Backsights (and lava caves)

[Bill Frantz]

At 8:21 AM -0800 1/20/03, P A Hill & E V Goodall wrote:
>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?

My experience at Lava Beds National Monument (way northern California) is
that errors of between 10 and 20 degrees are not unusual.  I have heard of
a 175 degree error.  There have been places in the middle of a 20 foot by
15 tube (standing station) where we could not get the fore and back sights
to agree.

While the errors outside seem to be less than those inside, they are still
significant (to the 5 degree level).

We have used the fore and backsights as a way of calculating the "turn" at
each station in order to overcome some of these errors.  In order to do
this, you need to be sure that the compass is in the same place at a
station for both the fore and back sight.  (Even a few inches may introduce
a significant error.)  You also loose the self-checking that fore and back
sights normally give.

My current idea for a true solution to this problem is a new instrument.
It would hold two laser pointers, and sit on the station.  (Short tripods
might be used as stations with this method.  You would align one laser on
the previous station, and the other on the next and read the turn angle off
the instrument.  With 3 tripods, two reflectors, and the instrument the
survey would proceed by:

Pick up the back tripod and it's reflector.  Move to the middle tripod,
replace the instrument with the back reflector, and proceed to the forward
tripod.  Replace its reflector with the instrument, and take the tripod and
reflector that are left to make the new forward station.  This technique
would require that the reflectors and instrument easily pop on and off the
tripods.

Some outside stations, and a sight on stars, or geographic features would
be used to actually get your turns oriented to the main geographic grid.


At 11:43 PM -0800 1/19/03, Patricia N Kambesis wrote:
>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.

As a long time instrument reader, I avoided doing the arithmetic.  Only
after I started sketching did I practice enough so the answer started
coming to me without thinking.  However I have been involved in a number of
"negotiated" agreements, in both carbonate and lava caves, involving many
repeats of the shots.  I wonder how many of the failures to agree in
Lilburn (a marble cave), were the result of the many granite boulders which
have washed into the cave, rather than errors on the part of the instrument
people.

Good Caving - Bill


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