Thursday, December 1, 2016

Lesson Note On Carrying out a level traverse






Carrying out a level traverse
To determine the difference in level between points on the surface of the ground a 'series' of levels will need to be carried out; this is called a level traverse or level run.
There are two method of levelling:
  1. Rise & Fall method and
  2. Height of collimation (height of instrument) methods
Click the link to see the animated rise&fall methods then click next for the height of collimation method. Please note when the shifting of the staff or level can be done using the rise&fall method
Leveling or Field Procedures
The leveling or field procedure that should be followed is shown in Figure 1 below..
http://www.boeingconsult.com/tafe/ss&so/survey1/level/levelling.GIF
Figure 1
  1. Set up the leveling instrument at Level position 1.
  2. Hold the staff on the Datum (RL+50 m) and take a reading. This will be a backsight, because it is the first staff reading after the leveling instrument has been set up.
  3. Move the staff to A and take a reading. This will be an intermediate sight.
  4. Move the staff to B and take a reading. This also will be an intermediate sight.
  5. Move the staff to C and take a reading. This will be another intermediate sight.
  6. Move the staff to D and take a reading. This will be a foresight; because after this reading the level will be moved. (A changeplate should be placed on the ground to maintain the same level.)
  7. The distance between the stations should be measured and recorded in the field book (see Table 1)
  8. Set up the level at Level position 2 and leave the staff at D on the changeplate. Turn the staff so that it faces the level and take a reading. This will be a backsight.
  9. Move the staff to E and take a reading. This will be an intermediate sight.
  10. Move the staff to F and take a reading. This will be a foresight; because after taking this reading the level will be moved.
  11. Now move the level to Leveling position 3 and leave the staff at F on the changeplate.
Now repeat the steps describe 8 to 10 until you finished at point J.
Field procedures for leveling
All staff readings should be recorded in the field book. To eliminate errors resulting from any line of sight (or collimation) backsights and foresights should be equal in distance. Length of sight should be kept less than 100 metres. Always commence and finish a level run on a known datum or benchmark and close the level traverse; this enables the level run to be checked.

There are two main methods of booking levels:
  • rise and fall method
  • height of collimation method
Table 1   Rise & Fall Method
Back-
sight
Inter-
mediate
Fore-
sight
Rise
Fall
Reduced
level
Distance
Remarks
2.554




50.00
0
Datum RL+50 m

1.783

0.771

50.771
14.990
A

0.926

0.857

51.628
29.105
B

1.963


1.037
50591
48.490
C
1.305

3.587

1.624
48.967
63.540
D / change point 1

1.432


0.127
48.840
87.665
E
3.250

0.573
0.859

49.699
102.050
F / change point 2

1.925

1.325

51.024
113.285
G
3.015

0.496
1.429

52.453
128.345
H / change point 3


0.780
2.235

54.688
150.460
J
10.124

5.436
7.476
2.788
54.688

Sum of B-sight & F-sight,
Sum of Rise & Fall
-5.436


-2.788

-50.000

Take smaller from greater
4.688


4.688

  4.688

Difference should be equal
The millimeter reading may be taken by estimation to an accuracy of 0.005 metres or even less.
  1. Backsight, intermediate sight and forsight readings are entered in the appropriate columns on different lines. However, as shown in the table above backsights and foresights are place on the same line if you change the level instrument.
  2. The first reduced level is the height of the datum, benchmark or R.L.
  3. If an intermediate sight or foresight is smaller than the immediately preceding staff reading then the difference between the two readings is place in the rise column.
  4. If an intermediate sight or foresight is larger than the immediately preceding staff reading then the difference between the two readings is place in the fall column.
  5. A rise is added to the preceding reduced level (RL) and a fall is subtracted from the preceding RL

While all arithmetic calculations can be checked there is no assurance that errors in the field procedure will be picked up. The arithmetic check proves only that the rise and fall is correctly recorded in the appropriate rise & fall columns. To check the field procedure for errors the level traverse must be closed. It is prudent to let another student check your reading to avoid a repetition of the level run.
If the arithmetic calculation are correct, the the difference between the sum of the backsights and the sum of the foresights will equal:
  • the difference between the sum of the rises and the sum of the falls, and

  • the difference between the first and the final R.L. or vice versa.
    (there are no arithmetic checks made on the intermediate sight calculations. Make sure you read them carefully)

Back-
sight
Inter-
mediate
Fore-
sight
Height of
collimation
Reduced
level
Distance
Remarks
2.554


52.554
50.00
0
Datum RL+50 m

1.783


50.771
14.990
A

0.926


51.628
29.105
B

1.963


50591
48.490
C
1.305

3.587
50.272
48.967
63.540
D / change point 1

1.432


48.840
87.665
E
3.250

0.573
52.949
49.699
102.050
F / change point 2

1.925


51.024
113.285
G
3.015

0.496
55.468
52.453
128.345
H / change point 3


0.780

54.688
150.460
J
10.124

5.436

54.688

Sum of B-sight & F-sight,
Difference between RL's
-5.436



-50.000

Take smaller from greater
4.688



  4.688

Difference should be equal
  1. Booking is the same as the rise and fall method for back-, intermediate- and foresights. There are no rise or fall columns, but instead a height of collimation column.
  2. The first backsight reading (staff on datum, benchmark or RL) is added to the first RL giving the height of collimation.
  3. The next staff reading is entered in the appropriate column but on a new line. The RL for the station is found by subtracting the staff reading from the height of collimation
  4. The height of collimation changes only when the level is moved to a new position. The new height of collimation is found by adding the backsight to the RL at the change point.
  5. Please note there is no check on the accuracy of intermediate RL's and errors could go undetected.
The rise and fall method may take a bit longer to complete, but a check on entries in all columns is carried out. The RL's are easier to calculate with the height of collimation method, but errors of intermediate RL's can go undetected. For this reason students should use the rise and fall method for all leveling exercises.

Always commence and finish a level run on a datum, benchmark or known RL. This is what is known as a closed level traverse, and will enable you to check the level run.
http://www.boeingconsult.com/tafe/ss&so/survey1/level/o&c-traverse.gif







Closed level traverse
Series of level runs from a known Datum or RL to a known Datum or RL.
Misclosure in millimeter
http://www.boeingconsult.com/tafe/ss&so/survey1/level/leq.gif  24 x √km
Closed loop level traverse
Series of level runs from a known Datum or RL back to the known Datum or RL.
Misclosure in millimeter
http://www.boeingconsult.com/tafe/ss&so/survey1/level/leq.gif  24 x √km






Open level traverse
Series of level runs from a known Datum or RL. This must be avoided because there are no checks on misreading



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