On backsites and Hawaii caving

[Patricia N Kambesis]

As Hazel, I also stand close to the instrument reader and ask them to
quitely give me the data - so as not to bias a second instrument reader.

Also, it may seem like figuring out in your head whether back and front
sites agree can be time difficult and time consuming - but its not if you
do the following.

If the reading is between 0 and 179 degrees, subtract 20 degrees and add
200.
If the reading is between 180 and 359, add 20 degrees and substract 200. 


With a little practice you can figure out if the shots agree pretty
quickly.

For Hawaii caving, we do take backsites.  If you can stay away from the
highly magentic lava passage lining (easy in most places except small
passages), then the magnetics of the lava usually don't affect the
compass readings.  One has to be careful about setting stations on
breakdown since some pieces (but not all) are very magnetic.  But there
are some situations where one just can't get the back and fore sites to
agree.  However, one of the advantages of lava tubes in Hawaii is that
they have many entrances - so we make a point of surveying out of the
entrances and tying into a surface benchmark (or some recoverable
station).  We usually do a surface survey and then tie all of the lava
tube entrances to that survey.  The surface survey line is used as the
"backbone" survey.  This is really the only way to assure that the
magenetics of the lava don't completely throw off your survey line.

Pat K.