E. Hydrometric Levelling Procedures
Before stage/discharge curve can be developed, gauge heights must be confirmed or adjusted. This section sets out the procedures and note-keeping format to be used for leveling in hydrometric operations (measurement of stage and the maintenance of station datum). The methods described must be strictly adhered to so that the data can be properly reviewed at a later date. Many of the methods in the section could be described as basic survey practice; however, procedures and documentation peculiar to hydrometric operations require these basics be carefully reviewed as a part of standard hydrometric operational requirements. In addition, some less frequently employed techniques are described, such as reciprocal leveling, i.e. transferring datum across a lake.
The following section has been prepared from material supplied by Environment Canada staff R.A. Terzi, E. Mayert, and D.G. Goller.
The two-peg test ensures that the level is accurately calibrated, i.e. that the line of sight through the telescope is parallel to the axis of the level tube (Figure E-1).
Figure E-1. Principles of the two-peg test.
Establish two firm points, A and B, about 60 m apart. Each point can be marked with a peg or a stake. Set up and level the instrument exactly midway between A and B. Take a rod reading a on point A, and a rod reading b on point B. The correct difference in elevation between points A and B is a - b; this is true even if the level is not sighting a true horizontal line, because the instrument is set up exactly midway.
Next, move the level to point A.
Set up and level the instrument so the eyepiece will swing within one centimeter of the rod held on point A.
Looking through the objective lens (looking backward through the telescope) record the rod reading c at A. As the crosshairs will not be visible, use a pencil to locate the exact rod reading on the rod. Now take a reading on point B to obtain reading d. (The difference between these readings should equal the correct difference established earlier.)
If these `Differences in Elevation' are equal, the instrument is in proper adjustment:
i.e., a - b = c - d.
If the last reading does not check, the instrument must be adjusted so that the rod reading on point B is e = b + c - a. See the following examples.
Example 1: Level in adjustment
a = 1.510
b = 2.230
a - b = -0.720
c = 1.730
d = 2.450
c - d = -0.720
Example 2: Level out of adjustment
a = 1.390
b = 2.110
a - b = 0.720
c = 1.630
d = 2.310
c - d = -0.680
Adjust level so rod b reading is e = b + c - a = 2.350.
Any adjustments to the instrument must be carried out while the instrument is still in position at point A. The manual supplied with the level instrument will describe the method of adjustment for each type of instrument. If adjustment is required, it is recommended that the entire level check process be repeated.
