Set up and level the instrument.
Read all three crosshairs as projected on the face of rod number 1, with the bubble
continuously in coincidence.
Read and record each crosshair to the nearest 0.001 meter, as shown in the example in Figure
4-7, columns (7) and (13). Always read the three crosshairs in their order of magnitude on
the rod, beginning with the highest value. These values are averaged for the final values. In
this process, the half (the difference between the top and the middle and the middle and the
bottom crosshairs) and the total stadia intervals are determined and recorded. Any time that
the half-thread intervals do not agree to within 0.003 meter, reobserve all readings. If the
difference after reobservation is exactly 3 millimeters, the divergence is acceptable. A note
should be added in the remarks column that the foresight or backsight observation was
repeated.
(1) Check the readings taken on the Invar strip on the front of the rod by reading the center
crosshair on the back of the rod to the nearest hundredth of a foot. The mean of the three thread
readings, converted to feet, must check within 0.02 foot.
(2) Instrument stations are numbered consecutively for a day's work. At odd-numbered
instrument stations, take the readings at the backsights first; at even-numbered stations, take the
foresight reading first. In this method, the same rod is held on a turning point for both the foresights and
backsights, so the same rod is read first at each setup. It is the one used for the backsights at the first
instrument station of the day's work. Use leveling platforms or metal pins driven into the ground for
turning points.
(3) Sections are named in their forward direction. Forward or backward denotes the direction in
which the level line is run. The backward measurement on any section of a line running in both
directions should be made under different atmospheric conditions from those on the forward
measurement.
(4) Shade the instrument from the direct rays of the sun at all times. Since the wind and the sun
are possible sources of error, record their direction and intensity in the level book in the spaces provided
at the top of the page (as shown in Figure 4-7, (2) and (4)). An explanation of the numbers used after
the words sun and wind is shown in Figure 4-8, page 4-18.
EN0593
4-16
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