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Team Ballibeg Member
Joined: 14 Oct 2005 Posts: 150
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Posted: Wed Nov 30, 2005 6:42 pm Post subject: Getting good co-ords for a cache under trees |
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Just placed a new cache which had its first visit. Co-ords described as slightly off. The cache is under tree cover and I took a series of readings over a few days and averaged them out.
Is there a better technique? I am toying with the idea of using my old map and compass skills to measure a distance and bearing to a place of tree free reception and then using the 'project waypoint' on my GPSr to get more accurate co-ords.
Anyone tried it?
Dave |
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Wildlifewriter Founder member
Joined: 04 Aug 2005 Posts: 948 Location: Norn Iron
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Posted: Wed Nov 30, 2005 6:53 pm Post subject: |
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I wrote the following article on another forum, some time ago. It's rather rambling and incoherent - but some of the info might be useful -Wlw
Woodland and forest caches...
“Due to tree cover, my GPS wasn't working very well when I hid the cache, but you should be able to find it if you hunt around a bit.” How many times have you read this – or something like it – on a cache page? The placer doesn't know exactly where the cache is, but we're expected to be able to find it, anyway!
There's really no need for this kind of thing. With a little thought and some preparation, it IS possible to hide a cache in wooded area, and still be confident of listing it correctly. Here are some tips to follow, when planning your new cache.
(1) If the chosen hide area is under foliage, make sure that there is a broad clearing or other open space nearby. (Preferably, more than one.) This will enable you to work out accurate co-ords (see below) and give seekers some chance of doing the same. If decent sat-lock cannot be obtained ANYWHERE near the place (specially in summer), then choose somewhere else!
(2) If your GPSr has a 'project waypoint' feature, read the manual and figure out how to use it. However, there's a problem with some Garmin receivers, because this facility doesn't work very well for geocaching. (The measurement is in main units – Miles or Km)
(3) Make sure that you can give a sensible clue, based on some distinctive feature close by. “It's under a medium-sized rock” is no use if there are 200 such rocks within a 20-metre radius!
If all else fails, you can STILL work out a pretty accurate fix – provided that condition (1) above is satisfied. You will need: (i) A good magnetic compass – Silva, or similar. (ii) A ball of string – unless you are a golfer.
First, measure and cut two lengths of string – one 22m long and the other 37m. Find an open space where you can peg these out on the ground. You can use the back garden, which will provide endless amusement for the family.
Now... practice pacing off these distances until you can do it accurately and repeatably. (Keen golfers can do this already.) Do multiples and halfs of the distances, as well, and check them. If you find this impossible, you'll just have to carry the string around with you, when placing caches.
Having mastered pacing, the next thing is to memorise this: W5.6
5.6 degrees West is the current geodetic variation of the compass, in Ireland. What does this mean? It means that, if the cache is due north (0º True) from where we are, as the GPS sees it, our compass will point 5.6º West of this point. Therefore, we add 5.6º to any relative bearing taken. How does this all help? Let's take an imaginary example:
You found a good spot for a cache, just inside wooded area – but your GPS won't give an accurate reading in there. There's an open space due east of the position (that's lucky) where your GPS works fine.
Hang your hat on a convenient tree at the cache site, so that you can see it from the clearing. Now, pace off 22 metres – or 44 metres, or whatever multiple you need to get into a clear spot. Position yourself so that the cache bears 275.6º magnetic from where you are. You might need to re-check both until distance AND bearing are as accurate as you can get them. (0.1 of a degree is difficult to distinguish with a hand-held compass. Just let the the thing do its best.)
Mark this spot, and set your GPSr there, giving it time to “settle down” and get a good fix. Have a Kit-Kat.
Note the waypoint, and do a few repeats to get a decent average. Ideally, come back the next day, and plot it again.
Having established a good position for the marker, all this stuff about bearings and distances becomes clear (at last). Each 'unit' of 22 metres represents almost exactly 2/100ths of a longitude minute. Latitude has a different interval, and the precise values are shown in the table below.
This is why we needed to get an accurate distance AND why we needed to be either due N, S, E or W of the cache – because it's now easy to work out the cache co-ords without doing any nasty trigonometry. If you are like me, and forgot everything about trigonometry on the day you left school, this is A Good Thing.
Suppose the reference point we got was N53º 32.410 W7º 58.097 – because the proposed cache location is due WEST of this, and we cunningly corrected for the variation already, it's just a matter of ADDING the 22 metres we paced off. 22 metres, remember, is 0.02 of a minute, so the position we want is N53º 32.410 W7º 58.117
If instead, we'd had to go 66 metres east, it would be N53º 32.410 W7º 58.157
Alternatively, 37 metres south gives N53º 32.430 W7º 58.097
In use, this method isn't half as complicated as it sounds here – and it works.
-Wlw
NOTES:
1)You can type the symbol for “Degrees” on a PC by holding down <ALT> and entering 167
2) I realise that the positions used here are in the middle of Lough Ree. That happens with hypothetical examples. There is no cache there.
3) The 'projected' location will be no more accurate than the error in the marker position x the error in distance and bearing measurement. This is still closer than the width of most nettle patches, though.
4) The magnetic variation changes over time, but we're OK for a few weeks yet. |
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HighlandNick Founder member
Joined: 30 Aug 2005 Posts: 635 Location: Highlands, Scotland
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Posted: Wed Nov 30, 2005 10:39 pm Post subject: |
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Here's a map which gives a rough idea of how magnetic variation changes around the country:
The red blob is roughly where I am and is just less than 4 degrees west. |
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Naefearjustbeer Founder member
Joined: 20 Sep 2005 Posts: 118
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Posted: Thu Dec 01, 2005 12:49 am Post subject: |
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HighlandNick wrote: | The red blob is roughly where I am and is just less than 4 degrees west. |
my goodness Nick did the top of your head get sunburned? .
When I am searching for caches under trees I tend to walk to a clearing and then allow the gps to settle for a while then start walking towards it at a steady pace then look at the distance, look in the direction and forget about the gps and pace out what I think the distance is. After that start looking under every rock and branch LOL. However ABC for BT was one that needed the clue to find as I doubt anyone could find it easily purely by gps use. As for hiding uner tree cover, not many trees in Caithness plenty whin bushes though _________________ www.naefearjustbeer.co.uk |
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Team Ballibeg Member
Joined: 14 Oct 2005 Posts: 150
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Posted: Thu Dec 01, 2005 9:37 am Post subject: |
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Thanks for the tips.
I'll use the method and see what co-ords I come up with. Then I guess I'll be asking the etiquette for changing the co-ords on the cache page....
I'm somewhere on the learning curve!
Dave |
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ghiribizzo Member
Joined: 22 Nov 2005 Posts: 510 Location: Ferryport-on-Craig. The Kingdom of Fife.
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Posted: Thu Dec 01, 2005 10:27 am Post subject: |
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On the learning curve is OK. Problems arise when you find yourself abandoned on the learning plateau. _________________ Anyone for a Hot Cross bun and a Creme Egg? |
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HighlandNick Founder member
Joined: 30 Aug 2005 Posts: 635 Location: Highlands, Scotland
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Posted: Fri Dec 02, 2005 12:02 am Post subject: |
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I should have said that you have to go to this page:
http://www.geomag.bgs.ac.uk/gifs/gma_calc.html
and put in your OS grid reference. This produces the map as shown in my original posting which gives the magnetic variation and shows where you are with a red blob at the same time. Of course, the grid "lines" shown on the map are just lat / long coords and not the mag. variation.
You would probably need to look at an OS map paper copy to find your 12 figure grid ref that this web page requires. Naturally, whilst you are there, you can look up the magnetic variation which is on the map anyway.
Hey Donnie... Rearrange these words into a well known phrase or saying:
Pot, kettle, black etc etc.... |
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Billy Twigger Founder member
Joined: 30 Aug 2005 Posts: 352 Location: N55 51.686 W5 05.647
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Posted: Fri Dec 02, 2005 4:14 pm Post subject: |
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HighlandNick wrote: | You would probably need to look at an OS map paper copy to find your 12 figure grid ref that this web page requires. |
Is there another way of getting these. I'm at NS 06173 67526 What is that in old money? |
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HighlandNick Founder member
Joined: 30 Aug 2005 Posts: 635 Location: Highlands, Scotland
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Posted: Fri Dec 02, 2005 9:33 pm Post subject: |
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Billy Twigger wrote: | HighlandNick wrote: | You would probably need to look at an OS map paper copy to find your 12 figure grid ref that this web page requires. |
Is there another way of getting these. I'm at NS 06173 67526 What is that in old money? |
Yes, there is. You can use this image... here
Your square is NS. The bottom left hand corner of this has coordinates 200, 600. Insert 2 (from 200) in front of your first 5 digits to give 206173 and put 6 (from 600) in front of the second 5 to give 662576. Note that in far north latitudes, the second set would have 7 digits which gives a 13 digit grid ref overall.
Then go to the first link for the mag var calculation and put in your two sets of 6 digits. You can check that you are in the right place by looking at the map to see if the red (sunburned?) blob is in fact on Bute roughly.
I'll leave you to call this one up but I can tell you that your mag var there is 2 deg 47 min west.... |
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Wildlifewriter Founder member
Joined: 04 Aug 2005 Posts: 948 Location: Norn Iron
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Posted: Fri Dec 02, 2005 10:22 pm Post subject: |
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HighlandNick wrote: | I'll leave you to call this one up but I can tell you that your mag var there is 2 deg 47 min west.... |
Just for the sake of clarification (and at the very real risk of sounding like The Forester, for which I apologise in advance) Nick's map shows variation relevant to the OSGB Grid - not true North.
As Wuthered and I found out when doing THIS CACHE yesterday, the difference can sometimes be important.
(But not important enough to stop him finding the cache straight away... )
-Wlw. |
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HighlandNick Founder member
Joined: 30 Aug 2005 Posts: 635 Location: Highlands, Scotland
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Posted: Fri Dec 02, 2005 11:08 pm Post subject: |
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Wildlifewriter wrote: | Nick's map shows variation relevant to the OSGB Grid - not true North.
-Wlw. |
I'd better shut up then and not try to be so clever..... |
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Wildlifewriter Founder member
Joined: 04 Aug 2005 Posts: 948 Location: Norn Iron
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Posted: Fri Dec 02, 2005 11:13 pm Post subject: |
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HighlandNick wrote: | I'd better shut up then and not try to be so clever..... |
Darn it...
I'm sorry, I'm sorry - I promise not to be The Forester any more.
-Wlw. |
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barryhunter Member
Joined: 06 Oct 2005 Posts: 9 Location: Snowdonia, Wales
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Posted: Thu Dec 08, 2005 12:01 am Post subject: |
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Billy Twigger wrote: | HighlandNick wrote: | You would probably need to look at an OS map paper copy to find your 12 figure grid ref that this web page requires. |
Is there another way of getting these. I'm at NS 06173 67526 What is that in old money? |
Just enter virtually any coordinate* into www.nearby.org.uk and look for the "Magnetic Variation" link in the OSGB Grid Ref section, it will forward the location in the correct format for you
* grid ref, lat/long, postcode, even NAC or IARU Locator.... _________________ - www.nearby.org.uk - www.trigtools.co.uk - www.geograph.co.uk - |
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Jacobite Founder member
Joined: 02 Sep 2005 Posts: 459 Location: Longformacus, Scotland.
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Posted: Thu Dec 08, 2005 4:58 am Post subject: |
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1. Buy a chainsaw.
2. Fell the trees.
3 Place your cache.
4. Get the co-ords.
5. Problem solved ! |
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HighlandNick Founder member
Joined: 30 Aug 2005 Posts: 635 Location: Highlands, Scotland
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Posted: Thu Dec 08, 2005 9:56 pm Post subject: |
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barryhunter wrote: |
Just enter virtually any coordinate* into www.nearby.org.uk and look for the "Magnetic Variation" link in the OSGB Grid Ref section, it will forward the location in the correct format for you
* grid ref, lat/long, postcode, even NAC or IARU Locator.... |
An excellent set of sites in my opinion. Worth a visit!! Thanks to BarryHunter. |
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