[Team Ballibeg]

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

[Wildlifewriter]

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.

[HighlandNick]

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.

[Naefearjustbeer]

[Team Ballibeg]

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

[ghiribizzo]

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?

[HighlandNick]

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....

[Billy Twigger]

[HighlandNick]

[Wildlifewriter]

[HighlandNick]

[Wildlifewriter]

[barryhunter]

[Jacobite]

1. Buy a chainsaw.

2. Fell the trees.

3 Place your cache.

4. Get the co-ords.

5. Problem solved !

[HighlandNick]