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GPS Accuracy


3Tails

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Hi All,

 

We have a Garmin Summit that takes us within 5ft of a cache, then jumps 15ft in another directions, then puts us 10ft back to where we were, then 12ft in another direction from that. What is the typical accuracy of these things?

Should we try to find the WAAS setting and use that? We did load mapping for the area of Georgia we live in hoping it would narrow down the field to 5-7ft, but it hasnt' helped. As much as we love to get out and geocache, we are frustrated with the fact we are always within a 25ft area, and sent in circles. Any suggestions? Or is a wide area of searching normal?

Thanks for your help!!

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Hi All,

 

We have a Garmin Summit that takes us within 5ft of a cache, then jumps 15ft in another directions, then puts us 10ft back to where we were, then 12ft in another direction from that. What is the typical accuracy of these things?

Should we try to find the WAAS setting and use that? We did load mapping for the area of Georgia we live in hoping it would narrow down the field to 5-7ft, but it hasnt' helped. As much as we love to get out and geocache, we are frustrated with the fact we are always within a 25ft area, and sent in circles. Any suggestions? Or is a wide area of searching normal?

Thanks for your help!!

Your observation is normal for the consumer grade GPS.

 

You are attempting to find a single point on Earth based on a timing signal from moving satellites of which the distance is never constant from your GPS. Your GPS is attempting to compute this signal and can only provide an estimate of your position with a level of confidence of no better than 3 meters, and based on many variables can be worse. The EPE (Error Position Rate) will give you the device's confidence of its position within a certain distance (I think I read) 85% of the time.

 

Use the EPE display of your GPS to help you to determine when to put the GPS away and start looking around. This is what most people do when they get close and avoid doing the bumble bee dance like what you experienced. Getting within 25 feet isn't bad when you consider the error rate the hider may have been experiencing. This is where you become the search engine. The GPS got you there, its up to you to pinpoint the location.

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I agree with TotemLake. The bounce factor you describe is actually normal. Commercial grade GPS's have some drift. there are two error factors involved, Your location, and your GPS drift, and the cache owner's location and GPS drift. I've run into a few caches, where One GPS may say I'm within 10 feet, Another could say I'm 20 feet away. On average, I've had as high as 35 feet away from the placer's data. Sometimes, when you get real close, You have to rely on Geo-Senses, to find the final.. Is something out of place? Why is that rock differnt? Why is there a unnatural pile of sticks there, and there has been no flooding? Why is that seemingly brand new electrical outlet box on the side of that old rusty lamp post? (Don't laugh, I found one magnetically stuck to a lamp post. The property owner wasn't so pleased, when he tried plugging some device into the box, only to have the box move, though..)

 

Unless you're willing to spend thousands of $$$, (for a Trimbel surveying grade.) or somehow obtain a military grade reciever, You'll always get the drift factor. The idea then, is use the human computer to find it from there.

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It is worse under leaf cover, or in canyons (especially urban canyons between buildings). It is due to multipathing, which is (basically) the signal from the satellite arriving from the direct path, plus arriving after bouncing off one or more objects (leaves, buildings, etc), thereby getting the same signal separated by a fraction of a second. Some GPSr units are better than others at figuriong out which signal is the direct signal.

 

Even survey grade GPSr have inherent error at any given moment. I use Trimble survey grade GPSr units at work, and even they will "wander" a bit. You can actually see this when you average a point reading.

 

As others have said, if you get within 30 feet of a cache using your GPSr, that is typically pretty decent performance (at least usable). That is when your eyes and brain should kick into "geocache mode". Remember, the person who hid the cahe may have been using a cell phone or older (less accurate) GPSr to establish the point anyway. So the original coordinates could have been anywhere within a 15 foot (or 30 foot, or even 50 foot) radius of the listed coordinate. Then you come back with your GPSr, which also has error. Get close enough using the GPSr, then switch to "human geocaching mode"

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Hi All,

 

We have a Garmin Summit that takes us within 5ft of a cache, then jumps 15ft in another directions, then puts us 10ft back to where we were, then 12ft in another direction from that. What is the typical accuracy of these things?

Should we try to find the WAAS setting and use that? We did load mapping for the area of Georgia we live in hoping it would narrow down the field to 5-7ft, but it hasnt' helped. As much as we love to get out and geocache, we are frustrated with the fact we are always within a 25ft area, and sent in circles. Any suggestions? Or is a wide area of searching normal?

Thanks for your help!!

All the other are correct with their comments about consumer grade receivers. 2 things: your GPS receiver probably has no more accuracy than that of the person who hid the cache; when you get to within 5-25ft, look for an obvious hiding spot - you don't need to keep going in circles with the GPS.

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