How to see if your GPS is acurrate

[gm100guy]

After reading all the post here about how accurate is my gps I have a simple test for you.

 

1 Get a top map of you location

2 Learn how to use the map and reference youself to 3 locations on map.

3 Mark on the map where you are.

4 Get out your gps and take a waypoint of where you are.

5. Get a utm roamer for the scale of the map and read the utm location from your gps.

6 Plot the position on the map.

7 If the positions are close and you can shake hands then the gps is acurrate enough for most of us to use.

 

This was the test I used after first getting my gps after many years of just using a compass. map and roamer in the military before gps's came out for everyday use.

 

 

gm100guy

http://members.rogers.com/gm100guy/cachepage.htm

Ontario geocachers http://groups.msn.com/GeocachinginOntario/homepage

[gm100guy]

It takes a women to come up with a easier test.

 

My wife told me to go outside to the corner of the street and take a waypoint.

 

Come back inside and input the waypoint into a online map and see if it is at the corner.

 

If it is then the gps is working just like it should.

 

 

gm100guy

http://members.rogers.com/gm100guy/cachepage.htm

Ontario geocachers http://groups.msn.com/GeocachinginOntario/homepage

[+blindleader]

And if you actually want to know how accurate it is, search on geocaching.com for a nearby benchmark that is used for geodetic surveying (location published to sub cm precision), go there and compare your fix to the published coordinates. Of course any such accuracy measurement will be valid only for that location and moment in time. The BM I like to use for this is this one. It's an easy drive-up with very good view of the sky.

[+leatherman]

I don't agree with either of these methods.

 

First no maps are accurate online or off. Moun10bike was experiencing that in the Northwest forums. He did a cache around a lake that did not appear to line up with the track logs on their GPSr's. He used many different map sources online and off. I'm not sure if he figured it out.

 

I think the closest your going to get, is to find a good benchmark. In an open/clear area and compare your coords.

 

POWDER!!!!!!

[gm100guy]

I have a cache here in Toronto Test your GPS

 

This Thread is for those need a simple test and to promote that one should have mapping skills to go along with using there gps.

 

gm100guy

http://members.rogers.com/gm100guy/cachepage.htm

Ontario geocachers http://groups.msn.com/GeocachinginOntario/homepage

[+Searching_ut]

I agree with leatherman when it comes to maps, and possible, or even probable errors with the map. I've found even the aerial photo maps, which seem to be quite accurate most of the time can be off a bit on steep slopes. Going to an accurate benchmark is by far the best way of seeing what sort of accuracy you're getting. That said, I still tend to navigate more by map than GPS when hiking and backpacking.

 

I have a few examples of map errors on my website for those interested. You can get there from my profile.

[+geospotter]

Geez, this is too easy.

 

1) Take reading (you don't even have to leave your chair).

 

2) Go to www.lostoutdoors.com and enter the coords from step 1.

 

3) View image. Is that where you are?

[+CYBret]

quote:

---

Originally posted by blindleader:

And if you actually want to know how accurate it is, search on geocaching.com for a nearby benchmark that is used for geodetic surveying (location published to sub cm precision), go there and compare your fix to the published coordinates.

---

 

I actually saw my GPS go to zero on this benchmark...just for a second or two.

 

This was as good as I could get with this benchmark.

 

And I think I've seen everything inbetween those two readings. I lean towards the www.lostoutdoors.com method, but you're still relying on benchmark accuracy with that too, as I understand it.

 

Bret

 

"The kingdom of heaven is like treasure hidden in a field.

When a man found it, he hid it again." Mt. 13:44

[+ZachMJ2345]

The best way I know of is just to get your hands on the most accurate GPS you can find and test yours against it. Not to long ago someone was out in our area letting people test out a GPS that's MUCH more accurate than anything we use for geocaching.

 

Anyways, your GPS is not ever going to be perfectly accurate, nor will it have the same accuracy day after day. Just trust it to within about 40 feet and then take what it says with a grain of salt.

 

-Zach

-Team Zen-

[+Brian - Team A.I.]

For my money, I can't complain about being within 6' of EXACT on my Vista. However, the SUVs to either side of me might be a bit leery if I'm 6' off of where I really should be.

 

Brian

Team A.I.

[Kerry.]

Definition of GPS accuracy.

 

Positioning accuracy represents how well the position solution conforms to "truth". Truth is defined to be any specified user location where the position is known, within acceptable error tolerances and with respect to an accepted coordinate system, such as the World Geodetic System 1984 (WGS84) Earth-Centered, Earth-Fixed (ECEF) Coordinate System. Factors which affect positioning accuracy include geometry and URE variations unique to a given user location and the sample interval over which measurements are taken.

 

So basically neither maps or waypoints are appropriate for determining the accuracy of a particular GPS over a specific period of time.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+Og's outfit]

This is a great thread. I will have to agree that finding an accurate benchmark has to be the best method for a non-military-type to test accuracy. I wish I could mount my unit to an F-15's windshield but that's out of the question.

 

I found a famous Markwell Photographer's Cache this summer where I'm pretty sure the coords were found on the web. I tried like the devil to zero out my GPSr, but "ground zero" was behind the ranger shack that was the objective.

The closest I got was 1 foot. (not bad!) I handed the unit to my daughter for the pic and it mooved to 3 ft. My kids thought I was nuts! Making them stand behind the shack while I did Tai-Chi with a GPSr. LOL

Here's the pic.

 

OG

 

Prophetically Challenged (or is that Pathetically?)

[+w5waf]

Repeatability may be more of an issue. Go to a spot in the clear and take a mark. Then go out at intervals...days...and see how much the error is from the first reading. In the early days of GPS (1990-91) it wan not uncommon to get an error saying that the geometry of the satellites prevented an accurate reading. I'm sure the geometry even today plays a part in the accuracy of the readout.

 

Benchmarks may not be a good choice, because the lat/long on many is taken from a quad map and not from an accurate survey. Many of the benchmarks are precise for elevation, but less precise as location.

[Kerry.]

Bill, Repeatable accuracy (and Predictable accuracy) are no longer mentioned in any of the GPS signal specs.

 

Certainly was relevent in the old days but appears no longer an issue.

 

Back in 1990-91 (first gulf war) there were only 16/17 satellites in total and these weren't enough to provide 24 hours coverage even in the gulf. And yes the geometry still plays a part (to a point) in the accuracy, which is why accuracy is stated as a statistic and relative to a specific period of time.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go