Coord Accuracy

[+hogrod]

when ever i notice coords are off by 40-50ft on both my GPS's i take my own new coords. i have two meridian golds. i did a cache the other day and i was showing off by that much, so i took new coords(should also say this was in a big open field and i had WAAS). I only upload the new coords when i compare my coords and the original

waypoint.

(an example would be i get to waypoint then i find cache is 50ft to the north. i make a new waypoint at the cache and compare the two on my GPS map screen. if my coords are showing to the north of the original waypoint, i share this info with others by posting them on the cache log.)

 

a couple days after i logged my new coords someone posted how great the original coords were. no i don't think my GPS is more right that other people, but i do feel that by comparing the coords the way i do my coords must be more accurate(besides the fact that both my GPS tell me the same thing)

 

so has anyone ever made waypoints with a magellan and found them with a garmin? or with a garmin and found with a magellan? if so in either case was there any differance in where the two put you?

 

another example of this would be someone said the coords were off by 50ft on my cache. yet the same day a friend of mine (also a magellan user) walked right to my cache and showed only a few feet away.

[+Miragee]

I have a Garmin Vista and it does seem that the coords are off a bit more on some of the caches placed by people who have Magellans, however that could just be a function of the time of day, the position of the satellites, and atmospheric conditions.

 

Since some Magellan units have that "boomerang" effect that takes a while to settle down, there might be a difference if the unit doesn't have a long enough time to settle down.

[FiddlinFool]

I have both a Meridian and a 60C and they tend to be very close to each other if I wait around long enough for the Meridian to settle down. They both exhibit the "boomerang effect", however the Garmin settles MUCH faster (10 - 15 seconds) than the Magellan, which can take anywhere up to 5 minutes to decide where it really is.

 

I have used the Garmin to revisit caches I place using the Magellan and found them to be within acceptable limits of variation (0 - 30 ft).

[+hogrod]

i know all about the boomerang effect, and since i know my unit does this, i tend to slow down while geting close to the cache. what i am seeing is slightly differant than that. i have found a few caches where the posted coords were about 50ft off. i make a new waypoint at the cache and go to my map screen to compare the original coords with my waypoint. low and behold my waypoint i made is exactly the right distance/direction away from original coords as i am showing the 50ft diff.

I have WAAS, and both my GPS are showing the same thing. yet a few days later someone posted that the original coords were great. seems that if i was having poor signal i would see this on my GPS, also i am standing where the coords take me for quite some time.(this was in open area and could see sky/horizon in all directions)

so i guess my problem is since both gps show coords off, and i have great signal, they must be off.

i gues i should ask what type of GPS they are using.

Edited June 25, 2005 by hogrod

[+Hard Oiler]

I've run a couple of "closest to the pin" competitions where I establish a "virtual" point and have cachers put markers in where their GPSr says the point is. Closest wins. I take a lot of care to get the coordinates right by averaging and the markers generally form a circle round the point - most within 10-15 feet. Not done a true statistical test but there has never been any suggestion that one brand is any different from another - last time I did it I placed it with a Magellan and a Garmin won.

 

So I find it hard to believe that there are really any differences between GPSr brands - whether there is variation within brands is another story - I did run into one cacher where we almost consistently differed by some 30 feet. Don't know enough about the GPSr electronics to know if it's possible to have an off-spec unit.

 

If you want to check, find a benchmark with measured coordinates and test your GPSr. I have a good one near me with coordinates established with cm accuracy - and have hidden a cache there just so people can check their GPSr accuracy.

[+Timpat]

...so has anyone ever made waypoints with a magellan and found them with a garmin? or with a garmin and found with a magellan? if so in either case was there any differance in where the two put you?

As stated by Hard Oiler, our Geocaching group held a "closest to the pin" contest as well at an annual event.

 

I noticed a marked difference in how a Garmin and Magellan behaved. Here is my question/comment to my GPS guru Joe Mahaffey of GPSInformation.net regarding differences in a GPS location determined by a Garmin unit and a Magellan unit:

 

"Recently our local Geocaching group held a picnic event where we established GPS contests, among other things. One was a contest to test who could place a pin flag the closest to two established control points hidden below grade. I used a Trimble Pathfinder ProXR sub-meter receiver to set two points for this very informal test. I occupied each for 30 minute sessions and post-proccessed the data to make them as accurate as possible. I feel I established the two points to within 1' to 1.3' horizontally.

 

We gave Garmin (of all model receivers) users one color flag, and Magellan (of all model receivers) another color flag. Each user had a chance to place a flag where their unit indicated ground zero.

 

The informal test results showed the Garmins clustered in a general area around each point, with the flags trending off to the north of said control points about 10'-15', and the Magellans generally off in their own cluster, in this case to the south of said control points about 10'-15', for a general total position spread of 20' to 30'. Some varied closer to 40' to 50'. Since we had about 60 participants the flag patterns were quite distinct. All flags were set within the same time period under identical conditions.

 

Then I realized that when I seek geocaches using my Garmins (I have a Legend and a Map60C) that were set by Garmin users, I generally fall dead-on the Garmin determined coords, and when I seek a Magellan user's Magellan determined geocache, I'm not dead-on. I often seem to see about that difference of least 20' off, more often 30' to 40' which isn't much, but interesting."

 

Now, here is Joe's reply to my letter above:

 

"Hi Pat,

As you can see from some of our experimental accuracy plots on the website, there is a "systematic difference" in the accuracy plots from Garmin and Magellan units. Both are "in specification" but there does appear to be some systematic error that is brand related.

 

We do not have any information that gives us a clue as to why these differences exist.

 

Joe Mehaffey"

 

I found this very interesting so thought to share it here.

Edited June 26, 2005 by Timpat

[+Miragee]

Very interesting. Thank you for sharing that information.

[+hogrod]

thanks so much.... i knew someone would know what i am talking about. having two meridain golds i noticed that both of these are not the same whith where the put you. thought this was kinda weird being these are both the same model/firmware. thanks again!

[+Renegade Knight]

Locally it seems if a cache is off 50' the owner is a magellan user. I think this all comes down to not letting them average the location long enough as some Magellan users are off and some are on. I don't think Magellan is less accurate but I do think you have to know that it takes a moving average of some type that can imapct accuracy.

[ImpalaBob]

I assisted in 3 geocaching classes this weekend. The initial hides were marked with a "non-Garmin" GPS and the coordinates supplied to me. I was looking as hard as everyone in my 1st group for the hides and re-marked the waypoints as we found them. The 2nd and 3rd groups were able to find the cache hides much ..... much easier!

 

My conclusion is that if I mark a point with my 60C. I can get back to it with minimal error. As far as anyone else .... the 30 foot circle rule usually applies!

 

On a fun note .... it was quite a laugh to see all the drunken ducks walk in the woods! I can still hear myself saying .... Good signal???.... distance and bearing! I will be bringing a tape measure next time to show everyone just how far 25 feet is!!!!

 

ImpalaBob