Definition of Commonly Used Terms in Leveling:
1. Datum. Datum is any convenient level surface coincident or parallel with
mean sea level to which elevations of a particular area are referred. Any
surface may be used as a datum when relative elevations over a limited
area need to be established. It is done by assigning an assumed elevation
to a reference point and determining the elevation of other points in the
vicinity with regard to this value.
2. Elevation. For a particular point, its elevation is the vertical distance
above or below mean sea level or any other selected datum. Points or
near the surface of the earth have either positive or negative elevations,
depending if the point is above or below mean sea level.
3. Mean Sea Level. Mean sea level is an imaginary surface of the sea which
is midway between high and low tides. It is taken as the reference to
which most ground elevations are referred. This surface is determined by
averaging the height of the sea surface for all its tides stages over a long
period of time which may extend to about 20 years. Readings are usually
taken hourly intervals on various properly distributed stations. Mean sea
level is not a steady frame of reference due to the melting of ice in the
Polar Regions, the effects of volcanic activity, and many other influencing
factors. It is for these reasons that it is necessary to have a continuing
observation of sea level fluctuations to detect changes. This surface,
which is considered to be at zero elevation, conforms to the spheriodal
shape of the earth and is perpendicular to the direction of gravity at every
point.
4. Difference in Elevation. The difference in elevation between two points is
the vertical distance between the two level surfaces in which the points lie.
There are various methods which could be employed to determine the
elevation of points and their differences in elevation. These methods may
be undertaken either directly or directly in the field. The principle involved
in each method differs in some aspects. Also, they may differ with respect
to the type of instruments used, the procedure employed, and the
attainable degrees of precision. Traditional methods of leveling have been
used for so many years and they still continue to be useful in the present
time. Such methods includes: direct leveling, trigonometric leveling, and
barometric leveling.
5. Direct or Spirit leveling. Direct leveling is the commonly employed
method of determining the elevation of points some distance apart by
series of set ups of a leveling instrument along a selected route. This
method of leveling is also referred to as a spirit leveling since the device
used is a spirit level. Differential leveling, double-rodded leveling, and
three wire leveling are forms of direct leveling. In direct leveling vertical
distances are measured above or below a level line and these values are
used to compute the elevation of points or their differences in elevation.
Being the most precise method of leveling, it is used when a high degree
of accuracy is required.
6. Reciprocal Leveling. Reciprocal leveling is the process of accuracy
determining the difference in elevation between two intervisible points
located at a considerable distance apart and between which points
leveling could not be performed in usual manner. This method is
commonly employed when leveling across a wide river, a deep ravine or
across canyons and gullies where it would be difficult or impossible to
maintain a foresight and a backsight distance of nearly equal lengths.
Reciprocal leveling provides a faster method of determining differences in
elevation and when it is carefully conducted; it could be as precise as
direct leveling. In determining the difference in elevation between the two
points, it will require two sets of observations and the mean of which is
used. Since there is inequality the sight taken, the process of reciprocal
leveling is expected to eliminate the errors resulting from curvature of the
earth ant the refraction of the atmosphere, provided that atmospheric
conditions do not change during the process.
7. Profile Leveling. This method of leveling is used determine differences in
elevation between points at designated short measured intervals along an
established line to provide data from which a vertical section of the ground
surface can be plotted. In the design of roads, railroads, canals, drainage
systems, and transmission lines, it is necessary to first obtain profile of the
existing ground surface. It is this method of leveling which will best suit
such requirements.
8. Trigonometric Leveling. This method of leveling is employed in
determining by trigonometric computations the difference in elevation
between two points from measurements of its horizontal or slope distance
and the vertical angle between the points. The required distances are
usually obtained by stadia, triangulation, or by direct taping. By most
leveling standards, trigonometric leveling only provides a very rough
determination of differences in elevation. Its degree of precision may be
improved by using precise measuring instrument such as optical
theodolites and total geodetic stations which measure angles to seconds.
9. Stadia Leveling. Stadia leveling combine features of direct leveling with
those of trigonometric leveling. It can provide reasonable accuracy for
preliminary surveys mapping, and rough leveling where quick
measurements are needed. In stadia leveling differences in elevation
between points are computed from observed vertical angles and the three
intercepts on a rod held at each point backsighted. Any surveying
instrument may be employed in stadia leveling as long as it has a
telescope to read vertical angles and is equipped with stadia hairs in
addition to the standard cross hairs.
10. Barometric Leveling. Barometric leveling involves the determination of
differences in elevation between points by measuring the variation in
atmospheric pressure at each point by means of a barometer. This
leveling method depends on the basic principle that differences in
elevation are proportional to the differences in atmosphere pressure. The
readings of a barometer at different points on the earth’s surface provide a
measure of the relative elevations of these points. It is an accepted fact
that the pressure caused by the weight of a column of air above the
observers decreases as the observer goes higher in altitude. The method
is particularly useful for low precision leveling over rough terrain where
extensive areas need to be covered and differences in elevation are large.
It is principally employed on reconnaissance surveys or other work
requiring only approximate values. However, it is not desirable to employ
when the atmospheric pressure in the area changes rapidly.
11. Borrow Pit Leveling. Borrow-pit leveling is a method of determining the
relative elevations of points in borrow pit excavations for the purpose of
calculating volumes of earthwork. This type of work is usually encountered
in the construction of roads and railroads.
12. Bench Mark (BM). A bench mark is a fixed point of reference whose
elevation is either known or assumed. They may be permanent or
temporary. Permanent bench marks (PBM) are those which established at
intervals the country. They serve as points of reference for level in a given
locality and their elevations are determined by precise leveling methods.
13. Backsight (BS). A backsight is a reading taken on a rod held of a point of
known or assumed elevation. It is a measure of vertical distance from the
established line of sight to the point sighted, and is always the first rod
reading taken after the instrument has been set up and leveled.
Backsights are frequently refer to as plus (+S) sights since they are added
to the elevation of points being sighted to determine the height of
instrument. The backsight does not necessarily mean that it is always a
sight taken backward.
14. Foresight (FS). A reading taken on a rod held on a point whose elevation
is to be determined is called foresight. It is represented as a vertical
distance from the line of sight of the instrument to the point observed.
Since the reading of a foresight is subtracted form the height of instrument
to determine the elevation of the point on which the rod is held, a foresight
is usually referred to as minus (-S) sight. Foresights are often taken
forward or in general direction in which the leveling work proceeds. It is
usually the last reading taken before the leveling instrument is moved to
another location.
15. Turning Point (TP). A turning point is an intervening point between two
bench marks upon which point foresight and backsights reading to enable
a leveling operation to continue from a new instrument position. It is
sometimes referred as a change point.
16. Height of Instrument (HI). The height of instrument or height of
collimation) is the elevation of the line of sight of an instrument above or
below selected reference datum. It is determined by adding the rod
reading on the backsight to the elevation of the point on which the sight is
taken.
Differential Leveling:
Differential leveling is the process of determining the difference in elevation
between two or more points some distance apart. It requires series of set ups of
the instrument along a general route and, for each set, a rod reading back to the
point of unknown elevation are taken. The basic equipment used in differential
leveling includes a graduated leveling rod and a leveling instrument. This leveling
instrument is used to establish a horizontal line of sight and to measure the
vertical distance between the line of sight and the point on which the rod is held.
1. Procedure of Differential Leveling.
The procedure followed in determining the difference in elevation between
two points is illustrated in Figure 1 in which the line of levels runs from
BM1 to BM2. There should be at least two persons to undertake
differential leveling; the rod man who carries and hold the rod, and the
instrument man who sets up the level and determines the required rod
readings. The instrument man can record in a field notebook or a third
person maybe employed as a recorder.
The leveling instrument is set up at any convenient location along the level
route and a backsight reading is taken on a leveling rod held vertically on
BM1. The backsight reading is added to known or assumed elevation of
the initial bench mark gives the height of instrument above datum or
HI = Elev. BM1 + BS
The rodman moves forward along the general direction of BM2 and holds
the rod at a convenient turning point (TP1). The instrumentman takes a
foresight reading on the rod. The foresight reading is subtracted from the
height of instrument that gives the elevation above datum of the turning
point or
Elev. TP = HI – FS
The level is then transferred and set up in another convenient location
beyond TP1, but still in general direction of BM2. The exact location at
which the level is set up is of little importance. The more important thing to
consider is that the level is set up at location where backsight and
foresight readings can be made. A backsight is then taken on TP1 to
establish a new height of instrument (HI2). The rodman finally moves
forward to the location of BM2 and a foresight is taken on the rod held on
it. Since the new HI has already been determined, the elevation of BM2 is
computed by subtracting the foresight reading form the height of
instrument.
If the terminal point (BM2) is still some distance away such that more
turning points have to be establish before it could be foresighted, the
procedure of reading a backsight, the rodman moving ahead to establish
another turning point and reading a foresight is repeated. This is finally
obtained.
2. Leveling Notes:
It is important that all data gathered in differential leveling work be properly
recorded in a special field notebook ruled with suitable lines. There is
standard format used in recording leveling notes.
3. Checking the Level Notes:
The calculations in differential leveling work are limited only to determining
the heights of instrument and the elevation of stations or points. As
pointed out earlier in the procedure of differential leveling, there are only
two steps performed in the computation. First the backsight reading is
added to the elevation of the benchmark to determine the height of
instrument; and second, the foresight reading is subtracted from the height
of instrument to determine the elevation of point foresighted.
A check on the addition and subtraction in each step is made adding
separately the backsight and the foresight readings. The difference
between the two sums should equal the difference in elevation between
the first and the last stations unless a mistake is made arithmetic. The
agreement between these two quantities signifies that the process of
adding and subtracting has been done correctly. It does not however,
provide a check against mistake in observing and recording data or
whether the fieldwork was performed satisfactorily. The rod can be
misread or an observed reading recorded incorrectly without affecting the
arithmetic check.
It is customary among surveyors to always make an arithmetic check on
all data which are calculated and recorded in the field notebook. Level
notes are considered incomplete unless the check is properly indicated.
The arithmetic check should be done before the survey party leaves the
site. It is only by using correct field procedures and giving careful attention
to the work being undertaken that the mistakes and errors can be
prevented.
Sources of Error in Leveling:
1) Instrument Errors
These errors are attributed to imperfections in the instrument either from
faults in their manufacture or from improper adjustment.
a) Instrument out of adjustment
The most common instrumental error is cause by the level being
out of adjustment. Particularly significant is when the line of the
telescope is not parallel to the axis of the level vial. The line of sight
will be inclined either upward or downward when the bubble is not
brought to the center of the tube. When a reading is taken on a rod,
the result is an error consistently either plus or minus and with a
magnitude proportional to the distance between the instrument and
the rod. This source of error can be eliminated or kept at a
minimum by frequently testing the instrument and keeping it always
in good adjustment.
b) Rod not standard length
It is possible to have inaccurate graduations or divisions on a rod.
This is usually due to imperfections in their manufacture. Inaccurate
rod graduations can cause errors in measured vertical distances
similar to those resulting from incorrect markings on tape. In any
case, it is important that the rod lengths are compared periodically
with a standardized steel tape.
c) Defective tripod
The movement of the level due to settling of the tripod legs can
cause possible errors in leveling work. The tripod usually settles in
soft ground or due to vibrations caused by passing vehicular traffic.
It is important to always set up the tripod rigidly as can lead to
erroneous measurements and waste of time.
2) Personal Errors
Although personal errors occur largely due to the limitations of the senses
of touch, sight, or hearing of individuals, the skills training and teamwork of
the members of a leveling party are also factors to be considered.
Personal errors are usually caused by erroneous manipulations and
careless handling of instruments when making observations.
a) Bubble not centered
Rod reading will be in error when the bubble is not centered in the
level vial. There are various conditions in the field which may cause
the bubble not to remain centered.
b) Parallax
If a pressure gauge or any graduated circular meter is viewed from
different angles, one will notice that a number of slightly divergent
values could be read. This due to the effect of parallax. However, if
the pointer and scale of the gauge were positioned at exactly the
same plane, parallax would be totally eliminated. A similar
conditioned occurs when sighting though a telescope to read a
leveling rod. The case of parallax is to cause relative displacement
between the image of the cross hairs and the image formed by the
focusing lens. Parallax can be eliminated by careful focusing of the
telescope until no apparent movement of the horizontal hair relative
to the rod can be detected when the eye is moved up and down
while sighting.
c) Faulty rod readings
The instrumentman at times may misread the number of meters
and decimals when taking a rod reading. An incorrect rod reading is
usually the result of the length of sight, poor weather conditions,
and the skills of the instrumentman and the rodman.
d) Rod not held plumb
Aside from holding the rod from a firm a definite point, it should also
be held as nearly as vertical as possible. If it is held off the vertical,
it will be intersected by the line of sight further form the base and
the reading will be much greater than what it should be. The
reading on the rod will lowest when it is held plumb.
e) Incorrect setting of target
It is important to handle the leveling rod carefully. The rodman at
times fails to set properly the target when a high rod reading is
made with it. During used the target may slip downward because it
is not clamped firmly at the exact position signaled by the
instrumentman. To avoid this type of error, the instrumentman
should always take a second sight on the target after it is clamped
by the rodman in order to be sure that it has not slipped.
f) Unequal backsight and foresight distance
3) Natural Errors
a) Curvature of the earth
b) Atmospheric refraction
c) Temperature variations
d) Wind
e) Settlement of the instrument
f) Faulty turning points
Common Mistakes in Leveling:
1) Misreading the rod
2) Incorrect recording
3) Erroneous computations
4) Rod not fully extended
5) Moving turning points
Setting Up the Level:
The leveling instrument may be set up at any suitable or desired location. When
starting a leveling operation the instrumentman should first consider where he
intends to position the instrument. The level is then pulled out form its box by
holding the level bar or base plate and screwed securely onto the tripod. Care
should be exercised screwing the level. The instrument should fit snugly and
bear firmly. If loosely fitted, the instrument will wobble and be unstable; if too tight
due to excessive pressure, it would be difficult to unscrew the instrument later
and may cause some damage.
Solid ground should be selected when setting up the instrument. Muddy and wet
areas should be avoided as these are unstable to stand on and may only cause
serious errors in leveling work. Tripod legs are spread so that the foot plate will
be approximately horizontal. It will be easier and quicker to level the instrument
later if the place is horizontally positioned. The legs should be far enough apart
for rigid set up, and they should be push firmly into the ground to make it stand
stable. The instrumentman should see to it that the telescope is at a convenient
height for sighting and he must be able to stand comfortably between the tripods.
Leveling the Instrument:
A considerable amount of practice in leveling the instrument will be needed by a
beginner. It is only by constant practice that one would really be able to feel and
experience the proper turning of leveling screws to bring the bubble in a level vial
to its center.

Instrument with three leveling screws.
For instrument with three leveling screws, the telescope is turn until the
bubble tube is positioned parallel to the line through any two of the
screws. The bubble is then centered on the level vial by turning these two
screws in opposite directions. As usual, the thumb and the index finger of
each hand are used to turn the screws. Also the bubble will still move in
the direction of motion of the left thumb.
The telescope is next rotated about the vertical axle of the instrument so
that the bubble tube is brought perpendicular to a line through the screws
turned earlier. Again the bubble is brought carefully to center by means of
the third screw alone. This process of leveling is illustrated in Figure 2.
The instrument is leveled if the bubble remains centered on the level vial
when the telescope is brought back to its original position. If the bubble
does not remain within the center graduations, the process is repeated
until it remains in the center in any position of the telescope. This method
of repeated centering will work only if the level vial is in adjustment.
Should it be out of adjustment, no amount of repeated centering will work.
Holding the Leveling Rod:
The leveling rod is held on a point by a rodman when the sight is to be
taken on it. To obtain the correct distance from the line of sight to the point
on which the rod is placed, it is extremely important that the rod be held
plumb when the reading is made. The instrumentman checks the rod by
observing through the telescope and noting if it is held parallel to the
vertical cross hair. If the rod is not correctly plumb the instrumentman
gives out a signal to plumb the rod. The rodman either stands beside the
rod or behind it. He should face the instrumentman and see to it that the
rod is held nearly right angles the line of sight. In high precision surveys
the leveling rods used are equipped with rod level.
Taking a Rod Reading:
Before readings are taken on a rod, it is important to first examine how the
graduations are indicated on it. The metric rod is graduated in centimeters
and numerals are indicated for every full meter.