Inaccuracy = Inevitable?

[+O'Cachin]

Scored a Gamin GPSmap 76 for Xmas, and have found three caches thus far. With all three, the exact coordinates given have placed me about 25 feet from the cache.

 

From what I've gathered in these forums, that's to be expected, and even adds to the fun. I'm just wondering if there's any way to reduce this margin of error as much as possible, or make sure my GPSr is "calibrated" correctly?

[+CYBret]

25 feet is well within the acceptable range of accuracy for a handheld GPS like that.

 

There's nothing much you can do about it, it all has to do with the wobble of the earth, the position of the satellites, the amount of signal you get, battery strength, yadda yadda yadda.

 

If you want to do better, I guess you could invest in one of these. Then again, if the guy who hid the GPS didn't have one, you're still going to have some inaccuracy.

 

 

Bret

Edited December 29, 2004 by CYBret

[skillett]

you might want to check to see if the WAAS feature on your gps is enabled( if it has one) . But from what i understand it requires a clear view of the sky from horizon

to horizon to work properly. The less sattellites your gps can "see" the less the accuracy will be. Hence an inaccurate reading in vallys and deep woods with lots of tree cover. you might want to go to a place with an unobstructed view of the sky and try relocating your gps ( consult manual). hope this helps.

 

God, these laptop keyboards stink!! gotta get an external if i am gonna hang around here!

[+Team Shibby]

Best way to "calibrate" your GPS would be to allow it to sit with a clear view of the sky for 20 to 30 minutes. Once the almanac has been completely updated, you will get the best accuracy as your current conditions permit.

 

25' sounds about right to me...

 

kar2n101

[+Sputnik 57]

I would only add to CyBret's comment that some benchmarks (but certainly not all) have pretty accurate GPS coordinates. You may want to check into benchmarking in your area, go to a well plotted benchmark and see how you do.

 

Sometimes my 60C is dead on (which probably only means that my position error and the cache hider's position error happened to coincide nicely for that cache). But 25 feet is about all you can expect. If you were consistently 200 feet off, I'd offer up some likely explanations (like wrong datum).

 

Edit to add: The satellite configuration that happens to be overhead when you are caching has a lot to do with accuracy. The Trimble site has a program that you can download that will give you best satellite telemetry for various hours of the day for your location. I just cache when I can and let the satellites soar where they may.

Edited December 29, 2004 by Sputnik 57

[+AuntieWeasel]

I'm just wondering if there's any way to reduce this margin of error as much as possible, or make sure my GPSr is "calibrated" correctly?

...and even if you get yours PERFECT, you're still have to deal with the other guy's margin of error, too. The biggest difference between what my unit said and where the cache was supposed to be so far is 297 feet (since nobody else complained about the coords, I assume my Garmin was just having a bad hair day). Though not common, I get differences of 170' fairly regularly. Usually, though, if the dingus says I'm more than 50' away, I go where it's pointing me.

 

Isn't it amazing how big a patch of woods a 100' radius is to a biped?

[dsandbro]

This link has been offered before, and it has a few pointers on field techniques -- like how to hold the unit.

 

http://www.safnet.org/archive/702_howtogps.cfm

[+NickL]

As has been stated run the GPSr for 20 - 30 minutes before going out. I usually plug mine into the power adapter and set it on the dash. By the time I make all the stops that I need and get to the first cache it is usually close to 30 minutes.

 

WAAS will help in some areas but only marginally so. Unless you are near a major airport that has a ground based WAAS transmitter then watch your error drop to less than 7 feet easily.

 

[silly mode: on]Of course if you are looking for sub millimeter accuracy you could get a Trimble but the computer, data logger, receiver, batteries and power converter will be a real bear when bushwhacking. Plus having to work off a known point-of-beginning and getting near real-time access to correction data to quickly post process the captured data. Lets see $15,000+ for the equipment and lets say 1 cache per day verse less than $500 and several caches in a day. [silly mode: off]

[+Kealia]

As has been stated run the GPSr for 20 - 30 minutes before going out. I usually plug mine into the power adapter and set it on the dash. By the time I make all the stops that I need and get to the first cache it is usually close to 30 minutes.

Another opinion - I'm using a Magellan SporTrak Pro and have never had to do this. I do give the unit about 5 minutes or so before I head out to "get it's bearings".

 

Never had a problem being off except for the $^@% redwood cover.

[+Kealia]

Isn't it amazing how big a patch of woods a 100' radius is to a biped?

I'm the mountains near Santa Cruz, CA and truer words have never been spoken!

[+Team Shibby]

As has been stated run the GPSr for 20 - 30 minutes before going out. I usually plug mine into the power adapter and set it on the dash. By the time I make all the stops that I need and get to the first cache it is usually close to 30 minutes.

Another opinion - I'm using a Magellan SporTrak Pro and have never had to do this. I do give the unit about 5 minutes or so before I head out to "get it's bearings".

 

Never had a problem being off except for the $^@% redwood cover.

I agree that this does not need to be done every time you go out, but if you do not use your GPS frequently, this is a surefire way to eliminate errors due to an old or incomplete almanac and you will get a lot less "lost signal" errors.

 

Magellans as I recall have to be set to alert you when you have lost signal, otherwise they will continue to average your location for a much longer time before it finally says "Hey bud...I think were lost"!!

 

kar2n101

[+DaveA]

Scored a Gamin GPSmap 76 for Xmas, and have found three caches thus far. With all three, the exact coordinates given have placed me about 25 feet from the cache.

 

From what I've gathered in these forums, that's to be expected, and even adds to the fun. I'm just wondering if there's any way to reduce this margin of error as much as possible, or make sure my GPSr is "calibrated" correctly?

With my WAAS enabled Magellan Meridian, assuming I have a WAAS lock and assuming I am in an area where the sat reception isn't getting bounced around (as happens around lots of rock) I generally have accuracy to about 7-10 feet, sometimes less.

 

Without WAAS I have 20-30 foot accuracy.

 

However, there are a lot of caveats.

 

First, you should test your unit under controlled conditions to see what it's accuracy is.

Make sure your unit is setup to receive WAAS signals when available. Some units you have to turn this on, other units it is always on. Go sit in a clear, open area and confirm you are getting at least 4 strong sat signals and a WAAS lock. Let your GPSr average for a minute or two. Save the location as a waypoint. Turn your GPS off. Turn it back on and wait for it to reaquire the sats and WAAS lock. Set a waypoint destination to be the waypoint you just saved. Let it average for a minute or two. Look at the distance to the waypoint the unit displays. That is your accuracy under ideal conditions since your GPSr hasn't moved. The distance should be 0ft if perfectly accurate. It won't be.

 

Second, you have to accept that the signals your GPSr recieves are not exact. There are things in the stratosphere that will throw it off a bit (WAAS does a reasonable job of correcting for this) and there are things on the earth that throw it off (water, rocks trees) Unless you are right next to a ground WAAS station there is no correction for this unless you have 10s of thousands of dollars to spend. Your accuracy will vary from one location to the next as a result, but it should always be within 30ft or something is wrong. Might be your GPS, the hider's GPS or an error entering coords, but 30' should be a practical maximum error except under really goofy conditions (like you are in the middle of 2 rock cliffs that go up hundred of feet in which case just turn the GPSr off ;-) )

 

Lastly you have to accept that the person who hides a cache may not have recorded the coords to the degree of accuracy you would like. A person might go to a general area, save the location as a waypoint and then look around for a hiding spot. Of course the coords will be off a bit. A person might go into a forest and hide a cache when the leaves are on the trees and a WAAS lock is elusive. You might seek the cache in the winter when the trees are bare and you have a WAAS lock. The hider may have an old GPSr that isn't WAAS capable.

 

Today I found 5 caches. In one case I was standing 1ft from the cache and the GPSr, after averaging said I was 1ft away and pointed in the correct direction. That is rare, but it happened. In another case I was walking on a narrow island with tons of rock and the GPS couldn't decide which side of the trail the cache was on. There was a 20-30 foot area the GPS indicated I would have to search, 10-15' on each side of the trail. In a third case I was in the clear, nothing obvious to interfere with the sat signals and the cache was 30' from the coords according to my GPSr. Looking at the logs for this cache, others indicated that the coords appeared to put the cache on private property. That matches what I experienced so more than likely the hider had some poor coords.

 

That's just how it goes. Test your unit to determine it's accuracy under ideal conditions and when out in the field as you use your GPS you wil get a feel for how accurate it will be in various conditions. Always keep in mind the coords listed by the cache placer have their own margin of error and there is nothing you can do about it.

[+DaveA]

As has been stated run the GPSr for 20 - 30 minutes before going out. I usually plug mine into the power adapter and set it on the dash. By the time I make all the stops that I need and get to the first cache it is usually close to 30 minutes.

Another opinion - I'm using a Magellan SporTrak Pro and have never had to do this. I do give the unit about 5 minutes or so before I head out to "get it's bearings".

 

Never had a problem being off except for the $^@% redwood cover.

I agree that this does not need to be done every time you go out, but if you do not use your GPS frequently, this is a surefire way to eliminate errors due to an old or incomplete almanac and you will get a lot less "lost signal" errors.

 

Magellans as I recall have to be set to alert you when you have lost signal, otherwise they will continue to average your location for a much longer time before it finally says "Hey bud...I think were lost"!!

 

kar2n101

The very first thing any GPSr does upon booting up is communicate with the very first sat it finds. From this it gets the 'almanac' if it needs to.

 

On a Magellan Meridian this is seen if you go to the "Position" screen upon boot up you will find it does one of 2 things. If the GPSr has been on recently and isn't far away from where it was last turned on it will find sats 1-4 and then will go to "collecting data" which means it is downloading info from the sats, the almanac and position info. It doesn't seek the almanac right away because it doesn't need to.

 

If the GPSr is being cold booted, meaning you are far away from the location it was last on or it has been a long time since being turned on it will find Sat 1 and then go to 'collecting data' which means it is refreshing the almanac because it has to, or thinks it has to.

 

Either way it doesn't take 30 minutes to figure out where you are with maximum capable accuracy.

 

Even with an incomplete almanac if 4 sats are locked the GPSr is as accurate as GPSrs can be. If WAAS is locked then it is as accurate as it can be with WAAS.

 

The idea that a GPSr has to be left on for 30 minutes to become accurate is just a myth. If your GPSr is locked onto 4 sats you are all set. The only thing that can improve the accuracy is a WAAS lock. Time isn't going to do anything for accuracy.

[+Team Shibby]

When you first turn on your GPS and it locks onto the first satellite, it DOES NOT download the almanac for all 24 satellites orbiting the earth. If you notice on the sat screen of your GPS the bar representing each sat in view starts out grey, as it downloads the almanac it turns black when and only when the download is complete.

 

If this wee true that it downloads the almanac when it locks onto the first sat, then all bars would immediately turn black.

 

Kar

[+DaveA]

When you first turn on your GPS and it locks onto the first satellite, it DOES NOT download the almanac for all 24 satellites orbiting the earth. If you notice on the sat screen of your GPS the bar representing each sat in view starts out grey, as it downloads the almanac it turns black when and only when the download is complete.

 

If this wee true that it downloads the almanac when it locks onto the first sat, then all bars would immediately turn black.

 

Kar

 

The bar you are speaking of indicates how strong the reception of the signal from that satellite is. It happens quite frequently that a solid black bar will raise and lower in height as well as go from black to grey or disappear altogether.

 

Every satellite contains the information on the location of all others.

 

If it was true that the almanac, when needed, isn't updated from the first satellite then the GPSr would have to manually search for each satellite rather than knowing where it is and only having to try and receive it

[+Allondro]

Scored a Gamin GPSmap 76 for Xmas, and have found three caches thus far. With all three, the exact coordinates given have placed me about 25 feet from the cache.

 

From what I've gathered in these forums, that's to be expected, and even adds to the fun. I'm just wondering if there's any way to reduce this margin of error as much as possible, or make sure my GPSr is "calibrated" correctly?

 

Please release my TB or send it back to me. I am becoming annoyed by people who grab a TB and then stop caching.

[+Markwell]

Scored a Gamin GPSmap 76 for Xmas, and have found three caches thus far. With all three, the exact coordinates given have placed me about 25 feet from the cache.

 

From what I've gathered in these forums, that's to be expected, and even adds to the fun. I'm just wondering if there's any way to reduce this margin of error as much as possible, or make sure my GPSr is "calibrated" correctly?

 

Please release my TB or send it back to me. I am becoming annoyed by people who grab a TB and then stop caching.

Hardly the appropriate way to contact someone. Did you really think they'll be reading a thread from 2½ years ago?

 

Closing