Finding Accuracy?

[Deanster]

What would you recommend to check the accuracy of your GPSr without getting into any kind of hi-tech measurement debacle, which I wouldn’t understand anyway? Could I use a benchmark? Aren’t benchmarks supposed to have accurate coords.? I really don’t have to do this; it’s just something I would like to do.

 

Thanks

Deanster

[+geospotter]

Here's an easy way.

 

Take a reading with the GPS. Go to www.lostoutdoors.com and click on Mapmaker. Enter your coordinates and see if they plot to your location.

 

In my case, it put the red dot right on the front part of my house (which is exactly where I was sitting).

[MTU_Cache_Spot]

One thing to watch out for if you do check on benchmarks is the accuraccy of the benchmark itself. Many of the benchmarks in the NGS database are mainly intended for vertical use, and do not have very accurate horizontal (lat/long) coords. Check to see whether it is a 1A, 1B, 3C, etc. (I forget what the actual abbreviations are) to see how accurate it is.

[Deanster]

geospotter, I have used the Mapmaker feature of that website many times and you're right, it's very good.

 

MTU_Cache_Spot, I guess what you are saying is that not all benchmarks listed is a true accurate coord.

 

What I'm looking for is an KNOWN object that... how can I say this... that is 'certified' to be true down to the last digit of the coords at this exact spot. Something you could use to calibrate with.. example- like a measuring device that you can set to exact zero because you KNOW it's a true zero. I hope I'm making myself clear here, but maybe I'm not

[+The Merman]

In 1936 the astronomer royal Sir George Airey recalculated the prime meridian and found that the previous line of East/West was actually out by 19 feet.

All survey maps are based on the original calculations. So therefore it could be fair to say that most maps would be out by that much on the longtitude.

Calculations made from space using sophisticated technology show that at the Greenwich prime transit, the following measurements are in force Airy Transit, GB36 = N 51 28 38.265 E 00 00 00.418 this is the actual lat long of the supposed true transit that we assume to be zero degrees.Airy Transit.

ETRF89 =N 51 28 40.1247 W 00 00 05.3101 This is the transit from the 1989 space mapping.

It would appear from this that to take a reading on your GPS at the Greenwich Observatory in London

your readout would not be the same as either figure, as your GPS is configured to be used with maps.

 

Confused.....I am!

 

try this site.History of the Prime Meridian

 

I woke this morning and my boat was not rocking...for one horrid moment I thought I lived in a house!

[k2dave]

One thing you can do to check the consistiance is to set a waypoint at a known location that you can get back to(like the crack in the sidewalk infront of your house) then check several times (maybe different days) to see how far off it is.

 

You could also compare it to other gps's (don't use them at the same time as they will interfere) - you could go to a geocache and see how far off you are from the cache.

 

----(sig line)---> Did you ever do any trail maintainence? - if so you will know that all but the most worn trails need continuous maintenance to prevent mother nature from reclaiming it. herd paths are quickly reclaimed - k2dave

[+V7L]

Having just read the History of the Prime Meridian, I decided I needed to test my GPSr... aimed it at my local bar... ended up facing a bottle of Lagavulin. Yep, my GPSr is right on.

 

V7L

[+Alan2]

Check this site. It'll tell you evrything you wanted and didn't want to know about benchmarks. The horizontal one are extremely accurate down to a few millimeters far more accurate than the GPS.

 

The site wil also show you how to find nearest ones. You might wish to check out the separate forum section on benchmarks. Lots of interesting info.

 

Alan

[Deanster]

Thanks for all the answers so far. My GPS'rs, if they have the birds locked, work just like the manufacturer said they would. This is all fine and good. BUT....

 

Say, if Larry said his unit X is better than Curly's unit Z, how is Larry going to test them to prove he is right? Now cache hunting isn't fair because Curly is just plain better at the hunting with any unit. So, a third person.. Moe, gives both a set of coords and tells them to place a dime on the ground where their units say it should be. The dime that is placed nearest the coords is the winner.

 

Now here's the question... what means would Moe use to 'officially' say Larry or Curly is the winner?

[+Alan2]

If Moe is using his GPS to determine the coordiantes, the test won't work cause Moe's coordinates are incorrect to begin with. You have to start with a proven location like a benchmark to measure accuracy.

 

Alan

[Deanster]

I agree with you 100% Alan2 with your last post.

 

On the same lines at what I'm getting at, are GPSrs calibrated somehow at the factory. If so, could some be 'better' calibrated that others of the same make and model. I know I sound like I know nothing about GPS and that's is wrong... I know less than nothing.

[Kerry.]

NO, GPSr's aren't calibrated anywhere, no such thing. It's all a function of the system and what the reciever does with the information it does get.

 

In Moe's case there is some "science" that could be used that "might" help decide who "might" be more right (or is that more wrong) than the other.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Eeyore and Shadow]

The error in a GPSr is not from a calibration as there is no actual calibration to a GPSr. The error comes from bad reception of signal due to exceptionally low angle. Or range variable in signal due to exceptionally high angle of satelites. Plus the extent the decimal is taken in doing the math in processign the speed at which processing of the math occurs. Spped of math in a computer doesn't seem like it would be much of an issue untill you think about it having to constantly do math to contend with movement and the doppler effect of the signal on the device. Anyone can plot thier position with a little knowledge of trigonometry and the distances to at least three known points. The acuracy comes into effect when you consider how long it akes you to calculate your position then factor in the time it takes to walk six feet. Thend do the math all over again. There are other factors to GPSr accuracy but in my opinion most of them would be negligable enough not to worry too much about unless I missed some variable. If such is the case I'm sure someone with a bigger brain than mine will point it out.

 

Eeyore

 

My other cachemobile is a broom!

[+Alan2]

All units basically have the same accuracy as was previously stated as long as he signals received are the same. It's kind of like a hand calculator. As long as you input the same numbers, the $3.00 calculator will add the same results as the $100. calculator. The processing is the same.

 

Check this Benchmark Cache for a report on the similar results between different GPS's.

 

Alan

[Deanster]

Thank you all again for your responses. So calibration is OUT! I got that.

 

Alan2, thank you for the 'Benchmark Cache' report page. That mainly answered my question. There is a place to test the accuracy of your GPSr unit is you so desire.

 

Moe could tell Larry & Curly to put their dime on the 'Benchmark Cache'. Moe wouldn't need any instrument but a tape measure to tell the winner.