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:

Rise & Fall method and
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

Procedure:

Set up the leveling instrument at Level position 1.
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.
Move the staff to A and take a
reading. This will be an intermediate sight.
Move the staff to B and take a
reading. This also will be an intermediate sight.
Move the staff to C and take a
reading. This will be another intermediate sight.
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.)
The distance between the stations should be measured
and recorded in the field book (see Table 1)
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.
Move the staff to E and take a
reading. This will be an intermediate sight.
Move the staff to F and take a
reading. This will be a foresight; because after taking this reading the
level will be moved.
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.

Booking
levels

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.

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.
The first reduced level is the height of the datum,
benchmark or R.L.
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.
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.
A rise is added to the preceding reduced level (RL)
and a fall is subtracted from the preceding RL

Arithmetic checks

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)

Table 2 Height of collimation method (height of instrument)

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

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.
The first backsight reading (staff on datum,
benchmark or RL) is added to the first RL giving the height of
collimation.
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
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.
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.

Closed
and open traverse

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.