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Wildlifewriter
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Joined: 04 Aug 2005
Posts: 948
Location: Norn Iron
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 Posted: Tue Sep 13, 2005 10:45 pm Post subject: Deciphering dodgy datums |
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First-to-finds: don'cha just hate 'em?
I mean, you skive off work with some pathetic excuse, drive all that way, walk to the location (and it hissing down like stair rods) - only to get to the co-ordinates and find that there's no "ynetr lryybj ebpx" there.
In fact, there are no ebpxf anywhere to be seen.
What could have gone wrong this time? Well of course, lots of things - but one of the commonest mistakes made by cache placers, (especially the less experienced) is a faulty map datum setting when taking the co-ordinates.
It's easily done. Folk like to use OSGB (or OSI) settings when referring to an Ordnance Survey map, and they forget that geocaching positions imply WGS84 - no matter where in the world this new cache is.
In my long, bitter and accident-prone caching career, I've been able to find five new caches which had duff co-ords - just by knowing the approximate datum offsets for the area I was in at the time.
Here's a table with some examples...
| Code: |
Site Position Err(Mtrs) Bearing ºT
Wick N58 26.923 W3 05.191 95 253 (to OSGB)
Fort William N56 49.430 W5 06.418 78 260 (to OSGB)
Bonnyrigg N55 52.506 W3 05.918 90 269 (to OSGB)
Dumfries N55 04.484 W3 36.859 88 275 (to OSGB)
Isle of Man N54 15.861 W4 27.658 84 281 (to OSGB)
York N53 57.484 W1 03.707 107 283 (to OSGB)
Derby N52 55.206 W1 28.400 107 288 (to OSGB)
Pwllheli N52 53.098 W4 24.556 87 290 (to OSGB)
Norwich N52 37.660 E1 18.163 126 289 (to OSGB)
Reading N51 27.332 W0 57.537 116 296 (to OSGB)
Plymouth N50 21.824 W4 08.138 102 305 (to OSGB)
Ballycastle N55 12.353 W6 14.797 67 269 (to Ir Grid)
Armagh N54 21.356 W6 39.020 63 279 (to Ir Grid)
Sligo N54 16.409 W8 28.582 47 283 (to Ir Grid)
Dublin N53 21.575 W6 19.693 71 291 (to Ir Grid)
Killarney N52 03.317 W9 29.933 57 319 (to Ir Grid)
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The maths of this stuff are too complicated to do in the nut, while standing on the edge of a peat bog in the pouring rain. However, a study of the table does show that the discrepancy gets smaller as we go further west, and the bearing to the correct point gets LARGER, the further south we are. (The error to Irish Grid follows the same pattern, though the actual numbers are different.)
From this we can make a stab at looking for a cache (or at least, something that matches the hint) by pacing off 80-90 metres (in Scotland Wales and IOM), an even hundred (in England), or 60-odd in Ireland, on an appropriate compass bearing.
So: if we suspect a datum error, it's worth a try... rather than coming back empty-handed... 
-Wlw.
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Billy Twigger
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Joined: 30 Aug 2005
Posts: 352
Location: N55 51.686 W5 05.647
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| Posted: Wed Sep 14, 2005 12:57 pm Post subject: |
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Ahhhhhhh...
So do you reckon that's what's happened here Corncrake Sound
If it is, -is there a way of guessing the location? |
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Wildlifewriter
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Joined: 04 Aug 2005
Posts: 948
Location: Norn Iron
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| Posted: Wed Sep 14, 2005 1:54 pm Post subject: |
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| Billy Twigger wrote: | | So do you reckon that's what's happened here Corncrake Sound |
Not in this case, no. That would put the cache at N58º 35.902 W4º 46.362 - which it isn't, as you know.
It's always suspicious when published cache co-ords are a nice round number - as in: N58° 35.920 W004° 46.280 for this one. I can't say what happened, though.
Given the duff co-ords, AND the remoteness of the site, it's amazing that so many people have found it!
-Wlw.
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HighlandNick
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Joined: 30 Aug 2005
Posts: 635
Location: Highlands, Scotland
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| Posted: Wed Sep 14, 2005 9:10 pm Post subject: |
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| Wildlifewriter wrote: |
Given the duff co-ords, AND the remoteness of the site, it's amazing that so many people have found it!
-Wlw. |
Probably because people read the logs and use Pooters coordinates plus a few pointers that I posted as well.
What is more annoying is that the original cache setters don't take the trouble to edit their incorrect coords to those that everybody is now using.
This is a grandfathered cache set by a vacationer I think who may not be doing much caching anymore??
Some cache setters do get very hot and bothered if you dare suggest that their coords are wrong...(not mentioning any caches in particular  ) |
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Haggis Hunter
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Joined: 29 Aug 2005
Posts: 2487
Location: The building site formally known as Edinburgh!
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| Posted: Thu Sep 15, 2005 8:38 am Post subject: |
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Perhaps a note to the approvers may get the co-ords changed?
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Naefearjustbeer
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Joined: 20 Sep 2005
Posts: 118
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| Posted: Tue Sep 20, 2005 11:03 pm Post subject: |
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| Wildlifewriter wrote: | | Billy Twigger wrote: | | So do you reckon that's what's happened here Corncrake Sound |
Not in this case, no. That would put the cache at N58º 35.902 W4º 46.362 - which it isn't, as you know.
It's always suspicious when published cache co-ords are a nice round number - as in: N58° 35.920 W004° 46.280 for this one. I can't say what happened, though.
Given the duff co-ords, AND the remoteness of the site, it's amazing that so many people have found it!
-Wlw. |
As one of the finders I believe that when i got onto the hill I stopped looking at the gps and used my cachers eye for a spot and found it fairly quickly. Quite funny really as my cachers eye sometimes leads me on wild goose chases at easyer caches, this time it was a corncrake chase and it scored a bullseye.
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Firth of Forth
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Joined: 29 Aug 2005
Posts: 1493
Location: East Lothian, Scotland
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| Posted: Sun Oct 02, 2005 8:51 pm Post subject: |
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I wonder if that's the problem with this cache at Lochnagar.
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Wildlifewriter
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Joined: 04 Aug 2005
Posts: 948
Location: Norn Iron
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| Posted: Thu Oct 06, 2005 1:21 am Post subject: |
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| Firth of Forth wrote: | | I wonder if that's the problem with this cache at Lochnagar. |
Been too busy for a while, to study this one... 
It doesn't look like a datum problem here, either. All their other quoted waypoints are wrong, but by varying amounts and in different directions.
The whole things a mess. Found:2, Hidden:1 - it's the old story, I'm afraid...
-Wlw. |
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Wildlifewriter
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Joined: 04 Aug 2005
Posts: 948
Location: Norn Iron
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| Posted: Wed Nov 09, 2005 7:23 pm Post subject: |
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Just to prove that this sort of this DOES happen: here a newly-approved cache Trostan Trove with the classic error.
-Wlw. |
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HighlandNick
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Joined: 30 Aug 2005
Posts: 635
Location: Highlands, Scotland
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| Posted: Wed Nov 09, 2005 11:10 pm Post subject: |
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| Wildlifewriter wrote: | | Just to prove that this sort of this DOES happen: here a newly-approved cache Trostan Trove with the classic error. |
How can you tell - have you been or do you know just by looking at the page?? |
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Wildlifewriter
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Joined: 04 Aug 2005
Posts: 948
Location: Norn Iron
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| Posted: Wed Nov 09, 2005 11:28 pm Post subject: |
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| HighlandNick wrote: | | How can you tell - have you been or do you know just by looking at the page?? |
Instinct...
In this case, it's easy peasy lemon squezy - the cache page gives a grid ref in the "hint" section. Plotting grid ref against the listed co-ordinates, shows that it is 68 metres away, bearing 269º.
Just as indicated on my "ready reckoner" table at the top of this thread.
-Wlw. |
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Haggis Hunter
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Joined: 29 Aug 2005
Posts: 2487
Location: The building site formally known as Edinburgh!
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| Posted: Thu Nov 10, 2005 10:16 am Post subject: |
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| Wildlifewriter wrote: | | HighlandNick wrote: | | How can you tell - have you been or do you know just by looking at the page?? |
Instinct...
In this case, it's easy peasy lemon squezy - the cache page gives a grid ref in the "hint" section. Plotting grid ref against the listed co-ordinates, shows that it is 68 metres away, bearing 269º.
Just as indicated on my "ready reckoner" table at the top of this thread.
-Wlw. |
So it could be that the co-ords are good, and the grid reference is out?
To be honest reading a map on the ground, I don't think I would get any better than 100 metres, unless there is a definite object that you can get down to an 8 figure grid reference.
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Wildlifewriter
Founder member
Joined: 04 Aug 2005
Posts: 948
Location: Norn Iron
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| Posted: Thu Nov 10, 2005 11:43 am Post subject: |
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| Haggis Hunter wrote: | | So it could be that the co-ords are good, and the grid reference is out? |
That is possible, of course, though I doubt it. There's clearly a datum error in one direction or the other, because the correspondence is exact - which is (as you rightly point out) almost impossible to do just from looking at ground features on a map.
The only way of checking this is for someone to go and find the cache - if it ever stops raining here, which seems unlikely at the moment... 
-Wlw. |
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Seacon
Founder member
Joined: 16 Sep 2005
Posts: 56
Location: Northern Ireland
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| Posted: Mon Nov 14, 2005 11:49 pm Post subject: |
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| Wildlifewriter wrote: | | Haggis Hunter wrote: | | So it could be that the co-ords are good, and the grid reference is out? |
That is possible, of course, though I doubt it. There's clearly a datum error in one direction or the other, because the correspondence is exact - which is (as you rightly point out) almost impossible to do just from looking at ground features on a map.
The only way of checking this is for someone to go and find the cache - if it ever stops raining here, which seems unlikely at the moment...
-Wlw. |
Found this cache with coordinates as listed. Yes, they may have been a bit out but the photo clue made it quite an easy find. I suspect that even with accurate coordinates that this photo clue may have been required.
This cache appears to have been hidden by a rambling club. The log book was signed initially by about 10 people many of whom are undoubtedly watching the cache page with anticipation of finds. These people are all potential new converts to geocaching and I'm delighted that they have hidden the cache in what is an outstanding location with excellent views over the County Antrim plateau and beyond to Scotland.
We certainly enjoyed the walk over peat bogs, mountain streams and mini cascading waterfalls. I'd settle for "more of the same the same please" anyday.
We should encourage more people into geocaching. They may make a few initial mistakes but we should not discourage their efforts.
I say, "the more participants the better!"
Trostan Trove Cache http://www.geocaching.com/seek/cache_details.aspx?guid=d1ac48ff-51d8-4b49-9c6f-7bff2fcf73f9 |
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Wildlifewriter
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Joined: 04 Aug 2005
Posts: 948
Location: Norn Iron
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| Posted: Tue Nov 15, 2005 12:43 am Post subject: |
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| Seacon wrote: | We should encourage more people into geocaching. They may make a few initial mistakes but we should not discourage their efforts.
I say, "the more participants the better!" |
I agree absolutely.
However, if no-one points out fundamental errors like happened in this case, (and the co-ords WERE out by the exact figure that I expected.) then how are beginners to know that a mistake has been made?
-Wlw. |
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