Figure 2-3. Line Leveling

TM 5-6675-329-13 & P

At the first instrument set-up, the telescope is pointed at a leveling staff held on station A and a reading taken. This is

the initial backsight reading R1. After the reading has been

Figure 2-3. Line Leveling

taken, the staffman carries the leveling staff to the instrument, counting his paces as he goes.. He then proceeds away

from the instrument the same number of paces and sets up the instrument-to-staff at station B. Instrument and leveling

staff stations must be selected carefully on firm ground so that neither instrument nor leveling staff will sink into the

ground. The observer then points to the new leveling staff station and takes the initial foresight reading V1. The

observer then carries his instrument to the next set-up 2 and the staffman carefully turns his leveling staff on station B to

face the new instrument station. Now the observer takes the second backsight reading R2 and the leveling continues in

this manner towards station E. At the final instrument set-up 4 the observer must ensure that the sighting distances to

staff stations D and E are of equal length. The reason for having equal backsights and foresights is to eliminate

instrumental and physical errors (collmination error and the influences of curvature and refraction). This is important for

accurate leveling.

The difference in elevation between stations A and B is obtained from the difference of staff readings at stations A and B,

i.e. R1-V1; this is, of course, a positive value if station B is higher than station A. Similarly the difference in elevation

between stations B and C is R2-V2, and so on. Now, the difference in elevation between stations A and E is equal to the

sum of all R-V differences; it is also equal to the sum of all R readings minus the sum of all V readings. The identity ∑

(R-V) = ∑ R- ∑ V provides a useful check. To guard against gross errors, the line is usually leveled in both directions, i.e.

from station A to E and then from station E to A. To check against reading errors, it is useful to read the two stadia hairs

as the mean of the stadia readings should equal the reading of the horizontal hair. If the best performance of the

instrument is desired, it is advisable to shade it with an umbrella in sunny conditions.

2-10