Establishing Horizontal Control

Establishing Horizontal Control

1. Identify and mark a number of key fixed points that will serve as control (reference) for all subsequent surveying. These might be points located on the boundary of the area to be mapped, points of maximum or minimum elevation, significant features, etc.. At least 3 points should be clearly visible from each point. Consider the visibility (e.g. background) of each control point and its susceptibility to disturbance, since they will remain for the duration of the survey or longer.
2. Establish the baseline, an accurately measured distance between 2 control points. The baseline should occupy fairly level ground (< 5º). Measure the baseline distance to the nearest 10 mm using a 50 m tape. If the distance is greater then 50 m mark the tape lengths with survey pins. Measure the baseline length in both directions. If the 2 measurements differ by more than 50 mm repeat both measurements, otherwise use the mean of the two distances. Measure the vertical angle between control points, so that the surface length can be reduced to a horizontal distance.
3. Station the tripod over a control point.
4. Mount the transit (theodolite) on the tripod. Slide the transit laterally until it is positioned directly over the control point as viewed through the optical plummet. Turn the 3 leveling screws unit the bubbles are centered in the 2 orthogonal spirit levels.
5. Release the upper set screw. Rotate the telescope about the vertical axis until the 0 mark on the vernier scale is aligned with the proper magnetic declination on the outer horizontal scale. Small adjustments can be made by tightening the set screw and using the tangent screw. Tighten the upper set screw and release the lower set screw. Release the magnetic needle and rotate the telescope about the vertical axis until the needle is suspended in the centre of its housing. Tighten the lower set screw. The telescope is now oriented in a magnetic north-south direction and the reading on the horizontal scale should be the magnetic declination. Thus all readings on the horizontal scale will now be in degrees east of true north
6. Record the horizontal angle to each of the control points visible from the instrument station. Sight on a stadia rod or ranging pole. The focusing screw is mounted on the outside of the standard not housing the vertical circle. The focusing ring for the cross hairs is located on the eyepiece. Rotate the telescope clockwise and close the round of observations by repeating the measurement on the first target. If the 1st and last readings differ by < 15 ", use the mean value. If they differ by > 15 ", the round of measurements should be repeated.
7. Rotate the telescope about its vertical axis by 1800. The telescope set screw is adjacent to the telescope. The tangent screw for fine adjustment of the inclination of the telescope in located on the standard with the focusing screw. Conduct another round of angle measurements according to instruction 6. The face-left and face-right angles can be averaged unless they differ by > 15", in which case, all the horizontal angle observations must be repeated.
8. Move the transit and repeat instructions 3-8 until horizontal angles have been established between all control points.