7.0 Introduction 

What is a topographical survey? 

1. A survey of your fish culture site can help you do one of two things: make a map to help you plan your work; or lay out marks on the ground that will guide you as you work. 
Site


2. Topographical surveys will help you to make plans or maps
of an area that show:

Map


Vertical profile 
What do topographical surveys involve? 

3. The purpose of the first type of topographical survey is to establish, on a horizontal plane, the position of one or more points in relation to the position of one or more other points. To do this, you will measure horizontal distances and horizontal angles or directions. You will use a method called plan surveying, which will be explained in this chapter. 
Site


4. The purpose of the second type of topographical survey is to find the elevation (or vertical height) of one or more points above a definite horizontal plane. To do this, you will measure horizontal distances and height differences; you may also need to lay out contour lines. You will use a method called direct levelling, which will be explained in Chapter 8. 
Map


5. You will learn how to make plans and maps based on the results of plan surveying and direct levelling in Chapter 9. 
Contour map

Planning your topographical surveys 

6. When you plan a topographical study, the most important rule to remember is that you must work from the whole to the part, keeping in mind all of the work you will need to do as you begin the first steps. Different types of survey require different levels of accuracy, but you should lay down the first points of each survey as accurately as possible. You will adjust all the work you do later to agree with these first points. 
Primary points


Example You need to plan survey a fishfarm site. (a) First, you must make a perimeter survey ABCDEA. Besides these summits and boundaries, add several major points and lines, such as AJ and EO. They run across the interior to create right angles, which will help you in your calculations. This survey gives the primary survey points, which you should determine and plot very accurately. (b) Then, lay out minor lines such as FP and TN. They go between the major lines to divide the area intoblocks. This gives you the secondary survey points, which you may determine less accurately. (c) Finally, survey details in each block using tertiary points, for which less accuracy is also acceptable. 
Secondary points


7. The way you plan a topographical survey will also depend on its purpose. You will use a planning method similar to the one described for soil surveys (see Volume 6, Soil, Section 2.4).

8. The way you plan a topographical survey will depend on
the subject you need to survey, such as:

Centreline of a dike


Adjacent pond dikes


Pond area


9. In open country, you will have no problems in plan surveying with the methods explained in the next sections. Any of the following methods should work well. In country with thick forests, however, you will not be able to use methods for which you need to see several points at the same time. In such areas, you will also need to rely on existing paths and roads much more than usual, and you might even need to clear lines of sight through the vegetation. 
Clearing land for a survey

What are the main methods used in plan surveying? 

10. There are four main methods used in plan surveying. You can fix the position of a point on the horizontal plane:

Open traverse


Each of these methods will be explained in the next sections. When you are choosing a method, you will also need to consider which methods are suited to the measuring devices you have available. Table 9 will help you select the most suitable plan surveying method, considering your equipment and abilities, the kind of information you need from your survey and the type of terrain you are surveying. 
Radiation survey

TABLE
9

Which method should be used for traversing? 

4. When you survey by traversing, you need to make measurements
to find information on:


5. If you have a theodolite (also called a transit), you can make a transit traverse. You will measure horizontal distances using the stadia method (see Section 2.8), and you will measure horizontal angles using the method described in Section 3.5 for use with the theodolite. Similarly, but with much less accuracy, you could use a clisimeter (see Section 2.7) and a graphometer (see Section 3.1).  
6. If you have a magnetic compass, you can make a compass traverse. You will measure the horizontal distances by pacing (see Section 2.2) or by chaining (see Section 2.6), and you will measure azimuths with the magnetic compass (see Section 3.2). Compass traverses are very useful for getting a general picture of the terrain. They also help to fill in details on surveys that have already been done. 7. If you have a planetable (see Section 7.5), you can make a planetable traverse. You will measure distances either by pacing or by chaining, and you will measure horizontal angles using a graphic method (see Section 3.3). 
Compass traverse


8. When you need to make a quick reconnaissance survey, you can traverse with a simple compass (see Section 3.3, steps 19) and by pacing (see Section 2.2). 9. In this section you will learn about compass traversing. You may use similar procedures for transit traverses. Further details on planetable traverses will be given in Section 9.2. 
Planetable traverse

Choosing the route of a traverse 

10. When selecting the route a traverse will follow, you should try to:


Surveying an open traverse with a magnetic compass 

11. You need to survey traverse AF for a future water supply canal. First, walk along the traverse. Mark its course by placing high stakes about every 50 m. If necessary, place additional stakes at important traverse stations, such as where the traverse changes direction, where hills or other changes in elevation reduce visibility between traverse stations, or where there are particular landscape features such as a road, a river, or rocks. 
Mark the main points


12. If necessary, clear any tall vegetation from the path of the traverse, so that you will be able to see each marked point from the one before it. 
Clear the path and mark details

13. Start traversing at the first point A. Remove the ranging pole and stand at point A. With the magnetic compass, measure the azimuth* of the line joining point A to point B, the next visible point. Point A becomes station 1. The direction you measure from there to point B, or station 2, is called a foresight* (FS) because you are measuring forward. Note down this value in a table (see step 17). 
FS=AB


14. Replace the ranging pole at station 1 (point A) and move to station 2, while measuring the horizontal distance AB by pacing or chaining. Note this distance down in the table (see step 17). 
Distance AB

15. At station 2 (point B), remove the ranging pole and stand over the point holding the compass. Look back at station 1 and measure the azimuth of line BA, which is called a backsight (BS). Then look forward at the next point C, or station 3, and measure the azimuth of line BC, a foresight (FS). Measure distance BC while moving forward along the traverse. Note these values down in the table (see step 17). 
BS = BA


Note: the difference between the foresight and backsight should be 180°. A difference of only 1 or 2 degrees between the FS and BS is acceptable and may be corrected later (see step 19). If the error is greater, you should make the measurement again before moving on to the next station. 
FS = BC


16. Repeat this procedure, measuring horizontal distances from station to station and measuring two azimuths (a BS and a FS) for each point. However, from the last station at the end of an open traverse, you will only have a BS measurement, just as you had only an FS from station 1. Note: if the land slopes and you need to use a more accurate method, you can use a special method to measure or calculate horizontal distances (see Sections 2.6 and 4.0). 
Distance BC

17. You should carefully note down all the measurements you have made in a field book. You can use a table like the one shown in the example or you can make a rough sketch of the open traverse on squareruled millimetric paper, noting down your measurements next to the correct stations in it.
Example
Measurements observed for the beginning of compass traverse AX made of 12 stations:


18. You must always check on such a compass traverse, particularly if you do not know the exact position of its starting and ending stations beforehand from studying previous surveys or existing maps. To check on your compass traverse, do the following: 
Observed traverse AX



Observed traverse XA



Observed traverse AX


Draw XX'



Draw the other segments parallel to XX'



Measure the distance BB', CC', DD' and EE'



Join the points of the adjusted traverse









Note: during the traverse, you may be able to see one or more additional stations from the station where you are standing. If you do, measure the azimuths of the lines running toward them. An example is line BD from station B. These additional observations are useful checks on your work.  
22. In a field book, carefully note down all your measurements. You can use a table similar to the one suggested for the open traverse (see step 17). You should also make a sketch of the traverse, on a separate squareruled page, and write in the measurements. At the same time, check to see that the foresights and backsights differ by 180º.
Example
You have surveyed site ABCDEA with a closed traverse and your field notes
are as follows:
Stations

Distance (m)

Azimuths (degrees)

Calculated difference FS/BS (degrees)


From

To

FS

BS


1

2

90.8

136

315

179

2

3

53.5

78

259

179

3

4

68.7

347

168

179

4

5

44.6

292

110

182

5

1

63.7

241

63

178


23. You have learned that in any closed polygon* of N sides, the sum of all the interior angles should be equal to (N  2) x 180º (see Section 3.0). This rule will help you to check your azimuth measurements after you calculate the interior angle for each station (see Section 3.2, steps 10 and 11).
Example
Using the observations given in the previous example, calculate the sum
of the interior angles of polygon ABCDEA as follows:
Station

Azimuth differences (degrees)

Interior angle (degrees)

1

AB AE = 136 63

73

2

(BA  BC = 315  78 = 237)

123^{1}

3

CD  CB = 347  259

88

4

DE  DC = 292  168

124

5

EA  ED = 241  110

131


Sum of interior angles

539

^{1} Since the magnetic north falls inside the angle, you must calculate it as 360º  (the azimuth difference) or 360º  237º = 123º,
According to the general rule, the sum of the five interior angles should be equal to (52) x 180º = 3 x 180º = 540º, which closely agrees with the above result. 
Check: Sum of angles = (5  2) x 180º = 540º


Adjusting a closed traverse 

24. Starting from station 1 (A), draw the observations of your compass traverse on squareruled paper. Use a protractor to measure the azimuths (see Section 3.3), and an adequate scale for the measured distances (see Section 9.1). If there is a closing error, adjust your drawing by using the graphic method described for an open traverse (see step 19, above).  
Example For the above example, the closing error is FA. Adjust it as foIlows: 


Draw AF to scale



Draw FA' perpendicular to AF



Draw AA'



Find points BCD and E



Draw and measure the perpendiculars




Draw FA



Draw the other segments parallel to FA



Measure the appropriate lengths



Join the points of the adjusted traverse

3. You should be able to reach the observation station easily. This station should also be a located so that:
4. When choosing the observation station, you should be particularly careful to avoid any points from which very small radiating angles (less than 15 degrees) might result.
5. The observation station 0 can be in a central position, inside the polygon to be surveyed. In this case, you will measure as many triangles as there are sides of the polygon. 
Number of triangles = number of polygon sides
N = 5 

6. The observation station 0 can also be in a lateral position (off to the side). In this case, 0 will be one of the summits of the polygon*. The number of triangles you need to measure will be the number of sides to the polygon, minus 2. 
Number of triangles = number of sides minus 2
N = 5  2 = 3 
Choosing a method for radiating surveys 

7. If you have a transit (a theodolite), you can measure horizontal angles more precisely than with the other instruments (see Section 3.5). A transit equipped with stadia hairs can also be used to measure distances rapidly (see Section 2.8).  
8. If you have a magnetic compass, you can use it to measure the azimuths of the horizontal angles at the observation station (see Section 3.2). You will usually measure horizontal distances by chaining (see Section 2.6). To learn further details of this simple method, see steps 1014, below.  
9. If you have a planetable, you can use it for mapping the area directly from the observation point (see Section 9.2). You will then usually measure the horizontal distances by chaining. 
Carrying out a radiating plan survey with a magnetic compass 

10. Walk over the area you need to survey and choose a convenient central observation station 0. Clearly mark all summits of the polygon. Clear any high vegetation along the future radiating lines of sight.  
11. With your magnetic compass, take a position over the central station 0. Measure the azimuths of the six radiating lines OA, OB, OC, OD, OE and OF. 12. Measure the horizontal distance over each of these lines. 

13. Carefully note down all these measurements in your fieldbook. You can use the first three columns of the table given in the example. Then make a sketch of the area, with the lines and angles and their measurements, on squareruled paper. 14. Calculate the value of the angles between successive points (see 4th column of the table and Section 3.2). Check this by adding all the values: if you find 360° or a figure close to that, the calculation is correct. Example Table for field observations from a radiating survey.
^{1}Since magnetic north falls inside angle AOB, it is calculated as 360º minus the difference of the azimuths. 

Surveying by offset 

3. While chaining line AB, you see two points of interest on either side of it, X and Y, whose exact positions you want to record.  
4. From these points, drop XC and YD perpendicular to line AB (see Section 3.6). Lines XC and YD are offsets. 
Drop perpendiculars from the points of interest


5. Measure horizontal distances AC and CD on line AB. Measure horizontal distances CX and DY along the offsets. 6. From these measurements you can plot the exact positions of points X and Y on paper, if line AB is known. 
Measure the distance to plot the points

Using the triangulation method 

4. On terrain with many obstacles such as hills, marshes or high vegetation, where traversing would be difficult (see Section 7.1), you can use the triangulation method successfully. 5. When you are traversing, and cannot measure a line directly, you can use the triangulation method instead. 6. Triangulation makes locating points on opposite sides of a stream or a lake very easy. 
A good site for a triangulation survey


Using the triangulation method in the field 

7. The simplest way to use the triangulation method in the field is with a planetable(see Section 7.5). You will learn how to survey by triangulation, using a planetable, in Section 9.2. 8. When using the triangulation method, avoid very large angles (over 165°) and very small angles (under 15°). The method works best with angles of about 60°. 
A planetable is useful in triangulation

7.5 How to use the planetable 

What is a planetable? 

1. A planetable is a horizontal drawingboard mounted on top of a vertical support. You use it with a sighting device, a spirit level and a magnetic compass. 
Simple planetable


Making a very simple planetable 

2. You can make a very simple planetable for reconnaissance surveys from a wooden board and a strong pole.  
3. Get a 50 x 60 cm board of soft wood, about 2 cm thick. With sandpaper, polish one of its surfaces well until it is very smooth. Draw two diagonal lines lightly across this surface to find the centre of the board. 

4. Get a straight wooden pole about 5 cm in diameter and 1 m long. Shape one end into a point. This will be firmly driven into the ground at the observation point when you use the planetable.  
5. Preferably using a brass screw, fix the board, smooth side up, by its centrepoint to the top of the pole.  
6. You can make a simple sighting device from an ordinary ruler about 50 cm long by driving two thin nails vertically into it along the centreline for sighting. 
Make a sighting device


7. You will also need a simple magnetic compass to use with the planetable. If you have a spirit level, use it to set up the top board horizontally. Or simply lay a rounded object such as a small ball, a glass marble or a pencil on the board's top surface. When the object remains still, the board is horizontal. 
Make sure the board is horizontal

Making an improved planetable 

8. To survey more precisely, you will need a more complicated planetable than the one just described. This planetable will be mounted on a tripod (a threelegged support) so that:
9. You can build a tripod with legs made out of single pieces of wood, or with adjustable legs. A tripod with adjustable legs is more difficult to make, but it is better since you can set up the planetable more easily on sloping ground by changing the length of the legs. 
An improved planetable


10. A planetable with a normal tripod is adequate for surveying
horizontal areas and areas with small slope gradients, which you must
often survey in aquaculture. To make this type of planetable, you will
need the following materials^{1}:
^{1}Adapted from Using Water Resources, Maryland, USA, VITA Publications, 1977, pp. 137140. 

11. Get a piece of 40 x 55 cm plywood 2 cm thick to use for the drawing board. If the plywood you have is thinner than 2 cm, make two battens (wooden supports) from two pieces of wood 30 x 8 cm and 2 cm thick. Attach these battens parallel to the 40 cm sides of your board, a few centimetres in from each side. The wood you use for the board should be soft enough to allow drawing pins and ordinary pins to go in easily. You should smooth the top of the board with sandpaper if the surface is irregular. 
If the board is thin, strengthen it with battens


Sand the surface smooth

12. Make the three legs from the 1.4 m pieces of wood. Shape each into a point at one end. On the other end face of each leg, mark a centreline parallel to the 2.5 cm sides. Continue this line 5 cm down either side of the leg. At these two points, mark a centred perpendicular line 2.5 cm long; connect the endpoints of this 2.5 cm line up the sides of the leg and over the top. Cut out this block you have marked, which will measure 2.5 x 2.5 x 5 cm, and discard it. Round off the edges of the two remaining "prongs" of wood which face toward the 2.5 cm side of the leg, using a knife and sandpaper, for example.
13. On these prongs, drill a 6 mm hole at a point 1.3 cm from the top of the leg.
Cut out the blocks

Shape the ends into points


Round the tops and drill two holes 
14. Make the rotating connection between the drawing board and the legs with the two circular pieces and the three small blocks of wood. Drill a 6 mm hole in the centre of one of the 15 cm wooden circles. Put a 6 mm bolt through the hole making sure the head of the bolt is even with the top surface of the circle. 
Put the bolt through the centre of the disc


15. Find the centre of the lower surface of the drawing board by drawing two diagonals across it from opposite corners. Hold the wooden circle on this side of the board, with the head of the bolt touching the centre mark. Nail or screw the wooden circle in place. 
Nail the disc to the board so that the bolt sticks
up


16. Take the second 15 cm circle and mark the points where you will attach the legs. To do this, first draw two perpendicular lines across the circle. They should intersect at the exact centre of the circle. Call them diameters a and b. With a protractor, using line b as the 0 to 180° line, draw two more lines from the centre of the circle to the edge at 45° and 135°. Call them radiuses c and d. They should divide one half of the circle into four equal, wedgeshaped sections. Then drill a 6 mm hole in the centre of the circle. 

17. Drill a 6 mm hole on the centre line of the 4.5 x 7 cm face of each 7 cm wooden block, 1.3 cm in from one end. Nail or screw these three 7 cm wooden blocks to the surface of the second wooden circle, so that they join around the centrehole in a Yshape. To do this, align the centrelines of the blocks' 2.5 x 7 cm faces over the lines a, c and d that you drew in step 16. The ends with the holes should be towards the edge of the circle.
Drill a hole in each block

Attach the blocks to the disc, following
the lines you have drawn 

18. Place this wooden circle, with the blocks facing you, against the circle already fixed to the underside of the board. Pass the bolt in the first circle through the centrehole of the second circle. Add a washer and a wing nut to it and tighten them securely. 
Mount the disc on the board


19. Align the holes in the three legs with the holes in the three blocks of wood on the underside of the board, and attach the legs with bolts, washers and wing nuts to the blocks. Your planetable is now ready to use. 20. You will also need a small spirit level, a magnetic compass, a sighting device called an alidade. You have already learned a one kind of alidade (see Section 3.1), but this one will be slightly different.

27. On each metal piece, use a nail to draw a fairly deep line perpendicular to the centreline, at a point 2 cm from the end without a slit or window. Then make three small holes parallel to this line and between the line and the end of the piece, using a hammer and nail. Sharply bend this end of the metal along the deep line, until it forms a right angle with the rest of the piece. 

You will use the alidade set flat on the planetable. You will sight through the slit at the wire. You will draw the line along the centreline of the wooden strip. 

Using the planetable 

29. You can use the planetable in two different ways, depending
on the type of survey you are making:
The planetable can also be used for measuring horizontal angles. 30. Before you plan survey with the planetable, you will need to:
You will learn more about each of these procedures later (see steps 3447). 
31. When you are ready to start surveying with your planetable, you will then:


You will learn more about each of these procedures later (see Chapter 9). 

What are the advantages of planetabling? 

32. Compared with other methods of plan surveying, planetabling is better in some ways because:

Setting up the planetable 

40. If you decide to start the survey from a selected station, first set up the planetable over this station. Note: you may need to set up the planetable so that a point drawn on it is exactly over a corresponding ground point. You can use a Vshaped metal arm and a plumbline, which you can easily make yourself. 

Otherwise, you can use calipers and a plumbline. The metal arm or calipers should be placed with one tip touching the point on the planetable and the other tip on the underside of the table. Hang the plumbline from the point indicated on the underside of the table, and move the table until the plumbline is directly over the ground point. 41. Spread the tripod legs well apart, and plant them firmly in the ground. The drawing board should be waisthigh, so that you may bend over it without resting against it. 

42. Rotate the table top so that the paper is in a position that allows you to draw the whole area you need to survey on it.  
43. Choose the scale you will use (see Section 9.1), making sure it will allow you to plot even the most distant point on the paper. You can first walk quickly over the terrain you will survey to check the distances by pacing so you can decide on the right scale to use (see Section 2.2). 
Level the tabletop in both directions 

44. Level the board with the spirit level, making it as horizontal as possible. To do this, first place the spirit level along one side of the board, parallel to two legs of the tripod and adjust the table to a horizontal position. Then place the level along the side perpendicular to that pointing toward the third leg of the tripod and adjust again. Repeat this process until the board is horizontal. 
Orienting the planetable 

45. You can orient the planetable either by using a magnetic compass or by backsighting. Usually, the board is first oriented roughly by compass, and then more precisely by backsighting.  
46. If you use a magnetic compass (see Section 3.2), rotate the compass until the direction of the needle lines up with the direction of southnorth, or the 180° to 360° direction. Draw a line on the drawing paper showing this direction. Draw another line in the same direction on another part of the paper. Mark the north direction on these lines with an arrow and the letter N. Note: remember to keep away from any materials which could have an effect on the magnetic needle of the compass (see Section 3.2, step 17). 

47. If at a surveying station you know the direction of a line which you have already plotted on the board, you can use that line to orient the planetable by taking a backsight. It is the most precise way of orienting the plane table and you should use it whenever possible.  
Example From station A, you have already plotted line ab. Set up the planetable at station B. Place the centreline of the alidade along line ba on the board. Rotate the board until the line of sight on the alidade lines up with line BA on the ground. The table is now oriented. You can proceed to survey and plot new points. 
Station B

48. During reconnaissance surveys, you can use planetabling to quickly map out areas and open traverses. The survey will proceed by one of the methods described earlier in this chapter or a combination of them. This method may be:



You will learn more about mapping with a planetable by these surveying methods in Chapter 9. 
Planetabling for plotting details 

49. When you have finished the reconnaissance survey and accurately mapped the main stations, you can further use planetabling to locate details such as rocks, buildings, a well or a group of trees.  
50. To do this, set up the planetable at each of the main stations in turn, and draw sighting lines to each of these features. 51. You can locate each detail on the drawing board by finding the intersection point of at least three sighting lines. You will not have to take any more measurements. 
ABCD main stations

Example
During a reconnaissance survey you have accurately mapped the fishfarm site ABCDA using your planetable. You want to add the exact positions of a rock outcrop X and a group of buildings Y. Proceed as follows:
Site ABCDA

Sight from point A


Sight from point B

Sight from point C


Sight from point D

The intersections determine points X and Y

Measuring horizontal angles by planetabling 

52. You can measure horizontal angles fairly accurately by drawing sighting lines on a planetable and measuring this angle with a protractor (see Section 3.3). 
Draw ab


Example

Draw ac



Measure bac
