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GPS Tips for accurate coordinates


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Ok, I need some help here please...

 

What is the trick to getting a good cooridinates when making a Geocache? Some of my recent drops have had coords off enough to have 'cachers make comment.

 

Garmin ETrex Legend.

 

I usually let it sit on the ground for a couple of minutes before making a waypoint. Any other tips?

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How far off?? What did the complaints say??

 

The general accuracy of most handheld GPS devices is around 15 to 25 feet. When you add your potential error to any finders - it wouldn't be terribly surprising to find a cache 30 or more feet from where your unit says it should be. If they are complaining about less than 30 feet - don't let it bother you.

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If you're having a hard time getting a good fix or you're really concerned about accuracy, take 6 or so readings each day for five or six days. Throw out any that are way off and average the rest. Maybe even try a few different brands/models of GPSr.

 

Ok, I need some help here please...

 

What is the trick to getting a good cooridinates when making a Geocache? Some of my recent drops have had coords off enough to have 'cachers make comment.

 

Garmin ETrex Legend.

 

I usually let it sit on the ground for a couple of minutes before making a waypoint. Any other tips?

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Here's what I do.

 

Set the GPS flat at GZ and mark coordinates

Take GPS 30 feet one direction and then return to GZ and allow to settle, mark coords

Take GPS other direction 30 feet, return, allow to settle and mark coordinates.

 

Average of three readings.

 

Thats what I do too only I do it 8-12 times just to be sure.

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for my PDA running beelineGPS: turn off static navigation, set the software to start averaging coordinates and let it sit there for a while. 10 minutes gives it 600 samples to average, got very good results this way. in circumstances with good reception less samples (~200) will do too.

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for my PDA running beelineGPS: turn off static navigation, set the software to start averaging coordinates and let it sit there for a while. 10 minutes gives it 600 samples to average, got very good results this way. in circumstances with good reception less samples (~200) will do too.

I go fore 15 to 20 minutes, then if you do it for 5 or 6 days and then average you can completely ignore anyone who complains.

If I didn't have this ability then I would take 20 a day for at lest 7 days before I was happy.

I have a buddy who complains that I spent to much time getting my initial coords and spend to much time making sure that I'm not getting some kind of float.

 

Another thing to remember, a persons gps might give a read out like "N 42 26.7154 W 083 53.0744" but gc.com will turn it into "N 42° 26.715 W 083° 53.074".

Notice the north and west are both 1 digit short?

Where I live Groundspeak automatically supplies up to 3.7 feet of error and if I remember correctly, the further south you go the bigger the difference becomes, however insignificant the amount may be.

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An expensive solution is to buy a better GPS unit. Some GPS units are more accurate than others, I'm not sure about your model. Otherwise, averaging coordinates is certainly the way to go, if you are patient enough to wait a while, or come back another day. In any event, it is hard to get a good reading in canyons or by a lot of buildings. I haven't picked up any of yours yet, but a canyon like the one Ulric goes down is going to make it harder to get a good fix. But if you get enough logs telling you better coords, you can just do a log of type "update coords" to fix them.

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Where I live Groundspeak automatically supplies up to 3.7 feet of error and if I remember correctly, the further south you go the bigger the difference becomes, however insignificant the amount may be.

yes. because the accuracy of the GPS system is much lower than the difference this last digit makes. so listing it doesn't really make much sense.

 

that being said, there are ways to get that last digit in your GPX downloads, even if you don't see it on the website :blink:

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Check it against a good aerial photo. :blink:

You can check your coordinates using the Sattelite mode of Google maps to zoom in as closely as possible (look for the Green Arrow). Depending on the resolution, you can fine tune the coordinates to point to the cache location. Of course, this assumes the Sattelite image has the proper resolution to zoom all the way in.

 

Most of the newer model GPSr's will zero out to this point (sorry but the eTrex Legend doesn't have the accuracy). I always do this check before submitting a cache.

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...

Most of the newer model GPSr's will zero out to this point (sorry but the eTrex Legend doesn't have the accuracy). I always do this check before submitting a cache.

 

Common misconception. With a nice clear view of the sky and a WAAS fix - the old blue legend is as accurate as any of the new units on the market. The newer units have lots of bells and whistles and sensors and excel at holding onto a signal in tough conditions but realistically are no more accurate with a clear view of the sky.

 

Averaging doesn't always work either. If the sat geometry is lousy - all you are doing is averaging bad data. If sat geometry is good - no need to average because you have good data. If the sat geometry is so-so - then maybe averaging will improve things a bit.

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What about the Garmin Etrex H? the little yellow unit was telling me I was accurate to 1 m (about 3 feet). Think thats good enough for just tagging one point for my caches?

 

Where im at the sat. geometry always seems superb. At least one in each cardinal and one right above it seems at all times. (for the most part)

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You can check your coordinates using the Sattelite mode of Google maps to zoom in as closely as possible (look for the Green Arrow). Depending on the resolution, you can fine tune the coordinates to point to the cache location. Of course, this assumes the Sattelite image has the proper resolution to zoom all the way in.

um, no. the resolution alone doesn't mean anything, there's always a chance the images on google maps/earth aren't aligned/projected properly, which means the images can be off by any unknown amount, even if the resolution is good. in fact, the images will always be off by an unknown factor, this can be anywhere between a few centimeters to several hundred meters (or even more). you just don't know, which is why you should never trust the aerial images too much.

 

What about the Garmin Etrex H? the little yellow unit was telling me I was accurate to 1 m (about 3 feet). Think thats good enough for just tagging one point for my caches?

that's another thing you should never trust. those numbers are purely mathematical. if the GPS tells you 1 meter of accuracy, it doesn't mean that you're within 1 meter of the coordinates it shows you. it's an estimate, an approximation, meant to give you a feel of how good the reception is, but the number itself doesn't mean much.

 

in fact, manufacturers have a habit of making the accuracy look better than it really is. like halving the EPE just because you have a WAAS lock, while in reality it may not even mean anything and the GPS chip actually reports a totally different number.

 

Where im at the sat. geometry always seems superb. At least one in each cardinal and one right above it seems at all times. (for the most part)

the closer to the equator you are, the better your chances for having good sat geometry. but it's not gonna be "always", especially when you have some failed/disabled satellites up there.

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Keep in mind that SOME GPSr's are more apt to give errors in areas where there is high EMF issues.

 

I have one cache placement that is in a safe location but is close enough to a high power transformer and high power lines that the EMF interference causes big issues with GPS accuracy with MOST GPSr's on the market.

 

Depending on the day. Accuracy is between 10 & 50'. The iPhone reports GPS accuracy. Many GPSr's are able to show accuracy as well.

 

If your in an area where obtaining accurant coordinates is difficult because of EMF, then use very good hints, or supply several hints. Also NOTATE in your cache page description & info, that this cache is in an area with EMF interference as well. This will let geocachers know that they might have issues with accuracy in that area.

 

In case someone hasn't mentioned. SOME GPSr's also have an "Averaging" feature. I have the Garmin Rhino 530HCX. It has averaging capabiliteis in it it as well. When I place a cache I put my unit in Averaging mode at GZ and let it sit there unti it has obtained 1000 readings. (Around 10 min). In really difficult areas. I will make 2 or 3 trips out and average another 1000 readings. So that I get about 3000 to 4000 readings on 3 to 4 days.

 

TGC

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yes. because the accuracy of the GPS system is much lower than the difference this last digit makes. so listing it doesn't really make much sense.

Like I said, "however insignificant the amount may be".

 

that being said, there are ways to get that last digit in your GPX downloads, even if you don't see it on the website :anibad:

You going to have to clarify that.

Stripping the WGS84 out of the gpx then converting it yields the exact same results.

Once the site drops the "extra" data, it is lost to us.

Something like 42.506417 -83.85255 will always return N 42° 30.385 W 083° 51.153.

Unless there is some psychic conversion program out there. B)

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I have one cache placement that is in a safe location but is close enough to a high power transformer and high power lines that the EMF interference causes big issues with GPS accuracy with MOST GPSr's on the market.

 

 

How does that work?

I can see the transformers having an effect on compasses, and emf possibly adding noise to the signal, but not clear on how the emf is going to give a false signal ?

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You going to have to clarify that.

Stripping the WGS84 out of the gpx then converting it yields the exact same results.

Once the site drops the "extra" data, it is lost to us.

Something like 42.506417 -83.85255 will always return N 42° 30.385 W 083° 51.153.

Unless there is some psychic conversion program out there. ;)

B):DB)

 

i've only seen it on caches created by other people until now. i've always had a suspicion on how it's possible to do that, but never bothered to check. now you made me :anibad:

 

you have to enter the coordinates as DegDec. when you do that, gc.com stores the coords as you enter them, without cutting off/rounding anything. on the website you'll still see it rounded to 3 digits after the comma and you'll also get that when you go to edit the coords afterwards (as DegMinDec is default) thus losing the additional digit when you do that, but in the GPX download you'll get the full numbers.

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I have one cache placement that is in a safe location but is close enough to a high power transformer and high power lines that the EMF interference causes big issues with GPS accuracy with MOST GPSr's on the market.

 

 

How does that work?

I can see the transformers having an effect on compasses, and emf possibly adding noise to the signal, but not clear on how the emf is going to give a false signal ?

 

Transformers opperate at 60Hz, 120Hz, or 400Hz. Even the very high harmonics will not interfere with GPS frequencies (>1GHz). That said, the actual power lines and metal in the transformer could block the signals, I don't know...

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Here's what I do.

 

Set the GPS flat at GZ and mark coordinates

Take GPS 30 feet one direction and then return to GZ and allow to settle, mark coords

Take GPS other direction 30 feet, return, allow to settle and mark coordinates.

 

Average of three readings.

 

To add to that, some GPS systems and cell phone apps do this for you, others don't.

But just sit and watch the cords pop around, and gather up a handful of them, then avg them together.

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