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Bill Morris wrote:
> When I checked it, the theodolite turned out to have a substantial index 
error, quite enough to account for most of the differences between the dip 
from the NA and the dip I measured.
>
>  I have negotiated another visit to the beach with the staff (wo)man and 
have removed nearly all the index error, a none-too-easy task with a bubble 
that has a sensitivity of 15 seconds per 2.5 mm movement. I will take 
readings off each face (not strictly necessary with the T2) if my assistant's 
patience will allow it.

Bill, a mean of the readings on both faces is always good technique. It
simultaneously reveals and eliminates index error. The T2 manual
discourages single face vertical angle readings, except when one minute
of error is acceptable and the theodolite is known to be in good
adjustment (index error not more than 30 seconds).

That is from the Wild manual for the late type T2 with automatic
compensator. I understand yours has the manually centered vertical index
bubble. However, I believe the recommendation above is still applicable,
since the compensator only removes the effect of minor dislevelment, not
index error.

Even for their first order theodolite, the T3, Wild says the horizontal
collimation and vertical index errors should be left alone unless they
exceed 15 or 30 seconds, respectively. I have been able to adjust to
that modest accuracy indoors by sighting through a window - far more
comfortable than fiddling with the instrument outdoors.

For your dip measurements, perhaps a reasonable compromise would be to
do half on one face, half on the other. Or, begin by taking the vertical
angle on both faces to some suitable target (not necessarily the
horizon). Then do all dip measurements on one face. Finally, take
another pair of vertical angles on both faces. The final target need not
be the same as the first. In fact, a different target would be better -
you would be unlikely to make the same reading blunder in the
microscope. Before leaving the site, calculate and compare the initial
and final index errors.

I agree with your remark that the scale reading microscope "has its
limits and variations." The U.S. Coast & Geodetic Survey first order
triangulation manual says two successive readings on a Wild T3 should
rarely differ by .3", and usually agree to .1". I'm not that good -
"usually within .3, rarely within .1" is closer to my standard!

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