4. Before you begin a topographical survey, you should try to get any available topographical plans and maps of the area, even though they may not be exactly the kind of plan or map that you need. General topographical maps are available from governmental organizations which are responsible for geological surveys or land surveys, for example. National geographical institutes, soil survey departments and agricultural development agencies can also usually provide existing topographical maps. The cadastral department (that calculates land taxes) of your local government may provide local topographical plans.
5. You will often have to make the topographical plans and maps yourself, however. You will base them on a plan survey (see Chapter 7) and direct levelling (see Chapter 8). In the following sections, you will learn how to:
6. On topographical plans or maps, you should always look for:
This information is often located in one corner of the map. It is called the legend.
2. There are three ways of expressing the scale of a drawing:
3. Table 11 gives the numerical equivalents of the most common scales, expressed as fractions. Scales for both distances (in metres) and surface areas (in square metres) are shown also.
4. General topographical maps usually have scales ranging from 1:50000 to 1:250000. These are small-scale maps. In most countries, 1 :50000 maps are now available. You can use these for general planning of aquaculture development, including the planning of your fish-farm.
5. To show greater detail, plans are drawn to a larger scale,
showing individual structures or land areas. The scales most often used in plans
are 1 :500, 1 :1000, 1 :2000, 1:2500 and 1:5000. Detailed engineering drawings
use scales much larger than 1:500, for example
1 : 100 or 1: 10.
Note: special rulers, called "Kutsch" scales or reduction scales, make it easy to transfer ground distances onto drawings.
Distances and surface areas expressed by scales
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9.2 How to make a map by plane-tabling |
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1. In Section 7.5, you read that you can use a plane-table to make a reconnaissance survey and to plot details. In this section, you will learn how to do this. It is best to use an alidade for this method (see Section 7.5, steps 21-28), but you can use a simple ruler, and a series of tailor's pins to show the observed directions, instead. 2. First, choose an appropriate scale for the map you will draw (see Section 9.1, steps 4 and 5). Get an estimate of the longest distance you need to map, and decide upon the size of the map you require. If the map is to be fairly large, you can draw it on several sheets of paper, and glue them together. |
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Example
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3. Cover the board of your plane-table with paper
(see Section 7.5, steps 34-39). Set up the plane-table
(see Section 7.5, steps 40-44) on or near some major feature A
of the area you need to map, such as a large rock, a path, a river or a
tall tree.
4. Using a well-sharpened pencil with a hard lead, mark a small point and circle on your paper. This is point a, the location of the major feature, where you have set up your plane-table. Be sure to choose a section of the paper from which you can later map the entire area. For example, if you will be mapping only ahead of point A, begin near the centre of the bottom margin of the plane-table. Note: you will identify physical features in the field that you need to map with capital letters. You will identity the corresponding points that you draw on the plane-table sheet with lower-case letters. |
Set up the plane-table at point A
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5. Rotate the table so that you will be drawing the map in the orientation you need. Using your magnetic compass as a guide, draw arrows showing the magnetic north (see Section 7.5, steps 45-46). Note: you should always try to locate the north facing the top of your map. This is a rule which is always applied in professional topographical maps. You may not be able to follow the rule, however, depending on the direction of the longest distance and on the scale you select. |
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6. Using your alidade, sight from the first marked point a to another major feature B which you can see from the plane-table location. This could be a small hill, a bend in a path or a ranging pole. Draw a thin line ax in this direction. Note: you can use the alidade much more easily if you place a pin at point a on the board, and then swivel the alidade around the pin until you can sight the second point. |
Draw ax
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7. Measure the horizontal ground distance from the plane-table station A to the major feature B. Then mark this distance along line ax, starting at point a and scaling it down as line ab. 8. Without moving the plane-table from point A, repeat this process for all other major features C, D, etc. which you can see, and draw lines ac, ad, etc. |
Draw ay |
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9. Move the plane-table to one of the major features you have just mapped, such as C. Choose a feature from which you can easily map another part of the area, such as the route of a path or the course of a river. 10. Set up the plane-table over this point C. Reorient the table. Use the compass and the magnetic north arrows you have already drawn (see step 5 above), or, instead, use the alidade, backsighting along a drawn line which passes through the new station C and a known major feature such as A (see Section 7.5, step 47). |
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11. From this new station C, map in the new major features which you can see, as explained above. 12. If necessary, move to other stations to complete the mapping of the entire area. If you need more details in the map, go back to one of the mapped features, reorient the table by backsighting on another mapped feature, and map the details as required. |
13. You can use the above procedure for plane-tabling in several different situations in the field, such as:
Usually, you will use a combination of some of these surveying methods to map an entire area.
Open traverse
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Closed traverse
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Radiation
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Triangulation
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Mapping a closed traverse with a plane-table |
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17. You need to map a closed traverse ABCDEA. First, set up the plane-table at station A and plot this on paper as point a; choose a scale and a location on the paper which will allow you to plot the other stations within the limits of the sheet of paper. 18. Using the alidade, take a foresight through point a to station B and draw line ax. Measure distances AB and map point b on line ax. |
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19. Move the plane-table to station B, set it up over the point, and orient it by backsighting along line ba on station A. Take a foresight to station C, measure distance BC, and map point c. | ||
20. Using this procedure, map the locations of the remaining points on the closed traverse. At the end of the traverse, when you plot the initial station A again, you can see any error of closure. If this error is within reasonable limits, correct it, using the graphic method explained in Section 7.1, step 19. | ||
21. From one station on the traverse, you may be able to see
two or more of the preceding stations which are not on the same straight
line as the station where you are standing (for example, from C to A, from
D to B, or from E to B). In this case, check the other parts of the traverse.
Example From station C, station A is visible. You should check from C the position of point a by backsighting on Station A. |
Take additional backsights to check your work
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Choose the proper angle
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Too small
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Too large
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Best angle
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Mapping with a plane-table by combined methods |
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31. In most cases, you will map an area with a plane-table by using a combination of the methods of traversing, radiating and triangulation. 32. You need to map site ABCDA, which includes such features as a rocky area, a group of houses and a well. Clearly mark points A, B, C and D with ranging poles. |
Site ABCDA
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33. Set up the plane-table at corner A of the area. Locate the mapped position of A on the sheet of paper. Be sure to choose a point which will allow you to plot the other features of the map within the limits of the sheet of paper at the drawing scale you have chosen. Orient the sheet by drawing the direction of magnetic north. 34. From station A, you can see the rocks and the houses. By radiating, determine the directions of the rocks and the houses from this station. Then measure and map AB. |
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35. Move the plane-table and set it up over corner B. Roughly orient the board by backsighting to A, and check this orientation with the magnetic compass. 36. From station B, you can see the rocks and the well. By radiating and triangulation, locate and map the rocks, and determine the direction of the well. Measure and map BC. |
At B
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37. Repeat this process at point C, from which you can check on the position of the rocks and locate the well and the houses. Measure and map CD. |
At C
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38. Repeat this process at point D, from which you can check on the positions of the houses and the well. Measure and map DA. |
At D
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39. Check the error of closure of traverse ABCDA and correct it, if possible. If the error is too great, repeat the survey. 40. Finish the map, checking that you have included all the information you need (see Section 9.0, step 6). |
Finished map |
9.3 How to map by protractor and scale |
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1. When you map in the office, using field records, you will usually plot horizontal distances with a ruler/scale, and the horizontal angles with a protractor (see Section 3.3). 2. First, using the scale you have chosen, make a rough sketch of the area to determine its size and shape. From this sketch, decide how large a piece of paper you will need to make everything fit and determine the position of your map on the sheet of paper. |
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3. Draw the first line in the right place on the paper and determine its length AB, using the selected scale. Using a pencil with a hard lead, accurately mark points A and B on the paper as two dots with a small circle around each. Note: draw the line so that it will extend beyond the next angle-point B, a distance greater than the radius R of the protractor. |
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4. Place the protractor along line AB so that:
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5. Plot the angle, which you have obtained from your field notes, remove the protractor, and draw the second line. Locate and map point C according to the measured distance and scale. |
6. Place the protractor along this last line with its centre
opposite point C. Lay out the measured angle, and draw the third line. Locate
and map point D according to the measured distance and scale.
7. Repeat this process until you have mapped the entire traverse. |
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8. Locate the details on the plan from this traverse line. Plot the positions of buildings, fences, rocky areas, streams, paths, etc., using the scale for distances and the protractor for angles. 9. You can use a method similar to this one to map survey information which you have obtained by radiating, by triangulation, and by offsets. |
1. Contour lines are lines drawn to join points of equal elevation. On a plan or map, they represent the contours you found and marked in the field (see Section 8.3). Contour lines show the three-dimensional ground topography of a site on a two-dimensional map or plan.
2. As you have already learned (see Section 8.3, step 7), contours are surveyed on the basis of a selected contour interval. Similarly, contour lines are drawn at equal vertical intervals. You should always clearly state the contour interval of the mapped contour lines.
Hill
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Peak
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Two hills
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Valley
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Contour interval = 50 m |
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3. If you clearly understand the characteristics of contour
lines (CL), you will be able to survey, make contour maps and read
maps much more easily. The facts to remember are:
Note: when two contour lines of equal elevation are near each other, the land between them is often flatter than the general trend of slope but its slope is indeterminate (unknown). Areas of indeterminate slope |
7. Find the points of lower ground elevation. Then, according to the contour interval you have chosen, determine which elevation represents the first contour line you need to draw.
8. The first contour line will pass between ground points with elevations which are lower and higher than the elevation of the contour points. Carefully locate the path of the contour line between these higher and lower points, as you draw. Note that contour lines are usually curved, not straight. You should draw them free-hand, rather than using a ruler to connect the points.
9. Using the same procedure, draw the other contour lines. Show the progressively higher elevations as multiples of the selected contour interval. Note: contour lines are only drawn for elevations which are multiples of the contour interval. Show the elevations of the contours by writing in numbers at appropriate intervals; the contour line is usually broken to leave a space for the number. |
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10. This general procedure may vary, depending
on the contour surveying method you have used in the field.
(a) If you have used a direct method (see Section 8.3, steps 10-29), the plan survey of the contours you have identified gives you all the information you need to map the corresponding contour lines. You will reduce the measured distances to scale, and use the parallel lines marked on the ground as a background to the contour lines. (b) If you have used an indirect method (see Section 8.3, steps 30-33), you will lay out the pattern of lines roughly in the drawing, map the points of known elevation and note their elevations. Then, estimate the position of the contour lines, as explained above. |
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1. Longitudinal profiles are plotted to show relative elevations on a plan. When you design a fish-farm, longitudinal profiles help you to determine the route and the bottom slope of such works as water-supply and drainage canals. They are also useful when you need to estimate the amounts of earth you need to dig out or build up on a site (called the volumes of earthwork), and when you choose sites for the construction of reservoir dams and river barrages (small dams that channel the water into ditches or canals).
Information from which longitudinal profiles can be plotted |
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2. You plot a longitudinal profile as a continuous line drawn through points of known elevations. The information you use for this can be:
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Scales to be used for longitudinal profiles |
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3. You need two different scales to be able to plot longitudinal profiles:
Both scales should use the same unit of length. This is usually the metre. 4. The horizontal scale of the profile should preferably be the same as the scale of the plan or map. Example If the scale of the plan is 1 cm per metre, the horizontal scale of the longitudinal profile should also be 1 cm per m. |
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5. In most aquaculture surveys, the differences in elevation are very small in comparison to the horizontal distances. When you plot longitudinal profiles for such a survey, you will therefore need to make the differences in elevation seem larger. You can use a vertical scale which is from 10 to 100 times larger than the horizontal scale. |
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Example
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Plotting profiles from contour maps |
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6. Get some sheets of square-ruled millimetric paper. Or, use one sheet as a guide only, placing it under a sheet of transparent tracing paper on which you will plot your profiles. 7. On the contour map, draw line AB along which you need to determine the longitudinal profile. Study the range of the elevations you will plot, choose the vertical scale, and decide where to start your drawing so that it will fit within the limits of the sheet of paper. Choose the horizontal scale equal to the scale of the contour map. |
Contour interval 2 m |
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Example Contour map with contour interval = 2 m; |
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8. Cut a strip of paper a little longer than the longitudinal profile AB you need to draw and about 2 cm wide. Place this paper strip on the contour map with one edge exactly on line AB. 9. Mark points A and B with thin vertical lines to indicate the end-points of the longitudinal profile. In a similar way, mark the position of each of the contour lines along the edge of the strip. Note the elevations of the main contour lines next to their mark. |
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10. Place the paper strip on the drawing sheet. Its marked edge should line up with the horizontal line representing the lowest elevation present (484 m) in the longitudinal profile. Align point A on the strip with the starting point of the drawing. 11. Transfer all the pencilled marks from the paper strip to the drawing and note the main elevations next to their marks. |
12. Using the vertical scale as a reference, transfer each of these marks vertically up to the horizontal line that corresponds to its elevation. Using a sharp pencil with a hard lead, make a small circled dot at each of these points on the lines.
13. Join these points with a continuous line, which represents the longitudinal profile of the ground along selected line AB.
Note: you can only apply this method if the horizontal scale of your drawing is the same as the distance scale of the contour map.
Plotting profiles from your own field survey |
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14. You can use measurements of distances and elevations from a field survey to plot profiles. Along the horizontal axis, first plot the positions of the survey stations which you have located, for example at regular intervals along a centre-line (see Section 8.2) using the horizontal scale (here 1 cm = 10 m) as a basis. Next to each of these points, mark its distance from the starting point of the profile, the cumulative distance* (in m). | ||
15. For each of these points, plot the elevations on vertical lines, using the vertical scale (1 cm = 5 cm) and the two extreme elevations (1.34 m and 1.06 m) as bases. 16. Join these points with a continuous line, which represents the profile of the ground along the centre-line. 17. Add more information, such as the elevations of the bench-mark (BM) and of any turning point (TP). If you also plot the proposed canal slope (0.15 cm/m = 7.5 cm/ 50 m), you can use the drawing to easily locate areas where you need to raise the land to a required level (called a fill*), or places where you need to dig a channel (called a cut*). Then you can use the drawing to estimate the amount of earthwork these will require. |
9.6 How to plot cross-section profiles |
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1. You can plot cross-section profiles either from contour maps or from levelling-survey information. 2. A good example of when to use a cross-section profile plotted from a contour map is for a study of a river valley when you want to create a water reservoir, or build a small barrage that will raise the water level and fill the fish-ponds by gravity. 3. If you use the information from a levelling survey, you can plot cross-section profiles to calculate volumes of earthwork when you are building water canals and fish-ponds, for example (see next manual on Constructions, in this series). |
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Plotting cross-section profiles from contour maps |
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4. On the contour map, draw the lines along which you will study the profiles. These lines should be perpendicular to a longitudinal profile. 5. Get several sheets of square-ruled millimetric paper, or use one sheet as a guide only, under transparent tracing paper. Plot the cross-section profiles with the help of a marked paper strip (as described in Section 9.5, steps 8-13). |
Scale: 1 cm = 20 m
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6. Remember that:
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Cross-sections of a valley
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Plotting cross-section profiles for earthwork estimates |
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7. To estimate how much earthwork you need to do, you can usually plot cross-sections to a scale of either 1 cm per metre or 1 cm per 0.5 m. Use the larger scale when the amount of a cut or fill is small. Horizontal scales and vertical scales should be identical, so that you can obtain a true surface area from the scaled dimensions. 8. You can plot best on square-ruled millimetric paper or use one sheet of such paper as a guide placed under a sheet of transparent tracing paper. 9. Draw a vertical centre-line (LL) representing the centre-line of the cross-section profile. LL should follow one of the heavier lines of the squared-ruled paper. 10. On both sides of this centre-line, draw the ground profile EFD on the basis of your levelling data, using the horizontal scale for distances and the vertical scale for elevations. |
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11. From your longitudinal profile, locate point A on line LL. In the example, it represents the elevation of the bottom of the canal at this particular levelling station (see Section 9.5, step 17). 12. Through point A, draw a horizontal line BAC to show the canal bottom. Make sure that AB = AC, and each is half the width of the canal bottom.
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13. Through B and C, draw lines BE and CD representing the sides of the canal (for example, with a slope of 1.5: 1). These two lines intersect the ground surface at points E and D. 14. The cross-section EBACDFE represents a vertical section of the earth. You can then easily calculate the area of this cross-section (see, for example, Section 10.3). Using this area as a basis, you can estimate the volume of earth you need to remove from this location along the centre-line of the canal. |
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