Backsights (and lava caves)

[P A Hill & E V Goodall]

Ken Grimes wrote:

I'll leave John H. to give the lecture about using turned angles as a 
possible solution.

> In the Lava caves of Victoria (Australia) I have had differences of up
 > to 29 degrees between fore- and back-sights

[...]

 >As you move the compass around the rock the needle swings to always point 
 >to the top of the rock.

All I can say is excellent! These are great examples.

So how from from the rock did you have to be to get what appeared to be a 
clean reading?

> Fortunately my main objective has always been to produce
 > a picture of what the cave looks like rather
 > than an accuracte location of its parts for engineering/survey purposes.
 > So the survey line is just for control of the sketching.

But the problem may be that random shots are 'bent' depending on how close 
you are to something with its own magnetict alignment that is different 
than magnetic north, so you could have a cave that has many shots bent in 
different directions as a function of nearby anolmolies, be they walls, 
rocks, ceiling or floors.

Pat, your description of Hawaii surveying suggested that the problem falls 
off fairly quickly when you are not near the walls.  Your suggestion that
surface surveys don't have this problem would suggest you are comfortable 
that the effect at head height from a lava ground is negligable.  Does 
anyone have data to support this idea that about 1 meter (or maybe less) is 
where the effect falls off to the order of precision of a hand-held survey?

No, I am not suggesting that Pat is only 1 meter tall!  I'm just using a 
rough value based on what she said.

So is 1 meter a reasonable safe distance?

Someone reassure me, am I right that the walls in a lava tube freeze the 
current magnetic direction at the time lava solidified?

-Paul