Claibration Question

[+FisherMatt]

OK - First post here. Found my first cache with the Cub Scout Den last weekend. We had a great time. However, I would not have found it with my GPS.

 

Thankfully, one of the other guys had a unit that was more accurate.

 

I have a Magellan Meridian Platinum. It has always worked great relative to the waypoints it recorded, i.e., locating a place I have marked with the unit.

 

I have always been suspect of its accuracy to published coordinates. For example, when I am traveling on a road that is mapped into the unit with the Map Send software, my track it always slightly off the road.

 

My GPS was marking the location of the cache this weekend to be about 30 feet from where it was.

 

I know how to calibrate the unit, and I had done so just before this trek.

 

Any suggestions? Should I send the unit in for repair?

 

Thanks,

Matt

[peter]

Doesn't sound like there's any problem with your Meridian Platinum. 30' off at a cache site isn't unusual since the hider could have been 15' off in one direction and you're 15' off the other way which is about as accurate as you can expect in real-world conditions.

 

Current street maps aren't nearly as accurate as our GPS receivers so when there's a discrepancy between the map and the GPS coordinates the fault is most likely with the map. If you really want to check on the accuracy of the coordinates you could look for an accurately surveyed USGS benchmark - there's more detailed information on the main site.

 

Note that the 'calibration' you do with the Platinum is just to let it compensate for any stray magnetic fields and only impacts the directional accuracy of the magnetic compass sensor. It has no effect on the accuracy of the GPS coordinates shown or how far away the unit may say you are when you reach the cache.

[+Renegade Knight]

Your GPS sounds fine. The maps are not 100% accurate to begin with, and your GPS calculated position will vary from 10-20 feet normally. Add it up and your GPS will show you driving 10' to one side of the road and it's no big deal.

[+NWMOhunter]

You can't fix normal! Everything sounds fine. I can't tell you the number of times I've driven down a highway and been in the middle of a river or something like that. There will also be quite a few discrepencies with road turns and stuff like that.

[Alphawolf]

I have people ask this question about "calibration" routinely. It is important, I think, to clearly spell out: You can't "calibrate" a GPS. You can only calibrate the compass in those models that have internal compasses (most don't). The GPS part of the receiver is only ever as good as it is at the time...You can do nothing to calibrate it to better performance.

[+FisherMatt]

Thank you all for your responses. I will go learn about the USGS benchmarks, and then check my accuracy against one.

 

I just figured that I had an issue, because my friend's unit was so accurate to the location. Then the next day , I took my son to look for another cache, but we could not find it. We were in a grass field, adjacent to a parking lot and 50-60 feet from any kind of cover that may hide a cache. The cover was thick, and I was not sure we were looking in the right place, so we gave up rather quickly.

 

Will try again and get back with you pros!

 

Thanks again,

Matt

[+Night Stalker]

You might want to do some caching with an experienced cacher in your area. After a while you will depend a little less on the GPS and more on your geocaching 6th sense.

[+blindleader]

Thank you all for your responses.  I will go learn about the USGS benchmarks, and then check my accuracy against one.

If you expect to discover the "accuracy" of your GPS receiver by going to a NGS GPS point or "benchmark", as they are called on geocaching.com, it's a complete waste of time. The accuracy of a GPS fix is a result of a whole raft of factors, few of them having anything to do with your receiver. The exact location of the satellites the unit is receiving relative to each other and the receiver, and ionosphere activity are the two biggest. The accuracy of the atomic clocks on the satellites is the basis for the inherent accuracy of the system.

 

If you haven't really taken to heart what others in this thread have said, I'll hammer it home:

 

-It is completely normal for your receiver to tell you the cache is thirty feet away from where it really is.

 

-Magellans display a number called EPE, which is an estimate of how uncertain the fix is. You should not be surprised to find the cache twice that far away. That's assuming the hider didn’t' have even worse conditions, or either of you don't orient the antenna for best reception.

 

Movement under tree cover, on hillsides and among buildings can cause temporary behavior that will have your fix be hundreds of feet to miles off. Yesterday, both me and a friend had our Garmins tell us we were in the Pacific Ocean 500+ miles Southwest of where I'm pretty sure we actually were.

[+Muddler]

For what it is worth, I have found several benchmarks and found that after letting the GPS set for a while ( 15 min) I got pretty close to the stated location of the BM. The EPE was down to 10 feet and I got a warm fuzzy that I could pretty well count on the readings I was seeing. I agree that there may be causes for erroneous readings but after checking several BMs, I try to believe the GPS.

Edited October 22, 2005 by Muddler

[Grasscatcher]

Also be sure to double check that the datum setting on your unit and the datum of the coordinates that you entered are the same. Probably the most common of GPS "error"'s. Been there...done that