University Of Washington
Department of Civil Engineering
CEE 317
ThreeWire Leveling
During this lab you will learn how to compute and adjust the elevation of unknown points knowing the elevation of one vertical control point (bench mark). The starting point will be point (909), which is located at the north west corner of More Hall and of elevation 108.92 ft. Follow the loop described in the lab lecture to collect rod readings ( backsights and foresights ), compute the elevations of the unknown points, and finally adjust their elevations.
Definitions
Backsight: is the first reading on the rod after setting up the level.
Foresight: is the last reading before removing the level.
Turning point: is a point where both backsight and foresight readings are taken. The rod should rotate when the level changes position, without moving the rod.
Closed loop: is the one that starts and ends at points of known elevations, it may and it may not start and end at the same point.
What To do
Set the rod on point 909 and the level at a reasonable distance at the direction you choose your loop to proceed. Read the rod and record that backsight reading at 909. Move the rod to a point in the direction of the loop at a distance almost equal to the distance between the level and 909. Record the reading at that point: a foresight at turning point 1 (T1). Move the level around the rod, without moving the rod, to a new point as close as possible to 100 ft, turn the rod to face the level and read the backsight reading at T1. Move the rod to another point, without moving the level and take a foresight. Iterate that procedure until you reach point 909 again. The last reading will be a foresight at 909.
The table of observations is as follows:
Rod Point 
BS 
D 
FS 
D 
909 
Upper(U) Middle(M) Lower(L)
First Reading 
100(UL) 
Last Reading 
100(UL) 
T1 
Third Reading 
… 
Second Reading 
… 
….. 
…. 
… 
…. 
… 
Tn 
Next to last reading 
… 
… 

SBS 
Sd_{BS} 
SFS 
Sd_{FS} 

Remarks
1 Use the following equation to compute the elevation of any point knowing the elevation of the previous point:
Elevation of point (i) =
elevation of point (i1) + backsight reading on point (i1)  foresight reading on point (i) ...... (1)
2 When you are done with a closed loop, use the following equation to check for errors in readings:
å B.S  å F.S. = the difference in elevation between the first and the last point.......... .(2)
If the loop starts and ends at the same point, the right hand side of the equation will be zero. When you substitute in equation (2) with actual readings, the two sides will not be equal, that is due to the errors in the measurements. The difference between the two sides is the overall misclosure (w).
3 Use the following equation to distribute (w) over the readings, according to each distance (di) between the level and the rod at the time of reading:
Correction per unit length = w / di = C .............(3)
Correction for each reading (i) = di * C = Ci .............(4)
Use the values of Ci to correct each reading, paying attention to the signs.
4 To reduce the reading error due to the deviation of the line of sight from the horizontal, try to keep the backsight and foresight distances as equal as possible for each position of the level and less than 100 ft. When choosing the rod positions use any approximate method to estimate the distance between the rod and the level.
5 When taking readings on the rod, record the upper, middle, and lower readings. Use the upper and lower readings to compute the distances, using the stadia equation:
di = 100 (U  L) ...................(5)
6 The level doesn't have to be on the line connecting the two rod points.
7 Your loop should start and end at point (909). The unknown points are points (24) and (20). The loop will go around More Hall, Wilcox Hall, and Robert's Hall.
What to check out
A level, a tripod, and a stadia rod.