Backsights

[P A Hill & E V Goodall]

John Halleck wrote:
>>I've been horribly guilty of doing this myself, when we're in a rush, it's
>>pizza for dinner - and it's such a pain to try and get the reading to agree
>>within a degree. Of course, these are usually my projects, so I deny
>>everything!!
> 
> 
>   For some surveys, they won't agree within a degree...  because magnetic
>   north doesn't agree within a degree between stations.
>  

Given that:
1. Garry Petrie is in on this thread
2. I know he has many kilometers of  lava tube/flow survey.
3. One of the theories is that the magnetic driection 'frozen' in the lava 
when it cooled can effect the compass when held close enough to the rock.

I was wondering:

Garry, have you ever seen the foresite and backsite not agreeing because of 
a magnetic bias do to a instrument too close to either the 
wall/floor/ceiling of the cave or too close to a peice of breakdown?

Anyone else ever actually observed this in the field?

Pat, you have had some involvement in Hawaii right?
Do they fore- and backsite on those surveys?

Curious to know,

-Paul