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Discussion Starter #1
I've been playing with my new WAAS enabled GPS making maps for our propertys and for an over all understanding of what a property is all about.

I have a couple problems, the first being that the only GPS mapping program available that I know of is a clunky freware thing called map tracker.

The other is the conversion of geodetic cooridinates to GPS.
 

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You're probably interested in cadastral survey using a GPS. Cadastral surveying is property surveying. Its usually plane surveying unless the traverse length is large compared to the Earth's curvature. Geodetic survey is accounting for the curvature of the Earth. The Earth curves away as a second-order function so the longer the traverse, the greater the rate of accumulation of error.

Almost nobody needs geodetic surveying. Consult a civil engineer if you really need that.

Set you GPS for UTM (universal transverse mercator projection). You're in either Zone 16 or Zone 17, I think. This is pretty important if your computer software can't figure it out from an uploaded GPS file.

If you're interested in plane surveying using a hand-held, you may be better-off using the GPS for points and just plotting that on graph paper. The returned values can be in either meters or feet.

Your GPS will return coordinate values with precision of probably a foot or less. Don't confuse that with accuracy. For example, you could sight-in your rifle to have a very tight grouping (high precision) but not hit the target (low accuracy). That is what your GPS gives, high precision, low accuracy. I doubt you'll get an error ellipse smaller than 10 meters, more likely 15 meters. This is essentially saying that about 67% of readings, over the course of several hours from your GPS, will fit within a circle about 10 meters radius. You can fool yourself into thinking its more accurate by quickly taking a bunch of readings over the course of a moinute or so. This will return the precision of the given constellation of satellites positioned nearby and given atmospheric conditions. It may appear that you're getting 2-3 meters or better accuracy. Don't believe it. It s a lie.

I have never used any low-end GIS programs, so I can't help you there. But if what you're doing is plane surveying with large error, then get an old copy of AutoCad. I like Autocad, or at least old Autocad (I haven't used it in many years). The older the better because the old DOS-based or Winders 3.1-based program was easy to use and reliable for modest industrial or civil engineering drawings and databases. I've seen old versions selling for next to nothing when folks decided they had to upgrade real bad. Or if you have more money than God, go ahead and buy the latest Autocad. In eitehr case, you'll need the manuals and tutorial.
 

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Discussion Starter #3
Thanks for the reply I've been playing with GPS since the first units were available and have checked accuarcy at known geodetic stations, I picked the unit I have because it enables you to average a waypoint for as long as you want to sit there or the battery dies I have found that with long averages 910 minutes) I get pretty close to x marks the spot. I am also aware that they can be way off but using witness markers and a laser range finder and a good compass on known points I can get what I'm looking for and that is a rough survey.

By no means am I looking to do an acutal survey. Thanks for the detailed post there are several points that help me out in my quest.

I might give you a call Chris for help decifering the civil engineer cooridates. I'm shopping ebay for a used sub meter survey GPS because I really enjoy this hobby.
 

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Look for an Ashtech or Trimble unit. I've not used Ashtech, but they're suppsoed to be top-o-the line along with Trimble. I heard that the guts of most GPS are actually built by Racal, but I don't know for sure. Racal builds the DoD model (PLGR, you may have used it back when...) that reads an encrypted signal unavailable to public. It has better reception under poor conditions. It's accuracy is fine if you're needing a 5m kill radius. Or to guide a Tomahawk into a particular building.

The high-end is about 2 cm accuracy but you have to log overnight if SA is in. If not, then the observation time per station is meassured in minuts, not hours. Also, Trimble sells very reliable, easy to use proprietary software that I highly recommend.

If you need submeter but don't need cm accuracy, look at the dual freq units that use either Coast Guard signals (free) or satellite signals (subscription) to correct the signal to submeter. These are very fast and very reliable systems. I use them often and I really like them. The Trimble unit is the Pro-XR. Ours is 6-7 years old and still very reliable. The newer versions are a lot lighter and the lgging units are superior. An even more interesting path (which we will follow this coming year) is to use a PDA as the logger. A good PDA can display a rectified image or a DOQQ as a backdrop and then plot your position on top of it. The Pro-XR can also take the laser rangefinder signal directly, which is a cool tool that I haven't used yet.

GPS is a lot like computers. You have your Magellan people and your Garmin people, and your Trimbles, etc. My favorite gen purpose unit is the Garmin. Light, small, rugget, fast, accurate, lots of memory, color display and very reasonably priced. Upper end I'm a Trimble man. You get lots of control over the signal that allow you to set various S/N parameters.
 

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You should be aware that the NGS survey markers are on the North American Datum of 1983 and the data from GPS is on the World Geodetic System of 1984. There are differences. I assume you know the difference between Mean Sea Level elevations and Ellipsoid Heights.

The best accuracy you will get out of a civilian handheld GPS unit using the broadcast ephemerides is 6m, 1 sigma. Anything better is coincidence.

Are you doing geocaching?

Perhaps you may be interested in this site. http://earth-info.nga.mil/GandG/
 

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Discussion Starter #8
I subscribe to Globeexplorer which is a little better than terra IMO.

I'm only doing it for a hobby I've found that it has a somewhat challenging learning curve and I can do it while I'm out hunting which is my #1 time burner.

and last but not least it's cheap.
 
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