SUPPLEMENTARY
READING (SKAA 1023) – LEVELLING
Levelling and how heights are defined
In surveying, three basic quantities are measured – heights, angles and distances –levelling is the
name given to one of the methods available for determining heights.
When levelling, it is possible to measure heights within a few millimetres and this order of precision
is more than adequate for height measurement on the majority of civil engineering projects. As well
as levelling, it is worth noting that heights can also be measured by using total stations, handheld
laser distance meters and GPS – these are described in subsequent chapters of the book. In
comparison to these, levelling offers a versatile yet simple, accurate and inexpensive field procedure
for measuring heights and this is the reason for its continued use on construction sites in competi-
tion with other methods.
The equipment required to carry out levelling is an optical, digital or laser level. All methods of
height measurement determine the heights of points above (or below) an agreed datum. On site
or in the office, surveyors, builders and engineers all use, on a daily basis, horizontal and vertical
datums as references for all types of measurement including levelling. To illustrate what is meant
by horizontal and vertical, Figure 2.1 shows a plumb-bob (a wei g ht on a length of string or
cord) suspended over a point P. The direction of g ravit y along the plumb-line defines the vertical at
P and a horizontal line is a line taken at right angles to this. Any horizontal line can be chosen as a
datum and the height of a point is measured along a vertical above or below the chosen horizontal line.
On most survey and construction sites, a permanent feature of some sort is usually chosen as a datum
for levelling and this is given an arbitrary height to suit site conditions. The horizontal line or
surface passing through this feature, with its assigned height, then becomes the levelling datum.
Although it may seem logical to assign a height of 0 m to such a datum, a value often used is 100
m and this is chosen to avoid any negative heights occurring as these can lead to mistakes if the
minus sign was omitted for some reason. The heights of points relative to a datum are known as
reduced levels.
Any permanent reference point which has an arbitrary height assigned to it or has had its height
accurately determined by levelling is known as a bench mark (BM). For most surveys and construction
work, it is usual to establish the heights of several bench marks throughout a site and if these have
heights based on an arbitrary datum, they are known as Temporary Bench Marks (TBMs). The
positions of TBMs have to be carefully chosen to suit site conditions.
The definition of a levelling datum given above is a horizontal or level line or surface that is
always at right angles to the direction of gravity. As might be expected, the direction of gravity is
generally towards the centre of the Earth and over large areas, because the Earth is curved, a level
surface will become curved as shown in Figure 2.2. On this diagram, the height of A above B is
measured along a vertical between the level surfaces through A and B.
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SUPPLEMENTARY READING (SKAA 1023) – LEVELLING
If heights are to be based on the same datum for the whole of a large area such as Malaysia, a curved
level surface of zero height has to be defined. Such a surface corresponds to the mean sea level
(MSL). Observations of the MSLs are entrusted to the Jabatan Ukur dan Pemetaan Malaysia
(JUPEM) – i.e. the Malaysian Mapping and Surveying Department. Observations of the MSL
are made at 21 tidal stations distributed over Malaysia (see Fig.2.3). The levelling datum
adopted for Malaysia is based on the MSL measured at North Port Klang. The datum is known as
the National Geodetic Vertical Datum (NGVD). As the datum corresponds to the mean sea level,
therefore, any heights which refer to this particular level surface as zero height are known as
heights above the mean sea level (MSL) or more commonly referred to as reduced levels. It should
be pointed out that, the zero height level surface is also known as the geoid.
All heights and contours marked on JUPEM maps and plans covering Malaysia are MSL heights.
The JUPEM have established, by levelling, bench marks (BM), known as JUPEM Bench Marks all
of which have a quoted MSL height. Figure 2.3 shows the distribution of these BM’s which are
normally located along major arterial roads in the country. Figure 2.3a depicts a typical example of a BM.
Height values of a BM can be obtained from the JUPEM with a small fee.
Some caution must be exercised when using BM’s as they may have been affected by local
subsidence or some other physical disturbance since the date they were last levelled. For these
reasons, if the heights given on maps or in bench mark lists are to be used for any survey work, it
is essential to include two or more BMs in levelling schemes so that their height values can be
checked for errors by levelling between them. Despite the existence of MSL, it is not always
necessary to use a MSL datum and many construction projects use an arbitrary datum for defining
heights.
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SUPPLEMENTARY READING (SKAA 1023) – LEVELLING
Figure 2.3 The levelling network and location of the tidal stations in Malaysia
Figure 2.3a A typical bench mark (BM). The reduced level is measured from the top of the brass
dome.
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SUPPLEMENTARY READING (SKAA 1023) – LEVELLING
The levelling staff
Levelling involves the measurement of vertical distance with reference to a horizontal plane or
surface. To do this, a levelling staff is needed to measure vertical distances and an instrument
known as a level is required to define the horizontal plane, both of which are shown in Figure 2.4.
Figure 2.4 Levelling equipment
A levelling staff is the equivalent of a long ruler and it enables distances to be measured
vertically from the horizontal plane established by a level to points where heights are required. Many
types of staff are in current use and these can have lengths of up to 5 m. The staff is usually telescopic but
can be socketed in as many as five sections for ease of carrying and use and it is made of aluminium or
non-conductive fibreglass. The staff markings can take various forms but the E-type is preferred by many
engineers and surveyors due to its legibility. This is shown in Figure 2.5 and can be read directly to 0.01
m and by estimation to 0.001 m.
Since the staff must be held vertically at each point where a height is to be measured most staffs are fitted
with a circular bubble (staff bubble) to help do this. If no bubble is available, the staff should be slowly
moved back and forth through the vertical and the lowest reading noted – this will be the reading when
the staff is vertical, as shown in Figure 2.6.
Source : Uren, J & W.F. Price (2006), Surveying for Engineers, Palgrave MacMillan Page 2.4
SUPPLEMENTARY READING (SKAA 1023) – LEVELLING
Automatic levels
The general features of the automatic level are shown in Figure 2.7. These instruments establish a
horizontal plane at each point where they are set up and consist of a telescope and compensator.
The telescope provides a magnified line of sight for taking measurements and the compensator,
built into the telescope, ensures that the line of sight viewed through the telescope is horizontal even if
the optical axis of the telescope is not exactly horizontal.
Surveying telescopes
The type of telescope used in automatic and other levels is very similar to that used in other surveying
instruments such as theodolites and total stations and is shown in Figure 2.8. The discussion that
follows refers mostly to the telescope of a level but is also applicable to measurements taken using
other surveying instruments.
When looking through the eyepiece of the telescope, a set of lines can be seen in the field of view and
these provide a reference against which measurements are taken. This part of the telescope is
called the diaphragm (or reticule) and this consists of a circle of plane glass upon which lines are
etched, as shown in Figure 2.9. Conventionally, the pattern of vertical and horizontal lines is called the
cross hairs.
The object lens, focusing lens, diaphragm and eyepiece are all mounted on the same optical axis and
the imaginary line passing through the centre of the cross hairs and the optical centre of the object
lens is called the line of sight or the line of collimation of the telescope. The diaphragm is held in
the telescope by means of four adjusting screws so that the position of the line of sight can be moved
Source : Uren, J & W.F. Price (2006), Surveying for Engineers, Palgrave MacMillan Page 2.5
SUPPLEMENTARY READING (SKAA 1023) – LEVELLING
within the telescope (will be discussed later in Adjustment of the level).
Source : Uren, J & W.F. Price (2006), Surveying for Engineers, Palgrave MacMillan Page 2.6
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The action of the telescope is as follows. Light rays from the levelling staff (or target) pass through the
object lens and are brought to a focus in the plane of the diaphragm by rotating the focusing screw.
Rotating the focusing screw moves the focusing lens along the axis of the telescope. When the
eyepiece is rotated, this also moves axially along the telescope and since it has a fixed focal point that
lies outside the lens combination, its focal point can also be made to coincide with the plane of the
diaphragm. Since the image of the levelling staff has already been focused on the diaphragm, an observer
will see in the field of view of the telescope the levelling staff focused against the cross hairs. The
image of the staff will also be highly magnified (see Figure 2.10) making accurate measurement of
vertical distances possible over long distances.
A problem often encountered with outdoor optical instruments is water and dust penetration. In order
to provide protection from these, the telescope and compensator compartment of some levels
are sealed and filled, under pressure, with dry nitrogen gas. This is known as nitrogen purging, and
since the gas is pressurised, water and dust are prevented from entering the telescope. The use of
dry nitrogen also prevents lens clouding and moisture condensation inside the telescope.
Typical specifications for a surveying telescope for use in construction work are a magnification of up to
about 30, a field of view of between 1 and 2° and a minimum focusing distance of 0.5-1.0 m. Some
telescopes use autofocusing, where focusing is achieved by pressing a button in a similar manner to
a camera.
Parallax
For a surveying telescope to work correctly, the focusing screw has to be adjusted so that the image
of the staff falls exactly in the plane of the diaphragm and the eyepiece must be adjusted so that its
focal point is also in the plane of the diaphragm. Failure to achieve either of these settings results in a
condition called parallax, and this can be a source of error when using levels, theodolites and total
stations. Parallax can be detected by moving the eye to different parts of the eyepiece when viewing
a levelling staff – if different parts of the staff appear against the cross hairs the telescope has not been
properly focused and parallax is present, as shown in Figure 2.11.
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SUPPLEMENTARY READING (SKAA 1023) – LEVELLING
It is difficult to take accurate staff readings under these conditions, since the position of the cross
hairs alters for different positions of the eye. For these reasons, it is essential that parallax is
removed before any readings are taken when using a level or any optical instrument with an
adjustable eyepiece.
To remove parallax, a piece of white paper or page from a field book is held in front of the objective
and the eyepiece is adjusted so the cross hairs are in focus. The paper or field book is removed from in
front of the objective, and the staff at which readings are required is now sighted and brought into
focus using the focusing screw. Next, the staff is observed whilst moving the eye up and down, and if
it does not appear to move relative to the cross hairs then parallax has been eliminated (see Figure
2.11). If there is apparent movement then the procedure should be repeated. Once adjusted, it
is not usually necessary to adjust the eyepiece again until a new set of readings is taken, say, on
another day. For all levelling, the focusing screw has to be adjusted for each staff reading, as focus
depends on the sighting distance.
The compensator
In an automatic level, the function of the compensator is to deviate a horizontal ray of light at the
optical centre of the object lens through the centre of the cross hairs. This ensures that the line of
sight (or collimation) viewed through the telescope is horizontal even if the telescope is tilted.
Whatever type of automatic level is used it must be levelled to within about 15' of the vertical
through the level to allow the compensator to work. This is achieved by using the three footscrews
together with the circular bubble.
Figure 2.12 shows a compensator and the position in which it is usually mounted in the telescope.
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SUPPLEMENTARY READING (SKAA 1023) – LEVELLING
The action of the compensator is shown in Figure 2.13, which has been exaggerated for clarity. The
main component of the compensator is a prism which is assumed to be freely suspended within the
telescope tube when the instrument has been levelled and which takes up a position under the influence
of gravity according to the angle of tilt of the telescope. Provided the tilt is within the working range
of the compensator, the prism moves to a position to counteract this and a horizontal line of sight
(collimation) is always observed at the centre of the cross hairs.
The wires used to suspend a compensator are made of a special alloy to ensure stability and
flexibility under rapidly changing atmospheric conditions, vibration and shock. The compensator
is also screened against magnetic fields and uses some form of damping, otherwise it might be
damaged when the level is in transit and might be affected by wind and vibration preventing readings
from being taken.
Fig. 2.3
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Figure 2.13 The compensator
Use of the automatic level
The first part of the levelling process is to set the tripod in position for the first reading,
ensuring that the top of the tripod is levelled by eye after the tripod legs have been pushed firmly
into the ground. Following this, the level is attached to the tripod using the clamp provided and the
circular bubble is centralised using the three footscrews.
When an automatic level has been set up and levelled in this way, the compensator automatically moves
to a position to establish a horizontal line of sight at the centre of the cross hairs. Therefore, at each
set up, no further levelling is required after the circular bubble has been set. As with all types of level,
parallax must be removed before any readings are taken.
In addition to the levelling procedure and parallax removal, a test should be made to see if the
compensator is working before readings commence. One of the levelling footscrews should be moved
slightly off level and, if the reading to the levelling staff remains constant, the compensator is
working. If the reading changes, it may be necessary to gently tap the telescope tube to free the
compensator.
A problem sometimes encountered with levels that use a compensator is that machinery
operating nearby will cause the compensator to vibrate, which in turn causes the image of the staff
to appear to vibrate so that readings become very difficult to take. This problem is sometimes
encountered on construction sites, particularly where the site is narrow or constricted.
Digital levels
Shown in Figure 2.17, the digital level is similar in appearance to an automatic or tilting level and
the same features can be identified including the telescope with focus and eyepiece, footscrews
with a base plate and so on. In use, it is set up in the same way as an automatic level by attaching it to a
tripod and centralising a circular bubble using the footscrews. A horizontal line of sight is then
established by a compensator and readings could be taken in the same way as with an automatic
level to a levelling staff, where all readings are taken and recorded manually.
However, this instrument has been designed to carry out all reading and data processing
automatically via an on-board computer which is accessed through a display and keyboard. When
levelling, a special bar-coded staff is sighted (see Figure 2.18), the focus is adjusted and a measuring
key is pressed. There is no need to read the staff as the display will show the staff reading about two
or three seconds after the measuring key has been pressed. When the bar-coded staff is sighted, it is
interrogated by the level over a short span of about 500 mm using electronic image-
processing techniques to produce a bar-coded image of the staff corresponding to the field of view of
the telescope. The captured image is then compared by the on-board computer to the bar codes stored
Source : Uren, J & W.F. Price (2006), Surveying for Engineers, Palgrave MacMillan Page 2.10
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in the memory for the staff and when a match is found, this is the displayed staff reading. In
addition to staff readings, it is also possible to display the horizontal distance to the staff with a
precision of about 20-25 mm. All readings can be coded using the keyboard and as levelling
proceeds, each staff reading and subsequently all calculations are stored in the level's internal
memory.
In good conditions, a digital level has a range of about 100 m, but this can deteriorate if the staff is
not brightly and evenly illuminated along its scanned section. The power supply for the digital level is
standard AA or rechargeable batteries, which are capable of providing enough power for a complete
day's levelling. If it is not possible to take electronic staff readings (because of poor lighting, obstructions
such as foliage preventing a bar code from being imaged or loss of battery power), the reverse side of
the bar-coded staff has a normal E type face and optical readings can be taken and entered manually
into the instrument instead.
The digital level has many advantages over conventional levels since observations are taken quickly over
longer distances without the need to read a staff or record anything by hand. This eliminates two of
the worst sources of error from levelling – reading the staff incorrectly and writing the wrong value for
a reading in the field book. As a digital level also calculates all the heights required, another source of
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error is removed from the levelling process – the possibility of making mistakes in calculations.
The data stored in a digital level can also be transferred to a removable memory card and then to a
computer, where it can be processed further and permanently filed if required. To do this, the
software provided by each manufacturer with the level can be used. As well as this, data can be
transferred from the memory card directly into one of the many survey and design software packages
now available. For on-site applications, design information can also be uploaded from a computer
and memory card to the level for setting out purposes.
Figure 2.18 A Leica digital level & coded staff
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