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4.3 Measuring Height Difference
(1) Set up the instrument at a point approximately halfway between points A and B (Fig 4.3A).
NOTE: The reticle stadia lines can be used to optically compare the distances.
See (Fig 4.2A)
(2) Position the rod vertically at point A. Take the reading
a
(backsight) on the rod at point A.
(3) Then sight the rod at point B and take the reading
b
(foresight).
(4) The difference (
a-b
) is the height difference (h) of B from A. (Fig 4.3A)
Example:
h=
a-b
=5.25ft-3.25ft =2.00ft
Therefore point B is 2.00ft higher than point A. (the value of h will be negative if point B is lower than point A.)
When the distance between points A and B is large or if the height distance is great:
(1) Divide the distance into a number of sections and determine the height difference of each section.
(2) The height difference between points A and B is the total of the height differences of all the sections.
The general formula is:
Altitude of the required point = altitude of known point + total of backsight values - total of foresight values. NOTE:
This simple leveling technique has no error check. It is better to measure from A to B and then measure back to
A so that the error of closure can be calculated.
