1.0 What is topography?

Topography is the science of measuring the earth and its features, and of making maps, charts and plans to show them. These features may be natural, such as plains, hills, mountains, lakes, streams, rocks or forests. They may also be man-made, such as paths, roads, buildings, villages or fish ponds. A topographical map can also show the slope of the ground. It can show not only which places are high and which are low, but also how steep the land is between high and low points.

Sometimes topography is also called surveying. A person whose profession is making topographical measurements and recording them on maps, charts and plans is called a surveyor.

1.1 Purpose of this manual

In this manual, you will learn what you need to know about topography to help you choose a good site for your freshwater fish-farm, and to design and build fish ponds, reservoirs for storing water, and small dams. You will also learn how to draw your own topographical maps and how to use topographical maps that are already available.

To do all this you will learn:

  • How to measure distances, angles, slopes, and height differences,
  • How to set out straight lines, perpendiculars, and parallels in the field;
  • How to determine horizontal and vertical lines;
  • How to survey an area of land to find its size and its high or low, flat or sloping features (called the relief);
  • How to make simple surveys that will help you when you are ready to build your fish-farm;
  • How to prepare and how to use topographical plans and maps;
  • How to calculate areas and volumes.

You will learn some of the technical language that land surveyors and civil engineers use. This way, you will be able to discuss your plans and projects with them more easily, and you will better understand books on topography, engineering and surveying.


1.2 Why do you need to know about topography?

Choosing a site

1. In other books in this series, Simple Methods for Aquaculture, FAO Training Series (4 and 6), you learned how to study water and soil on a site before deciding to build a reservoir for storing water and a fish-farm on it. Topography is also very important when you are choosing a site. Good fish-farm construction is possible only with the right topography.

2. After you choose a possible area of land for your fish-farm, you will need to measure:

  • its size;
  • the slope of its ground surface;
  • its elevation (height) in relation to the source of water you want to use.

You will also need to find out:

  • the distance between the source of water and the location of the ponds;
  • the best way to supply water to the ponds;
  • the easiest way of draining the ponds.

3. You will need to measure distances in different ways. You may have to measure very long distances; to do this, you must know how to measure distances along a straight line, and how to keep this line straight. In addition, when you measure on a slope, you will need to find the horizontal distance rather than the distance on the ground.

4. When you are looking for a site for a small dam, you will find that the best choice is a narrowing valley where the stream slope is not too great and the valley walls are steep. You can use an existing topographical map to help you find such a valley, or you can measure a valley yourself to see if it is a good dam site.

5. When you have chosen the best site for your fish-farm, topographical methods will help you to decide whether you can do all the work that is needed on it.

GR000004.JPG (33181 byte)

Surveying a site

6. After you choose a possible site for your fish-farm, you will need to survey the site, keeping in mind your plan for the farm. For this survey you will measure distances, directions, areas, slopes and height differences in more detail.

7. To do this, you will have to draw a detailed topographical plan. This plan will show the position of boundaries, the different heights of land forms such as hills, and the location of existing physical features such as paths, roads, streams, springs, forests, rocks and buildings. Such a plan is very important because it gives you the basic horizontal and vertical elements of the area, which will guide you in your design of the farm. It shows you the direction the water will have to take, from higher to lower points. It guides you in choosing where the water-supply canal, the ponds and the drainage ditches will go. It becomes the basis for estimating how much earth you will have to move as you build, and how much all the work will cost.

8. All the physical features of your fish-farm depend directly on the topography of the site. These features include the type, number, size and shape of the fish-ponds, and how they are placed in relation to each other. The supply of water and the type of drainage also depend on the topography of the site.

Fish farm

Making a construction survey

9. Once you have made a detailed survey of the site you have chosen, and designed the fish-farm or dam (see next volumes in this series), you will use topographical methods to help guide you as you build.

10. You will need to make sure that your fish-ponds regularly get the right amount of water. To do this, you must build a water supply canal with the right size and bottom slope. First, you will need to stake out the water supply canal, along its centre-line. You will then need to tell the workers helping you exactly how wide, deep and long the canal must be, and how much earth they must remove at each point of the canal.

11. You will need to stake out the bottom area of each pond and tell the workers how much earth to remove and where to transport it. This will allow you to drain all the water out of the ponds in a natural way. It will then be easy to harvest your fish and to manage your pond.

GR000890.JPG (10193 byte)

12. You will need to stake out the dikes of each pond and show the workers where to remove soil and where to add soil. You will also need to mark the location, height and width of each dike, as well as the slopes of their walls. Usually, you will need to set out perpendicular*(crossing) and parallel* (side-by-side) lines to do this.

13.You will need to follow the exact plan of your fish-farm as you work. To do this, you will have to be sure where each structure should be built, and you will have to check these locations during construction. You will need to measure differences in height between the different parts of the farm to make sure that the water will flow naturally in the right direction. The water will have to flow, for example, from the water source to the ponds, from the pond inlets to the outlets and from the outlets into a drainage ditch, which carries the water away from the farm site.

Working from the plan

Studying your water supply

14. In Water for Freshwater Fish Culture, FAO Training Series (4), you learned how to use simple topographical methods:

Measuring the volume of a pond
Choosing a site for a reservoir

Studying your soils

15. In Section 1.3 of Soil and Freshwater Fish Culture, FAO Training Series (6), you learned that the qualities of soils vary depending on the topography of the area. Shallow soil is found on sloping land and deep soil is found on flatter land, for example. You learned that alluvial soil, which is found in sedimentation plains, often contains large amounts of clay. The clay in this soil helps it to retain water, and makes it a good material for building dams.

16. You will need to use topographical methods to draw a map showing the different kinds of soil present in an area of land. In Soil and Freshwater Fish Culture, Sections 2.4 and 2.5, you learned how to use two of these methods: reconnaissance surveys and detailed soil surveys.

A map for studying your soil

1.3 There are two kinds of lines in topography

1. Almost all topographical methods are based on lines. There are two kinds of lines, lines of measurement and lines of sight.

  • Lines of measurement may be either horizontal or vertical or they may follow the level of the ground. These lines are clearly plotted in the field with markers to show the exact path along which you will measure. A line of measurement can be:
    • a straight line, which runs in one direction between two marked end-points;
    • a broken line, which changes direction more than once between two marked end-points, with each point at which the direction changes also marked;
    • a curved line, which is marked like a broken line, but with markers much more closely spaced so that the curve is clearly followed.
  • A line of sight is an imaginary line that begins at the eye of the surveyor and runs towards a fixed point. Lines of sight are either horizontal or oblique (between horizontal and vertical).
Straight line
Broken line
Curved line

1.4 How to plot lines in the field

1. Lines of measurement are always plotted on the ground either as one straight line or as many connected straight lines. The markers that show where the line goes can be pegs, small concrete pillars, simple wooden stakes or ranging poles (see next Section).

2. Lines of sight* are always straight lines. The object or point you look toward, called the point of reference, is marked either by a ranging pole or a levelling staff (see Section 5.0).

3.Vertical lines of measurement can be formed with the help of a plumb-line (see Section 4.8).

Some field markers
A ranging pole
A plumb-line

1.5 How to make and use markers to plot your line

1. You can make wooden pegs to use when you plot your line. Get straight pieces of wood 3 to 8 cm in diameter and 0.1 to 1 m long. With a knife, shape the pieces at one end to make sharp points so that they can be easily driven into the ground. Your pegs will last longer if you make them out of hard wood and coat them with used engine oil to prevent rotting.

2. You can use iron pegs, made of cut pieces of iron rod or tube about 1.3 to 2 cm in diameter. You can also use long wire nails. Iron pegs last longer than wooden pegs, but they cost more and they are heavier and more difficult to carry when you are working in the field.

3. When you have a point on the ground that you will need to refer to for a long time, you can mark it with a small, upright pillar made of concrete. Such pillars should be from 15 to 30 cm square, and 10 to 60 cm high. You can build them on the site, placed on a small concrete base.

Note: to make your measurements in the field more accurate, you will often need to mark a point on your peg or pillar. This will show exactly where you must take the measurement, or place a measuring instrument. To do this, you can drive a nail into the flat top of the wooden peg, or you can set a nail into the top of the concrete pillar.

The nail shows the exact point
GR13_a.GIF (11726 byte)

4. Ranging poles are the most commonly used markers in topographical surveys. Ranging poles are long, thin poles. You can use them to mark a point on the ground that you need to see from a distance. You can easily make your own ranging pole. Get a straight wooden pole, 2 to 3 m long and 3 to 4 cm thick. Shape the lower end into a point to make it easier to push into the ground. At the other end of the pole, cut a slit 5 cm deep into the top side. Then, starting from the top end, paint one 40 cm long section red; paint the next 40 cm section white. Continue painting the pole in alternating red and white sections until you reach the end.

5. Sometimes you will need to sight a ranging pole from a long distance. To make the pole easier to see, fasten two small flags of different colours, one above the other, near the top of the pole. Or you can place a 15 x 25 cm piece of white cardboard in the slit at the top of the pole.

Ranging pole
GR14.GIF (4856 byte)

White card for sighting
GR14_a.GIF (4585 byte)

Colourful flags for sighting
GR14_b.GIF (2939 byte)

6. You must always drive ranging poles vertically into the ground. To check that your pole is vertical, take a few steps back and look at it. If it seems straight, walk one-quarter of the way around the pole, and check that it also looks straight from the side. Adjust it if necessary until the front and side views are both vertical.

7. At times you will have to centre a ranging pole over a marker and leave it in position for some time. To do this, you can use a series of guys. These are ropes or wires which you will tie around the pole, and fasten to pegs in the ground. You can also use guys with the pole on hard ground, whenever you cannot drive the pole deep enough into the ground to keep it in a vertical position.

Check from both sides that the ranging pole is vertical
GR15.GIF (9897 byte)
A guy to hold the ranging pole steady
GR15_a.GIF (15019 byte)

1.6 How to set out a straight line between two points

1. When you carry out a simple survey, you will often need to set out straight lines between two given points, called A and B, which are more than 50 m apart. To do this, you will "range" line AB. This means that you will plot intermediate points along line AB at intervals preferably shorter than 30 m.

2. When you range a line, you will face one of two possible situations:

  • you can see point A from point B and vice versa;
  • you cannot see point A from point B. In this case, an obstacle (a forest, river, lake, etc.) is in the way.

Setting out a straight line between two points visible from each other

3. You want to set out line AB. Mark the beginning of the line, point A, with a ranging pole. Then mark the end of the line, point B, with another ranging pole. You will now need an assistant to help you.

4. Stand about 1 m behind ranging pole A and look at ranging pole B. Your assistant should stand at ranging pole B. Ask him to walk, carrying another ranging pole, about 40 paces from B towards A, and stop.

5. Ask your assistant to move slowly sideways while he holds the ranging pole vertically in one hand between his thumb and forefinger. When the ranging pole he is holding hides ranging pole B,ask him to stop and to drive his pole vertically into the ground. This is intermediate point C.

6. Ask your assistant to walk 40 paces toward you, from C toward A. Then repeat the same procedure as above with a fourth ranging pole. Mark the new intermediate point D.

Setting out line AB with ranging poles
GR16_a.GIF (8399 byte)

7. If the distance from D to A is greater than 50 m, you should repeat the same procedure and mark the next intermediate points E, F, G

Note: always make sure that the ranging poles are vertical.

GR17.GIF (7690 byte)
Make sure the ranging poles line up exactly
GR17_a.GIF (5625 byte)

Setting out a straight line between two points when you cannot see one from the other

8. You have to set out line AB, which runs through a forest. Mark points A and B with ranging poles. Choose a point X, which is beyond point B, and which you can see clearly from point A. Mark point X with a ranging pole or a marking peg. Then set out a line as above from point A to point X, avoiding the forest.

9. Look at Section 3.6 of this manual, and learn how to drop a perpendicular. Then, from point B, how to drop a perpendicular. Then, from point B, drop perpendicular BC onto line AX. The lines will cross at point C.


GR18.GIF (5486 byte)

GR18_a.GIF (2574 byte)

GR18_b.GIF (2500 byte)
Set out line AX
GR18_c.GIF (2892 byte)
Drop perpendicular BC

10. Choose a point D on line AX, close to the forest, and set out perpendicular DY. Point Y must be on the other side of line AB.

11. Measure horizontal distances horizontal distances AD, AC, and CB, using one of the methods described in Chapter 2.

12. Intermediate point E will be the place where line DY intersects line AB. To find its exact location, you must calculate horizontal distance DE using the formula:

DE = AD x (CB � AC)

13. To mark point E, you must measure this distance DE horizontally. Starting from D, pace off the distance DE along line DY. Mark intermediate point E with a ranging pole.

14. Walk along line AX to the other side of the forest. Set out a perpendicular FZ close to the forest, using one of the methods described in Section 3.6. Point F is on line AX; point Z is beyond line AB.

15. Measure horizontal distance AF (see Chapter 2).

16. Point G will be the intersection of lines AB and FZ. To find it you must first calculate horizontal distance FG as:

FG = AF x (CB � AC)


17. Measure this distance FG horizontally. From F, measure along line FZ to determine point G. That is, the intersection of line FZ and line AB. Mark intermediate point G with a ranging pole.

18. You have now clearly laid out and marked line AB in the field as line AGEB  

GR19.GIF (3872 byte)
Set out perpendicular DY
GR19_a.GIF (4567 byte)
Set out perpendicular FZ
GR19_b.GIF (3960 byte)
Connect the points you have found to form line AGEB
GR19_c.GIF (5462 byte)

1.7 How to prolong a line you have marked in the field

1. You will often need to make a straight line you have marked longer; this is called prolonging a line. As in the previous section, you will have to consider two different situations:

  • prolonging a line where there is no obstacle;
  • prolonging a line behind an obstacle.

Prolonging a line where there is no obstacle

2. Mark a straight line AB in the field with a ranging pole at each end. If you are working alone, take a ranging pole and walk away from point B, following the direction of line AB as closely as you can. After you have walked about 40 paces, stop and turn around to face ranging poles B and A.

3. Hold your ranging pole vertically in front of you between your thumb and forefinger. Then move slightly sideways, if necessary, until your pole seems to hide ranging poles B and A from your view. Drive your pole into the ground in a vertical position.

4. Step back 1 to 2 m along the line and check to see if ranging poles B and A are still hidden behind your ranging pole. If they are not, move your pole a little to the left or right, and step back and check again. Repeat this procedure until your pole is in the right position. This then marks point C, which prolongs line AB.

5. If you are working with an assistant, stand 1 to 2 m behind ranging pole A to determine a line of sight AB. Your assistant should stand by ranging pole B.

6. Ask your assistant to walk, carrying a ranging pole, about 40 paces from ranging pole B in the direction away from you. He should then stop and turn around to face you.

7. While your assistant holds his ranging pole vertically, ask him to move to the left or right until ranging poles A and B hide his ranging pole. At that point, direct your assistant to drive his ranging pole vertically into the ground. This marks point C, which prolongs line AB.

GR20.GIF (7593 byte)
Line up all the ranging poles alone
GR20_a.GIF (8562 byte)
... or with an assistant

Prolonging a line behind an obstacle


8. You want to prolong line AB to a point behind a forest. Set out perpendiculars AX and BY from points A and B respectively, using one of the methods described in Section 3.6.

9. On these two perpendiculars, measure equal horizontal distances AA' = BB' You must make sure that this distance is far enough along the perpendiculars so that the line joining points A' and B', when prolonged, will just clear the obstacle.

10. Prolong line A'B' through C' and D',well past the obstacle, using the method described above in steps 2-7.

11. At points C' and D' set out perpendicular lines C' Z and D'W (see Section 3.6).

12. On these two perpendiculars, measure horizontal distances equal to AA' (see step 9 above) and determine points C and D. Mark these points with ranging poles. You have now prolonged line AB with line CD.

GR21.GIF (5424 byte)

You must prolong line AB through the forest
GR21a.GIF (2507 byte)

GR21_a.GIF (3751 byte)
Set out perpendiculars AX and BY, then find points
A' and B' and join them with a line
GR21_b.GIF (4040 byte)
Prolong line A'B' to points C' and D'
GR22.GIF (5403 byte)
Set out perpendiculars C'Z and D'W

Find points C and D and join them to prolong AB

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GR22_a.GIF (8286 byte)