How To Calibrate A Gps

[+southcarolinahwypatrol]

Hi all! New to the hobbie and on my second find I noticed that my GPS seemed to be almost drifting in signal somewhat. Also, I was off around 50-75ft, is this normal? Is there a way to calibrate the GPS? I have a Magellan 310. THanks in advance.

[CoyoteRed]

Uh oh! It's the MAN! Just act normal...

 

JK

 

Welcome to the wonderful world of geocaching! I'm out of Summerville!

 

Anyway, the drift you see is normal. The atmosphere cause the signal from the satellites to use less than perfect paths and as a result the unit gets a little confused.

 

I'm not familiar with the 310, but make sure your unit is set to WGS84 and DD° MM.MMM. Other settings can put you off a little bit or a lot depending.

 

Hope this helps!

[ozarkray]

I am given to understand that the best way to calibrate the GPS is to leave it outside, turned on, without moving it for fifteen to twenty minutes. I haven't seen it listed anywhere how often the satellites update their ephemeris, but I usually recalibrate my GPS about once a month or anytime I travel more than 100 miles.

Ray

[+LthrWrk]

hmmmm....

 

How about going to a known benchmark. See how close your gpsr reads to the known cooridinates?

 

Something I have been wondering about. Especially as my regular cachemate and I get very different coordinates for the same location.

 

Her's a Garmin 60CS and mine the more accurate, spot on, 100 percent correct Magellan Gold heheheh.

[Keystone]

Moving this topic to the GPS Units and Software Forum.

[+Sagefox]

Hi all! New to the hobbie and on my second find I noticed that my GPS seemed to be almost drifting in signal somewhat. Also, I was off around 50-75ft, is this normal?

This is normal. Sometimes it will appear that the gps is over 75' off, most of the time it will be 10 to 50' off and occasionally it will read spot on.

 

The readings you get when you visit a site are dependent on the specific location and number of satellites available at the time of your visit AND at the time of the hider's visit. Many people talk about the accuracy of this or that gpsr when finding caches but seldom mention that we have no idea how accurate the readings were when the hider placed the container.

 

Is there a way to calibrate the GPS?

 

I haven't seen anything in writing that says a gpsr can be calibrated. As an earlier post suggested, setting it down outside (or on your car dash) for several minutes appears to be all you need to do. When the gpsr says it has a 3d lock you are ready to roll and it is as good as it is going to get – until a few minutes pass and all the satellites have moved around a bit. It is a very fluid process.

 

My manuals say it might take up to 15 minutes in a new location to get a 3d lock but I've never had it take more than three minutes

 

A good practice when you are in bad reception areas (trees, canyons, tall buildings & walls, etc) is to stop for a minute or two and hold the antenna end up. Moving around in these bad condition areas causes satellites in marginal locations to fade in and out.

 

If the geocache placement coordinates are sloppy then people will beat up on the cache owner a bit which might inspire them to come back out and do some waypoint averaging. Or often times a finder will post revised coordinates and subsequent finders will agree and then the owner will use those numbers to update the page.

 

I find that caching works best for me when I don't pay much attention to the tech aspects. I've used a Garmin eMap and Map60CS to place over 100 caches and find at least 800 containers and I've found that most gpsr discrepancies can be overcome by two other tools we use to find caches: " What is the interesting point at this cache site, where would a cacher place it?" and by reading the cache description and previous find logs for clues.

[+Sagefox]

How about going to a known benchmark. See how close your gpsr reads to the known cooridinates?

The operative word here is "known".

 

Most benchmark waypoints are "scaled" by someone sitting at a desk with maps and the benchmark description. I find that they are often 100 to 300 feet off. But occasionally you can find markers that are just about spot on. Not sure what that process is called on the benchmark page but it is fun when the marker shows up exactly when the goto says 0 feet.

[dave and jaime]

How about going to a known benchmark.  See how close your gpsr reads to the known cooridinates?

The operative word here is "known".

 

Most benchmark waypoints are "scaled" by someone sitting at a desk with maps and the benchmark description. I find that they are often 100 to 300 feet off. But occasionally you can find markers that are just about spot on. Not sure what that process is called on the benchmark page but it is fun when the marker shows up exactly when the goto says 0 feet.

that depends on the type of benchmark. some are for verticial control and those may have been scaled from a map or other source. try finding a horizontal control marker--those have known coordinates to the millimeter.

[+rusty_tlc]

On the subject of "known" coordinates. I am tempted to ask my surveyor friend to mark a reference point in my front yard using his differential GPS system.

[+blindleader]

Is there a way to calibrate the GPS?

No. Your GPSR is not a measuring device. The only thing in it that could theoretically be calibrated (in the instrumentation sense) is the clock, and it gets calibrated (in the correction sense) every time a fix is taken (once per second for these consumer devices).

 

How about going to a known benchmark.  See how close your gpsr reads to the known cooridinates?

That's an interesting exercise and might be fun, but the only two things it will tell you is that 1. The GPS system (including your receiver) is functioning, and 2. How good a fix you have at that particular time and place. Walk down the road a few hundred yards an hour later and the information you got at the "known" point is completely useless.

 

You can get a fair idea of how accurate your fix is by looking at the EPE. You can get a somewhat better idea by recording the exact time of the fix, write down the names (PRN) of the satellites you are receiving, and then calculate GDOP back at your computer. But only nut cases (not mentioning any names, here ) ever go to that much trouble.

[+Kewaneh & Shark]

All recreational GPSrs will 'drift' a bit, but 50-75 feet is a lot. One thing to check is to make sure the firmware is up to date. When the Magellan 310 and 315 (my first unit) were introduced, the signals from the satellites only had an accuracy of +/- 100 meters. (The satellite signal's selective availibity [sA] was turned on.) Consequently, the shown precision for the 310 & 315 was only to the second decimal place for the minutes (ie DD°MM.MM), or about 60-70 feet measured on the ground for my part of the world. They really couldn't get any more accurate anyway. Since then, in May of 2000 to be exact, SA was turned off and the satellites' signals have an accuracy of +/- 10 meters. If the firmware in your 310 is still set to show the minutes to the 2nd decimal, it could give you readings all over the place, particularly in the 50-75 foot range. After I updated the firmware in my 315, I was able to get shown readings to the third decimal place (ie DD°MM.MMM). That third decimal place makes a lot of difference.

 

As far as calibration of a GPSr goes, it's a relative thing. Blindleader explained it well. The signals from the satellites have an inherent error of +/- 10 meters, so any recreational instrument you may be using will have that error in it, and they can't be 'calibrated' to do any better. However, if your problem is in fact the firmware, updating it would be, in a sense, calibrating your GPSr.

 

You should be able to get the firmware update from the Magellan webite.

 

Hope this helps.

 

- Kewaneh

[+Learned Gerbil]

The 310 only shows two decimals of minutes so will always be this innacurate. even so, I know some very successful cachers who use a 310.

[flame1]

One of my friends had the same problem, he was off sometimes by 100 feet. I had called magellan helpdesk and they told me to leave the gps 310 outside for 1 to 2 hours in a clear view of the sky. So we did this in January of 2004 and the gps is working great since that. It had to initialised by itself in the open with the gps on. Hope this will help you.. Also with the 310 be sure you always have a minimum of 3 satellites because if you ahave less than 3 this will put you off a lot also chow Flame1

[+apersson850]

Ephimeris data (precise orbital information) is updated by the hours, so there's no point in leaving your GPS outdoors for a couple of hours at one instance, and then believe that it would affect the operation of the unit another day.

[+raouljan]

And your GPSr datum is set to the same datum as the co-ords you are comparing against...

 

75 feet is way too far off ...