GEOG 411/811 - Establishing Vertical Control
Identify a traverse (circuit) consisting of a series of survey control points that begin and end at the same point (closed-loop traverse) or a points of known elevation (closed traverse).
Station the tripod midway between the first 2 control points. Mount and level the automatic level.
Record a backsight to the first control point by reading the elevation on the stadia rod at the main (centre) cross hair. The focusing ring is on the right side of the barrel of the telescope. The cross hairs are focused by turning the eyepiece.
Release the set screw located beneath the telescope on the front of the level. Rotate the telescope in the horizontal plane until a stadia rod stationed at the next control point is visible through the telescope. Record a foresight.
Move the level to a position midway between the 2nd and 3rd control points. Record a backsight to point 2 and a foresight to point 3. Repeat this process until the final observation which is always a foresight to the last control point.
If the distance between control points is more than several hundred metres, intermediate survey points (turning points) will likely be necessary especially in areas of moderate to high relief. The location of these turning points is not important, except the backsights and foresights from a single instrument station must be measured over approximately equal distances. This condition can be satisfied by pacing the distances or by ensuring that the stadia intervals (spacing between upper and lower cross hairs on the stadia rod) are approximately equal.
Level data is 'booked' by recording the turning point number, a backsight and a foresight. Note that the control points are also turning points and that the first record always consists of a single backsight, while the last record always consists of a single foresight.
The rod person should hold the stadia rod between his/her feet and eyes, and slowly move the rod towards and away from his/her body such that it passes through the vertical plane orthogonal to the line of sight. The instrument person reads the minimum value off the stadia rod since this represents the plumb position of the rod.