I verified my GPS today

[solohiker]

I unofficially verified the accuracy of my GPS today.

 

I have an old Garmin 12 that I have owned for years. I received some comments about coordinates being off on a recent cache that I had placed. I tried bench marking for the first time and found I was way off on the posted coordinants. I was loosing faith in my old Garmin 12.

 

I did some more research on bench marking and found that there is a significant margin of error for some bench marks. One helpful person suggested I do a search on 'GPS' verified bench marks. I found a local bench mark that was GPS verified with a commercial GPS to an accuracy less than one centimeter.

 

I went out at lunch today and plotted the bench mark. My old Garmin was 4 meters off

 

Has anyone else found a way to verify the accruacy of their GPS?

[+Dixie's Crew]

Pretty impressive, even for military standards!

 

quote:

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I found a local bench mark that was GPS verified with a commercial GPS to an accuracy less than one centimeter.

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[Kerry.]

Ah but what about tomorrow .

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[+Logscaler and Red]

One of the local cachers here set this cache up and I think he is charting the difference between units as well as time of day.

 

Is there something like it in your area so everyone local can compare GPSr's ?

 

logscaler.

[+Criminal]

Uh.....what kind of benchmark was it? Most of the coordinates were determined by some guy looking at a topo map and figuring out the lat/long.

 

http://fp1.centurytel.net/Criminal_Page/

[+Team Shibby]

I have always wanted to put my gps to the test against a benchmark, but the one time I tried it was no longer there due to a new highway extension

 

I would be interested in trying it again against a benchmark that has been recently recovered and updated with the latest surveying equipment. My question is, how do you know it was GPS certified? Is it listed somewhere on the benchmark page? If not, is there a site that lists up to date coords of new or old bm's?

 

Thanks!! Kar

 

TEAM SHIBBY!!!!

 

Krs, Kar & Na

[+Marky]

Accuracy is a fun virtual cache which does just that, check the accuracy of your GPS. Mine was 1 foot off at the measurement location.

 

--Marky

"All of us get lost in the darkness, dreamers learn to steer with a backlit GPSr"

[Kerry.]

quote:

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Originally posted by Marky:

.... Mine was 1 foot off at the measurement location.

---

 

1 foot interesting and 1 foot based on coordinates to 3 decimal minutes, was it

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[BassoonPilot]

Every area should have a cache like Accuracy. Locally, we have Alan2's Time Capsule - Open Cache in 5000 Years.

[solohiker]

quote:

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Originally posted by Criminal:

Uh.....what kind of benchmark was it? Most of the coordinates were determined by some guy looking at a topo map and figuring out the lat/long.

---

 

You are correct, most are. Some are recorded by military grade GPS's. The benchmark was FG1961. YOu can look up the data sheet at the link below. The coordinates are posted to centimeter accuracy.

 

The link also allows a search of benchmarks adjusted by GPS.

 

http://www.ngs.noaa.gov/datasheet.html

 

[This message was edited by solohiker on March 12, 2003 at 04:54 AM.]

[+Team Shibby]

Its amazing how many have been GPS verified! (at least for the county I looked under) Time to make a list and check a few out! Thanks again!!

 

Kar

 

TEAM SHIBBY!!!!

 

Krs, Kar & Na

[solohiker]

My county only had two

 

Both were at the local Airport between taxiways.

[+Team Shibby]

Sorry to hear that. I live in NJ and I looked in Bergen and Passaic Counties and it probably listed 100 or more combined. Most are not accessible, I imagine. For example, some are located directly on the "Garden State Parkway" and there is NO pedestrians allowed on the GSP. Others mention taking readings from gravel rooftops of a rubber factory!! I won't be going to that one either!! I'll find one for sure!

 

Thanks again...Kar

 

TEAM SHIBBY!!!!

 

Krs, Kar & Na

[Desertphile]

quote:

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You are correct, most are. Some are recorded by military grade GPS's. The benchmark was FG1961. YOu can look up the data sheet at the link below. The coordinates are posted to centimeter accuracy.

 

The link also allows a search of benchmarks adjusted by GPS.

 

http://www.ngs.noaa.gov/datasheet.html

 

Hummm. I thought that Earth flexed, bulged, and moved around quite a bit. Any cache placed on Earth would move several feet in relation to other spots on the ground (let alone artificial satellites). I cannot imagine how the CM accuracy could be measured: a fixed location is required for reference, and there are no fixed references. I suspect the 1CM is theoretical and not actual.

 

But then, I dunno. The section of North America that I'm currently sitting on is moving NNW a meter or so a year.

[+MaxEntropy]

quote:

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Originally posted by BassoonPilot:

Every area should have a cache like http://www.geocaching.com/seek/cache_details.aspx?ID=17500. Locally, we have Alan2's http://www.geocaching.com/seek/cache_details.aspx?ID=10814.

 

Wow, was I the last person to log that one? Geez, I was out there in Nov. on vacation.

It's a cool spot, near the very impressive Unisphere and the flying saucers from MIB.

Hello, New Yorkers! Time to go outside!

 

Mickey

Max Entropy

More than just a name, a lifestyle.

[+Marky]

quote:

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Originally posted by Kerry:

quote:

---

Originally posted by Marky:

.... Mine was 1 foot off at the measurement location.

---

 

1 foot interesting and 1 foot based on coordinates to 3 decimal minutes, was it

 

Cheers, Kerry.

---

My GPSr read 1 foot, so it was 1 foot off. How can that be confusing? Are you saying that your GPSr never reads less that 10 feet?

 

--Marky

"All of us get lost in the darkness, dreamers learn to steer with a backlit GPSr"

[+smillersmiller]

I think Kerry is implying that 1 foot is less than 0.001 minutes.

 

How much is one thousandth of a minute? It's been discussed in several different threads such as this one. The short answer is: 0.001 minutes North/South (lattitude) is about 6 feet. 0.001 minutes East/West depends on your lattitude, but a halfway respectable rough number for the continental United States is something like 3 to 4 feet.

 

So in other words, if you're off by 0.001 minutes in either the lattitude or the longitude, you're going to be off by more than one foot (unless you're located near the North or South Pole).

 

Of course, there's no reason the GPS receiver would have to round to the nearest thousandth of a minute before performing the distance calculation. And given that many GPSr's (such as my Mag 315) have the option to display more precise coordinates such as Degree decimal to hundread thousandths of a degree, it's certainly possible the GPS receivers internally have the coordinates determined to a more precise level than one thousandth of a minute.

 

(OK, I think that may have been too much rambling. I hope that made sense to someone.)

 

ICQ: 5563417

 

[This message was edited by smillersmiller on March 13, 2003 at 12:00 AM.]

[Kerry.]

1 foot I'll stay away from some of the reasons I don't wear 1 foot as practical what so ever, regardless of what manufacturers think they can display on a screen.

 

Now apart from what errors are involved from the other end (satellites) before any signals actually reach a receiver it rather difficult to expect an accuracy of 1 foot when being compared to coordinates given to 0.001'.

 

But we're not talking about 10 feet (but I might/could even question that as well) but with the receivers being used (or any other for that matter) here I certainly don't get confused about what accuracy is all about.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go