Co-ordinate accuracy

thismechanican · Post by **thismechanican** » 29 December 08 10:56 am

What is the generally accepted distance that a cache should be from the given co-ordinates in a traditional cache?
This weekend we searched for several caches by the same cacher and all were between 10 and 20 metres away from GZ. We had 2 GPS' with us, and both confirmed GZ. When reading the logs, other people had also had a similar problem with these caches. It is quite time consuming and very frustrating (especially caching with geokids) to have to search such a wide area of scrubland to find the cache.

Big Matt and Shell · Post by **Big Matt and Shell** » 29 December 08 11:33 am

Many older GPS units are only accurate down to 5 to 8 m. Based on this you could be 10 - 16 m from the other users coordinates based on two circles with a radius of 5 to 8m. <P>This said, this is unlikely. If you find a cache is off by 10 + meters I would comment, more than that I may even add a waypoint to the log that may help other finders.

mtrax · Post by **mtrax** » 29 December 08 11:34 am

I would have said these days the accuracy should be between 4-10m

Papa Bear_Left · Post by **Papa Bear_Left** » 29 December 08 11:57 am

If they're old caches, they might have been spot-on when they were placed but have moved due to continental drift.

OK... we're drifting north at about 6cm a year, so for a 10m inaccuracy... let's see.. carry the two...

Were these caches placed more than 170 years ago by any chance?

But seriously, it's worth mentioning the error and taking the time to average a good set of coords and, if there's a lot of agreement in the logs, a responsible cache owner will check their coords with a newer GPSr (if they've upgraded) or even just accept the concensus of the finders' numbers.

Post by **caughtatwork** » 29 December 08 12:01 pm

Depending on whether the cache is hidden under a lot of tree cover or, say, in the city where building will block and reflect the GPS signals, you can vary from the hider, even if you have more than 1 GPS say that GZ is over "there".

Newer hiders do not always understand that GZ can move depending on the position of the satellites in the sky (aka the constellation), so when they hide their cache they take 1 reading and assume it's always going to be the same. It won't be. That means if they have a an error of 10m and you also have an error of 10m, you could be 20m from the cache.

As others have said 10m is OK. 20m (depending on the number of possible places to hide a cache, say on the forest floor) may be too far. See whether other logs indicate that the coordinates are off as well. Sometimes you might just be having a bad day. I get these frequently.

If you leave you GPS at the cache with a clear view of the sky while you log the cache, you may find that the distance from your original GZ may actually get closer to where you are now standing.

Do a little reading up on averaging and it may be clearer.

Post by **caughtatwork** » 29 December 08 12:22 pm

These may also be of assistance:
http://wiki.geocaching.com.au/wiki/Hiding_a_cache
http://wiki.geocaching.com.au/wiki/Finding_a_cache

There is also a link to a document that dak's Emu Mob created in 2005 (that as far as I know was never published, but has been now) which further explains the process to gaining accurate, reliable co-ordinates. It can be found linked from this page:
http://wiki.geocaching.com.au/wiki/Accurate_location

Jardry · Post by **Jardry** » 29 December 08 12:55 pm

Another "issue" is cache creep whereby a cacher doesn't quite replace the cache where the cache owner placed the cache.

I've placed a magnetic cache under a walkway and have when doing maintenance found the cache had moved about five metres or so!

rhinogeo · Post by **rhinogeo** » 29 December 08 4:37 pm

big_matt wrote:... If you find a cache is off by 10 + meters I would comment, more than that I may even add a waypoint to the log that may help other finders.

I got FTF on a cache that I found 12m away from where my GPSr was indicating GZ to be and mentioned the discrepancy nicely in my log

The owner indignantly emailing me demanding that I edit my log because when he hid the cache he had 2m accuracy

(obviously he has the most accurate consumer GPSr in the world

)

I revisited the cache a day or so later and found it to be 16m out and edited my log to mention that

After a number of subsequent finders mentioned that the coords were out in their logs the owner updated the coordinates then deleted my found log ... he must have gone off his medication that week

thismechanican · Post by **thismechanican** » 29 December 08 4:56 pm

I think either this cacher is deliberately doing it (as his descriptions / hints indicate quite clearly where the cache is) OR he doesn't know how to use a GPS....
One cache had GZ in the water of a boat ramp, but the hint said "you need to be a table dancer". About 16m away there was a table under a shelter and the micro was in the rafters of the shelter. After reading the clue I knew where it was...

Kerry · Post by **Kerry** » 30 December 08 7:51 am

....If you leave you GPS at the cache with a clear view of the sky while you log the cache, you may find that the distance from your original GZ may actually get closer to where you are now standing.

Then again it might also move further away couldn't it? The thing is you really don't know if the position is getting better or worse?

....Do a little reading up on averaging and it may be clearer.

Averaging over the period of time generally available or what users are prepared to spend is generlly a waste of time.

thismechanican · Post by **thismechanican** » 30 December 08 8:34 am

Thanx guys. I'm going to suggest that he changes his co-ordinates OR adds a waypoint. He gives the caches a difficulty of 4, but I am starting to think that this is simply because they are so far from GZ, not because they are extremely well hidden (because they aren't).

Nilbog_Aus · Post by **Nilbog_Aus** » 30 December 08 10:13 am

I think it is a good idea to mention it in a log but just be as nice as possible.

I have a acquaintance in the UK with a cache in a steep tree lined valley. Its coords are spot on over the winter but often showed 5m-20m out in the summer. The change was due to the trees dropping all their leaves over winter but being lush green walls in summer. He used to receive a couple of harsh logs each summer telling him he didn't know what he was doing placing caches. Then in winter get logs mentioning the accuracy of the coords. (Thankfully once SBAS was turned on the complaints ended.)

So again I think it is best just be nice. It is probably an honest mistake but it may be for some reason as a finder you could not be aware of.

CaleD · Post by **CaleD** » 30 December 08 12:20 pm

That's a good document, caughtatwork.

Out of interest, I see that WGS84 should expire around 2010... what happens after then?

Guest · Post by **Guest** » 30 December 08 12:39 pm

CaleD wrote:That's a good document, caughtatwork.

Out of interest, I see that WGS84 should expire around 2010... what happens after then?

Erm, I thought they kept updating other datums in relation to WGS84 and keep WGS84 based on the meridian running through the UK and in effect zero out plate drift across the whole system?

There is a 8 or so stations used for plate monitoring around Australia, I forgot where I found the list, but a friend of mine took photos of the one in Sydney to send me. Funny looking thing it is

CaleD · Post by **CaleD** » 30 December 08 12:46 pm

This the one? I think I stumbled across the same site while trying to gather topo information

http://www.ga.gov.au/geodesy/argn/