Coordinate Precision

[Amok Run]

Hi,

 

In an urban environment for example, using services such as google map/earth, you can easily get coordinates at a precision below one meter. Still, I often find caches which are several meters away from the zero-spot. What is the good-practice with respect to that? Is it acceptable? If yes, how many meter far from the zero-spot can a cache be hidden?

 

Thanks!

 

Amok Run

[AZcachemeister]

In some areas, the Google products are very accurate.

In some areas, they are very inaccurate.

Nobody knows what the accuracy really is at any particular location. (Well, maybe somebody knows, but they're not telling! )

 

In an urban environment with tall buildings blocking satellite signals and reflective surfaces bouncing them around, GPS accuracy can (and usually does) suffer.

 

The difference is that a good quality GPS receiver will tell you when it thinks it has poor accuracy.

 

It would be nice if you could right-click on a spot on Google Earth and get some sort of estimate of the accuracy, and maybe someday this will be possible. For now it is NOT safe to assume the co-ordinates provided on Google Earth are 100% correct.

[+Gitchee-Gummee]

While those maps may be that close in your area, they are notoriously poor in other locations. Their accuracy is best described as "spotty"; good here, not-so-good there, atrocious elsewhere.

 

Regarding the accuracy of cache placements, a lot of things come into play.

1] Time and effort that a cache hider takes in placing the hide;

2] Knowledge, practice and use of their GPSr device;

3] A phenomenon known as "cache migration" -- not returned to the same hiding spot (this can be multiplied with more cache finders);

4] Limitations of consumer-grade GPSr devices -- they simply are not designed for absolute pin-point accuracy;

5] Unseen and unnoticed outside factors -- (this plays into #1 and #2: current satellite constellation, signal reflection/bounce along with the possibility of signal interference [uncommon]);

----

6] Add to that, the same factors enter into the equation in regards to the cache finder. Perhaps even more-so.

 

In placing a hide, the best practice is to be as accurate as possible using a GPSr device -- after-all, those hunting the cache are also going to be using a GPSr device, although it isn't required for that.

[+T.D.M.22]

The co-ords; from a GPS or Google maps, or whatever, will only tell you where the CO put the posted co-ords for, not where the cache is. If the cache is at the posted co-ords and someone moves it then the co-ords are still correct, the cache just isn't there.

 

Anyway there is always inaccuracy in the devices used to place the cache, and find the cache. You might get really lucky and it may be unnoticeable. Or both devices may be 3M off. When you get within 10 feet or so stop using a GPS and use your eyes. 10 feet/3M seems to be the accepted average error.

[+The A-Team]

Hi,

 

In an urban environment for example, using services such as google map/earth, you can easily get coordinates at a precision below one meter. Still, I often find caches which are several meters away from the zero-spot. What is the good-practice with respect to that? Is it acceptable? If yes, how many meter far from the zero-spot can a cache be hidden?

 

Thanks!

 

Amok Run

There are two completely different terms here that I think you're using interchangeably: precision and accuracy.

 

Precision ≠ Accuracy

 

Sure, I can get a set of coordinates from Google Earth down to within a few inches (precision), but that won't do any good if the satellite imagery in that location is mis-aligned by 50

feet (accuracy).

 

Regardless, the accuracy or precision of satellite imagery has zero bearing on geocaching, since coordinates are required to be taken using a GPS device. Those GPS devices, unless you're using a commercial-grade surveying GPS, will only get you to within a few metres at best. A good rule-of-thumb is that the coordinates are most likely within about 30 feet of the cache.

[+Beach_hut]

Not to mention the fact that the thousandths-of-a-minute accuracy that co-ords are measured to on this site will give you an area of at least a couple of metres squared (about 4 metres by 2 in my part of the world)