Topographic surveying involves determining the positions and elevations of natural and artificial landscape features to represent them on a topographic map. The process has three main parts: establishing horizontal and vertical control points; locating contours; and locating details like rivers, roads, buildings. Contours are imaginary lines connecting points of equal elevation, with the difference in height between contours indicated by the contour interval. Contours can be obtained directly via leveling or indirectly by interpolating spot heights. Mathematical interpolation uses proportional calculations between spot heights to determine contour locations on a map.

1. Addis Ababa University Ethiopian Institute of Architecture, Building construction and City
Development TOPOGRAPHIC SURVEYING
Compiled by Ebisa Tesfaye Page 1
CHAPTER 7
TOPOGRAPHIC SURVEYING
7.1 INTRODUCTION
Is the process of determining the positions, both in plan and elevation of the natural and artificial features of a locality for
the purpose of delineating them by means of conventional signs up on a topographic map.
 The basic purpose of the topographic map is to indicate the three dimensional relationships for the terrain
of any given area of land. Thus, on topographic map the relative positions of points are represented both
horizontally as well as vertically. The representation of the difference in elevation is called the relief.
7.2 PROCEDURE IN TOPOGRAPHIC SURVEYING
The work in a topographic surveying consists of the three parts (operations)
 Establishing horizontal as well as vertical control (skeleton upon which the details of the map are placed);
 Locating the contour;
 Locating details such as rivers, streams, lakes, roads, rail ways, buildings, trees, etc
The term detail implies features both above and below ground level and on the ground. For instance; buildings, roads,
walls and other man made constructed features are called hard details where as natural features including rivers and
vegetation are known as soft detail. Other information include overhead details (for example power and telephone lines)
and underground details (for example water pipes and sewer lines)
LOCATING DETAILS
 After having located or established the horizontal and vertical control, the detail is located from the
control points by measurement of angles and distances to those points which are to appear on the
map.
7.3 REPRESENTING DETAILS
 The map is prepared by plotting first the control points and then the detail. The contour lines are
drawn next and then the relief is depicted by means of conventional signs
 A variety of special signs used to represent the different features on a map are explained in the
legend of the map.
 The location of the features is referred to as planimetry and the configuration of the ground is referred
to as topography.
School Mosque
Church Museum
Hospital Muslim Cemetry
Hotel
7.5 CONTOUR
 A contour is an imaginary line joining places or points having the same elevation (height) on the ground
surface. If the locations (height) of several ground points of equal elevation are plotted on drawing, a line
joining these points is called a contour line.
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2. Addis Ababa University Ethiopian Institute of Architecture, Building construction and City
Development TOPOGRAPHIC SURVEYING
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 On a map the difference in elevation between successive contour lines is known as contour interval. The
horizontal distance between two consecutive contour lines is called the horizontal equivalent. This can vary
from point to point in a given map depending upon their steepness.
Contour can be obtained either directly or indirectly using mathematical or graphical interpolation techniques.
Contouring methods
1. Direct Contouring
 In this method, the position of contours are located on the ground by leveling. A level is set up in the
area at any convenient location, so that as much ground as possible can be covered by staff
observations from the instrument position a BS reading is taken to a BM and the FS and IS are
calculated.
2. Indirect Contouring
 This involves the heightening of points that do not, in general, coincide with the contour positions.
Instead, the points leveled are used as a framework on which contours are later interpolated on a
drawing. The area to be contoured is divided into a series of lines forming squares and ground levels
are taken at the intersection of the grid lines. The plotted spot heights will not be at exact contour
value and it is necessary to locate points between them on the plan which do have exact contour
value. This is known as interpolation. Interpolation can be carried out by estimation which is used to
show or draw contours roughly, mathematical and graphical methods.
Mathematical interpolation
 The height difference between each spot height is calculated and used with the horizontal distance
between them to calculate the position on the line joining the spot heights at which the required
contour is located.
Example1:- Referring to fig a and data given below in which the positions of 2353,2353.5 and 2354m contours are to be
located on the map. To draw this contour line, we can use a simple proportion. The area to be contoured is subdivided in
a series of lines forming in squares of 50m interval. The ground elevation also taken at the intersection grid points are as
listed below. Compute the elevations to show contour lines of 2353,2353.5 and 2354 mts.
Collected data from field observation.
Station BS IS FS HI Elev
Bm1 1.022 2354.388 2353.366
Ao 0.888 2353.5
A1 0.588 2353.8
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3. Addis Ababa University Ethiopian Institute of Architecture, Building construction and City
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A2 0.588 2353.8
Bo 1.388 2353.0
B1 1.588 2352.8
B2 0.288 2354.1
Co 1.888 2352.5
C1 0.388 2354.0
C2 0.188 2354.2
BM1 1.021 2353.367
The area to be contoured is drown to a suitable scale.
50m
Ao
Co
Bo
1 2
2353.8
2353
2353
2352.8
2353
2352.5 2354
2354.2
2354.1
2354
2353.5
2353.5
2353.8
The ground area is 50mx50m. Horizontal distance between station on the plan =5cm; difference in elevation between A1
and B1 is =2353.8-2352.8=1m
The position of the 2353m contour is to be located between two spot heights A1 and B1. soln.
1. ΔH between the spot heights A1 and B1 is 1m
2. The difference between the higher elvn and the required contour is 2352.8m -2353=0.8
3. The difference between the required contour and the lower elvn is 2353m-2352.8=0.2
Let the 0.8m position from the higher elevation A1=x
Let the 0.2m position from the lower elevation B1=y
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4. Addis Ababa University Ethiopian Institute of Architecture, Building construction and City
Development TOPOGRAPHIC SURVEYING
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By simple proportion
1m/5cm=0.8/x=0.2/y
X=4cm and y=1cm
Check the total length A1B1=5cm
Length on plan=5cm
To locate 2353 contour measure 4cm from A1 to wards B1 or measure 1cm from B1 towards A1.
For remaining contour lines, follow the same method.
Characteristics of Contour
 A single contour line cannot lie between two contour lines of higher or low, elevations.
 As contour lines represent contours of different elevations on the ground, they cannot merge or cross-
each other on the map except rarely in the case of an overhanging cliff or cave.
 A contour line must close upon itself within or out of the border of the map if it is extended.
 A long plane surface (such as railway), the contour lines are straight and parallel to one other.
 If the contours are equally spaced, they represent a uniform slop. But if they are irregular , they
represent uneven ground
 A closed contour lines with the higher values inside the map indicate a hill and a depression if the
higher values are outside.
 The horizontal distance between contour lines is inversely proportional to the slope. Hence on steep
slope (such as at the riverbank) the contour. lines are spaced closely
 Contour lines cross a valley line at V shape towards a higher ground
 Contour lines cross a water shed line at U-shape towards the higher place
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