STANDARD - Survey Instrument Antique Center!

90
included between this and the other wire ;
this multiply mentally by 100 (the constant
&) which gives the direct distance D'.
In cases where it is not possible to read with both stadia wires, it is the custom
to use but one of them in connection with the center wire, and then to double the
reading thus obtained. "With very large vertical angles, this custom is not advis-
able, as the error may amount to 0.50 % .
Fig,4,
To find the height of the point where the stadia stands above that one of the
Instrument, simultaneously with the distance, we have the :
following
We assume in reference to figure 4,
9 = height of instrument point above datum.
MP= D = horizontal distance as derived from formula (3).
n = vertical angle.
h = FE = stadia reading of the center wire.
Q = height of stadia point above datum it is
;
Q = q -f D tan n h.
The substraction of h can be made directly by the instrument, by sighting with
the center wire to that point of the rod, which is equal to the height of the telescope
above the ground (which is in most cases =4.5") ;
the same instrument point ; then the formula :
Q = D tan n ;
this in connection with formula (3) gives
Q = c sin n -f-
or Q = csinn a.
large ;
& cos n. sin n.
j.
q will be constant for one and
The first term of the equation can be neglected, when the vertical angle is not too
hence the final formula for the height is
(5; Q_a.fc. 8in2n
The position of the stadia must be strictly vertical.
The error increases with the height of m; (m = height of center wire on the
rod). In shorter distances the result is seven-fold better when the center wire is
placed as low as one foot than it is at 10' ; in longer distances this advantage is
only double.
It is always better to place the center wire as low as possible. If the stadia is
provided with a good circular level, the rodman ought to be able to hold it vertically
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