Gps Which Unit Is Most Accurate

[+Turtletoes/Biggie]

I am wondering whcih GPS unit the most accurate. I have a Garmin Rino 120.

Somtimes it seems to be off up to 50 feet in coordinates. I have an old Garmin 12

handheld (I think). Seems to be more accurate at times. Any feedback.

[+NimravusHSSR]

What are you comparing it against? If it's another cache then who knows which GPSr is more accurate.

 

Have you compared it against a benchmark?

[Neo_Geo]

Gotta compare them together at the same time in the same location. Sometimes the constellations line up for better accuracy than other times. Best scenario is getting them out in the wide-open sky (no trees or buildings).

[+Marky]

Trimble! Edited April 6, 2004 by Marky

[+GeckoGeek]

I am wondering whcih GPS unit the most accurate. I have a Garmin Rino 120.

Somtimes it seems to be off up to 50 feet in coordinates. I have an old Garmin 12

handheld (I think). Seems to be more accurate at times. Any feedback.

Are the set for the same Map Datum? Have they both been on long enough to get a full satellite lock and download the full almanac? Try checking the EPE (Estimated Position Error) on the two units and see if they are in the same range.

[+SBPhishy]

Overall though, there is no super GPS, rather than the Military stuff, if they even have better stuff. You can argue antennas, patch or quad helix, etc. etc. You can buy an external antenna, but overall, you get what you get. For general use, GPS's are even overkill for accuracy, but caching though, we look for something so small, that the "inaccuracies" seem so much greater.

[4x4van]

Another thing to consider is the fact that you are basing your idea of a unit's accuracy on it's own accuracy statement, which also has to be taken with a grain of salt. That's like asking a Ford which is better, Chevy or Ford? Obviously, the Ford is going to tell you that the Ford is better, and the Chevy is going to tell you the Chevy is better.

 

In other words, the accuracy quoted by any unit is based on alot of different things (satelite strength, # of satelites visible, their orientation in the sky, view of the sky, atmospheric conditions, etc.) but also, and perhaps even more important, the given unit's software that is doing the calculations. Different software and/or brands may be more "optimistic" than others in reporting it's own accuracy. To accurately find out the error in position of any unit, you'd need to use something independent of that unit, like a benchmark, and even then, the accuracy will vary minute by minute with the earlier listed conditions.

 

I use a SporTrak Pro with WAAS, my son uses an ETrex yellow (no WAAS). Curiously, the yellow almost always "reports" better accuracy than my ST Pro, yet my Pro usually seems to get me to the cache a little more accurately. Because of that, I believe that the Garmin firmware is simply a bit more "optimistic" than the firmware in my Magellan. So simply comparing two different units side-by-side tells you nothing unless they are identical models running the same firmware. Otherwise, how do you know that the accuracy numbers are arrived at using the exact same criteria?

Edited April 6, 2004 by 4x4van

[+Marky]

Overall though, there is no super GPS, rather than the Military stuff, if they even have better stuff.  You can argue antennas, patch or quad helix, etc. etc.  You can buy an external antenna, but overall, you get what you get. For general use, GPS's are even overkill for accuracy, but caching though, we look for something so small, that the "inaccuracies" seem so much greater.

Did you follow my Trimble link? They have hand held units that get submeter accuracy (submeter accuracy means that you are within 3 feet of the actual coords - someone correct me if I'm wrong here). These are generally used by surveyors and cost way too much money for us mortals. But they are in fact much more accurate. One I was checking out has a built in/integrated Pocket PC! Man, that would be one sweet combo.

 

--Marky

[+JeepCachr]

Theres one of those on ebay right now... I wonder if my wife will notice it on the charge card?

[+bons]

I would love to see the technical details on how they can attain submeter accuracy. The only thing I can think of is that you sync the dang thing at the closest reference site (making a fixed offset) and then immediately drive to your project making sure the satelites being used don't change between here and there.

 

And I'm not sure how accurate that really would be...

[+Sparky-Watts]

Well, according to the guy on the DIY network show the other night, cheaper units are less accurate than expensive units....HA!!!!!!

 

To be honest, my GPSr is the most accurate one!

[+EScout]

The best way I know of is to take all of the units you want to test and turn them on and stand over one of the super-accurate "gps adjusted" benchmarks. I have done this with my Legend and Magellan and they are real close, one beating the other on a given day. By accurate, I mean the number of feet the GPSr says it is away from the known coordinate, when over that coordinate. The EPE or "accuracy" figure given on the GPSr screen is not really relevant in comparing one unit to another. The "true" accuracy is usually better than the EPE or screen "accuracy", especially if you stay in place for several minutes.

[Kerry.]

First some need to get past the misconception that military units are more accurate than civil units (as such). There days without Selective Availability there is very little difference between standard military and civil receivers.

 

Sub-metre accuracy of any receiver, be it a Trimble or any other has to be put into proper perspective. For sure with single freq receivers this can be achieved using augmentation or post processing of some sort. With dual freq using specific methods and techniques sub centimetre is quite easily achieved.

 

Accuracy is dependent on receiver type, software, antenna's and a lot of other things including technique and what they can with the information they receive and the information they receive is basically the same information that everybody receives (to a point relative to the receiver). When referring to accuracy there needs to be some definition/clarification of just what type of accuracy they are referring.

 

Basically you can have what you pay for and then generally need some understanding in knowing how to use it.

 

Cheers, Kerry.

Edited April 7, 2004 by Kerry.

[+Rosco Bookbinder]

I am wondering whcih GPS unit the most accurate.  I have a Garmin Rino 120.

Somtimes it seems to be off up to 50 feet in coordinates.  I have an old Garmin 12

handheld (I think).  Seems to be more accurate at times.  Any feedback.

Are the set for the same Map Datum? Have they both been on long enough to get a full satellite lock and download the full almanac? Try checking the EPE (Estimated Position Error) on the two units and see if they are in the same range.

Also make sure one of them isn't set for WAAS and the other is. That can make a difference if you are gathering WAAS data......

[Team GeoHoy]

I am wondering whcih GPS unit the most accurate. I have a Garmin Rino 120.

Somtimes it seems to be off up to 50 feet in coordinates. I have an old Garmin 12

handheld (I think). Seems to be more accurate at times. Any feedback.

I have the Rino 120 and have been real close. You won't get an actual on the spot reading. I have had an accuracy of 8 ft consistently with it for awhile too.

[lego_guy]

http://products.thalesnavigation.com/en/pr...t.asp?PRODID=40

 

down to millimeter accuracy. though it must be within a dozen killometers of 'the station' . . . whatever that means.

[+QuickKarl]

The most accurate gps is the one that each of us ownes.

Bronz

[+fizzymagic]

I would love to see the technical details on how they can attain submeter accuracy.

Well, to start with, they use two frequencies, which allows real-time determination of ionospheric delay, the major cause of position inaccuracy. The units also have very sensitive antennae and average data over a fairly long period of time to get that accuracy.

[+Allen_L]

Here is a pretty good article http://www.eomonline.com/Common/Archives/1.../98feb_gps.html which talks about the dual frequencies and how they process carrier phase data for even more accuracy.

[+bons]

And as I read that it strikes me that in order to get the accuracy in question you still have to set the differential immediately prior to performing the task. I also suspect that setting the differential properly involves insuring that the same sattelites used in setting the differential are the ones being used at the final location.

 

Or am I completely off mark?

[Kerry.]

Sub-metre accuracy isn't limited to dual freq receivers as many single freq receivers can achieve sub-metre even cm accuracy just that it takes a bit more time.

 

Once accuracy (with integrity) towards the sub-metre, cm mark is required then irt will require some form of differential/static/RTK/post-processing of some sort and the limiting factors basically come down to distance and/or time. For real-time systems (and generally these will be dual freq) the the limiting factor is basically distance with modern technology/software now being able to resolve ambiguities out to about 30km.

 

Static/post-processing can resolve sub cm for hundreds of kilometres providing there has been enough data recorded and this comes down to time. Longer the distance the more the time required and sometimes this can be many days, even with dual freq.

 

In the above link http://products.thalesnavigation.com/en/pr...t.asp?PRODID=40 this is a single freq receiver spec'd to sub cm (< 0.01m) accuracy but is limited to 15km and to achieve this accuracy requires a minimum recording time of 10 minutes+1m per km so for 15km <1cm accuracy would require "at least" 25 minutes of recording time. A dual freq receiver would know over less than 1cm over this distance in much less time than that but would still require a minimum amount of recorded data to achieve this.

 

Single freq receiver/s can achieve sub cm accuracy but takes time and is obviously a cheaper option but if the job required positions from the middle of a busy road then having to setup for 25 minutes isn't practical at all.

 

Bons, with differential the principle is that both receivers are recording/using the same satellites at precisely the same time (both receivers running and recording at the same time), hence any inaccuracies (in simple terms) in the atmospherics etc is assumed to cancel out so one has a relative (correct) vector and by knowing the actual position of the abse then then position of the required point can be determined absed on relatively. The further the distance between the 2 receivers then the more possibility there is of differing atmospherics (plus a few other issues) so the longer the distance the more time/data is required with the longer distances the less suitable are single freq receivers.

 

Cheers, Kerry.

Edited April 10, 2004 by Kerry.

[Prof. Y. Lupardi]

Indeed, I have used my old Garmin GPS12 for getting positons within a margin of 40 cm. But I had to put the GPSr for more than an hour on the same point and needed a laptop in the field to gather all the raw information coming from the GPS12.

And returning home with the data I had to postprocess it with data from a nearby reference station. And then the real tricky thing: are these results usable or not? Do I have to reject the measurement, and if so, on what grounds?

For more you can read my webpage Using the GPS12 for precise (sub-meter) accuracy