Levelling and Contouring Questions and Answers

Answer: A

The arithmetic check applies to back sights, intermediate sights and fore sights.

In the rise and fall method every reading, including each intermediate sight, is compared with the one before it to give a rise or a fall, and every reduced level is worked out from those rises and falls. The check

sum of B.S. − sum of F.S. = sum of rises − sum of falls = last R.L. − first R.L.

therefore tests the reduction of all the reduced levels, intermediate ones included. This is its advantage over the height of collimation method, whose usual check does not cover intermediate sights. Options that leave out the intermediate sights, or the fore sights, are incomplete.

Answer: C

The sensitivity is \(\frac{1}{500}\) radians.

The sensitivity of a bubble tube is the angle through which the axis turns when the bubble moves one division. For a small angle that is the length of a division divided by the radius of the tube, in the same units:

\[ \begin{aligned} \theta &= \frac{2\ \text{mm}}{1000\ \text{mm}} \\ &= \frac{1}{500}\ \text{rad} \end{aligned} \]

That is about 412 seconds of arc. The other options are out by a factor of 4, 10 or 100, the kind of error that mixing millimetres with centimetres or metres produces.