

Before you believe the accuracy your GPS unit reports, you need to calibrate it with a known marker.

It is likely that the GPS units are optomistic about their own accuracy.

I do some GIS/geospatial work, and for the majority of our academic applications we find regular GPS accuracy to be enough. When we need verifiable high resolution accuracy, differential gps equipment is a must.

WAAS helps, but if you need sub-meter accuracy, you're going to have to pay for it. Only GIS, survey, and related fields ever really use it.

Jim
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