INTRODUCTION TO LAND SURVEYING
A Brief Historical Account: It is not yet specifically known where surveying originated, but it is very probable that it has its origin in ancient Egypt. For instance a plan of the villa of a great Egyptian noble was found in a Thebian tomb of the eighth century dynasty. In the tomb of one Menna, at Thebes, there is a representation on the walls of two chainmen surveying a field of corn, and in Ptolemaic and Roman Papyri in the same country, measurements of plots of land are described.
The invention of printing and engraving in Europe in the fifteenth century and the age of discoveries which followed contributed much to the expansion of surveying and mapping activities.
Definition: Surveying is the science which has to do with the determination of the relative positions or locations of points on the earth’s surface. This involves the act of measuring horizontal and vertical distances and of determining direction and angular relationships by the use of surveying instruments in the field. Information from surveying is generally depicted graphically in the form of maps and charts. Since maps are regarded as the tools of geographers, it is imperative therefore for geographers to know the methods by which maps are produced.
Classification: Surveying can be classified on two main bases. First, on the basis of the instruments used as in case of compass surveying, chain surveying, plane table surveying, photogrammetric surveying and trigonometric surveying. Second, on the basis of the function of the surveying as in case of topographic surveying, hydrographic surveying, route surveying, etc.
Operation: The operation of any surveying can be classified into three phases, viz.
(a) Pre-Field Operation
• State the type of surveying.
• State where and when you are conducting the surveying.
• Assemble the instruments.
• The personnel to be involved in the fieldwork must be properly briefed.
• Proper knowledge of weather condition is essential in order to choose a favourable time when any disturbance like rain can be avoided.
(b) The Actual Field Operation
This is the phase where the actual field-work has to be carried out.
(c) Post-Field Operation
All the people involved in the actual field operation will now return to the laboratory with a view to making diagrammatic representation of what had been done on the field.
Chain Surveying and Chain Surveying Instruments
Chain surveying is a particular method of surveying in which no angles are measured, only linear measurements are taken in the field. Chain surveying is usually used in fairly open ground. Chain surveying method is used in towns, areas and on development projects.
It is a very useful method to the farmer who is surveying the area of his farm, but it is not suitable to adopt in a built-up or heavily-wooded area. Where visibility is obstructed along straight lines, chain surveying is impossible.
• The Chain: This is of two types – Engineers chain which is about 50 metres long and the Gunter’s chain which is about 30 metres long. The chain is made of steel wire, and consists of long links joined by shorter links. It is accurate enough to measure the chain lines and offsets of small surveys.
• Tape Measure: Tapes are used where greater accuracy of measurement is required, such as setting out of roads. They are marked in metres, centimetres and millimetres. They are usually 15 or 30 metres long, and are of different types.
• Station Pegs: These are made of wood or iron. They are about 32.5 cm long. They have pointed markers which can be stuck into the ground. They are used to mark the stations.
• Ranging Poles: They are made of wood or tubular steel and are usually 2 m long. They are painted in alternate bands of red and white, the bands generally being 0.5 m deep so that the pole may be used for measuring offsets.
• Cross Staff: This is used to set out lines at right angles to the main chain line.
• Optical Square: This instrument is used for similar purposes to the cross-staff, but it is easier to use and of greater accuracy.
(a) Why Chain Surveying is the Best for Beginners
• It is the best because of its simplicity.
• Many of its principles may be applied also to the other methods.
• The apparatus is cheap or alternatively readily improvised and can be used with complete accuracy.
• No advanced mathematical knowledge is required in this method.
• Apart from the portability of the instruments, the method is relatively more accurate than other methods.
• The method does not cost so much in terms of personnel and techniques.
(b) Procedure in Making a Chain Surveying
• Reconnaissance: Walk over the area to be surveyed and note the general layout, the position of features and the shape of the area. If the preliminary inspection is thoroughly carried out, the actual surveying is thus made easier.
• During the preliminary inspection, choose the main triangle. It is upon this that much depends and there must be plenty of time for consideration.
• Build up the secondary lines of the surveying framework, starting from the main triangle, and remembering that all points must be fixed by intersection or measuring along the side of triangle or that side produced.
• Take the chance to get an idea of the maximum dimensions of the surveying, by rough tape measurements or by pacing. This is useful when considering the scale.
• Make a key diagram of the lines you are going to use, lettering or numbering the stations, and possibly sketching in one feature to help identification.
(c) Possible Errors in Chaining
During a chain surveying exercise, it is possible to make anyone or some of the following mistakes:
• Omitting to book one or more chain lengths.
• Confusing tallies.
• Miscounting the links.
• Mistaking 9 m for 6 m on the tape.
• Incorrect marking of the chain end.
• Incorrect booking.
• Mistaking dimensions when called out.
(d) Measurement of Offsets
During the field-operation of a chain surveying, it is possible to discover that the outside boundaries of the area to be surveyed consist of curving hedges whose positions must be fixed by taking offsets; that is, to measure the perpendicular distances to the particular boundary at intervals from the chain line. Right angles are determined by using the optical square or crossstaff described above.
The measurement of offsets is of vital significance to any survey because by so doing one is able to get the actual configuration of the area to be surveyed. This is why the surveyor has to ensure that enough offsets are taken whenever he is on the field to conduct a chain surveying or even a compass surveying.
During any survey, all measurements should be carefully entered in the field book. The field book must be portable. To make the field book, an exercise book may be used, whereby two lines measuring 2 cm apart down the centre of the page may be ruled. Nothing should be written between the lines except the distances along the chain line. Generally, booking starts from the bottom of the page and as one goes along the chain line, one works upwards: right and left on the page that correspond to right and left on the ground.
The position of any building such as the shed in Figure 1.49 is fixed by making two measurements towards each of the nearer corners which can then be plotted accurately by intersecting arcs. The shed is then measured and plotted in order that the corners in question may fit the points which are already obtained.
(f) Chaining Round Obstacles
During a chain survey exercise, it might be possible for the surveyor to come across some obstacles. To any advanced level student, chaining round such obstacles should not pose any serious problem.
Two types of obstacles a surveyor may come across during a chain surveying exercise are discussed below: First, he may come across an obstacle across which he can see, e.g. a pond. The procedure for chaining round the pond may be in either of the two ways below:
To measure the distance BC, range AB through to C. Line through B to any convenient point D, that clears the pond. With a pole at D range in E, making DE = BD. Produce CD to F in such a way that DF CD. FE is parallel to AC. Therefore, <EFD = <BCD (alternate angles. Similarly, <DBC = <DEF (alternate angles) <EDF = <BDC (vertically opposite angles).
Therefore, it can be rightly concluded that triangle BDC = triangle FDE. By identical triangles FE = BC.
The second way of chaining round the pond is by sticking in poles or arrows exactly on the line at A and B, then set out equal perpendiculars AC, BD, long enough to clear the pond. Then CD will be equal to AB and measurement along it can proceed as though it were in fact AB.
Apart from the aforesaid, the surveyor may come across an obstacle through which he cannot see, e.g. a building.
Let A and B be two points on the line approaching the building. Therefore,
• set out equal perpendiculars BC, AD;
• focus along CD and fix two other points, E and F, exactly in line with it beyond the building;
• from E and F, set out perpendiculars EH and FG both equal to BC in such a way that G and H are in line with AB, and CE equals BH.
(a) Description of the Prismatic Compass
It is a circular magnetic compass which has, on one side a prism, with a slit in it, and on the opposite side a sight vane, with a vertical hair. The plane containing the prism slit and sight vane hair also contains the pivot of the compass card, the central point of the compass. The prism reflects the figures of the card immediately below it and enables the surveyor to read the bearing without taking his eye from the sights and theobjects being sighted.
There are several types of prismatic compass, but all areusedinmuch the same way. The cheapest and simplest to use and standing up best to hard-wear is the ordinary ‘service’ pattern as used in the army.
The compass is light and easy to carry. Bearings can be taken much more quickly than with the theodolite. It eliminates the need for check lines and so forms a means of rapid survey. In spite of these advantages, the compass is not without its disadvantages. For instance, the degree of its accuracy is not high, and the instrument readings can be affected by ‘local attractions’.
Rules Guiding the Use of a Prismatic Compass
• In using a prismatic compass, it is imperative to bear in mind the following rules:
• All magnetic materials should be removed from the pockets before the bearing is taken.
• The bearing should not be observed from a position close to strongly magnetic material such as corrugated iron shed or railway.
• The compass must be held horizontally so that the edge of the card does not rub against the top of the compass.
• To accord with the surveying principle of independent checks, the ‘back’ bearing should be taken from the other end of the line, and should differ from the ‘forward’ bearing by exactly 180?. On the other hand, if the forward bearing is below 1800, add 180?. This principle applies to both the magnetic and true bearings.
(c) Reading the Prismatic Compass
In reading the prismatic compass, efforts should be made to follow the undermentioned steps:
• Raise the lid and move the prism over the card.
• Pass the thumb through and use thumb and forefinger together to make a ‘platform’ on which the compass can be held level and steady.
• Looking through the sighting slot, bring the compass gently round until the centre of the object to be sighted is covered by the hair line in the lid which itself should be in exact centre of the field of vision given by the slot.
• Without disturbing the compass, lower the eye and read the graduation on the ring which is exactly in line with the hair.
(d) Correction of Closing Errors
As a result of minute errors in observation, measurement or drawing, a traverse may not close when plotted or drawn. Therefore, when a ‘closing error’ occurs, it must be distributed between each of the angles measured so that the traverse will close accurately.
The traverse shown in the above figure does not close by a distance AA1. It is essential, therefore, to determine whether the error AA1 is within the permissible error of 1 in 400. The total length of the traverse = 1000 + 1500 + 1000 + 2000 = 5500 m. Let distance AA1 be equal to 12 m.
Then error = closing error /length of traverse
The error of 12 m appears to be large, it is not necessary to repeat the whole exercise in view of the fact that an error of about 1 in 400 would still be present due to the limitations of the instruments.
To adjust the survey, therefore, Bowditch’s method may be used.
• Draw a line and set off to scale on it the distances AB, BC, etc. in figure 1.54 (b).
• At A1, on the line, draw a line A1, a perpendicular to AA1, and equal in length to the closing error A1A on the drawing.
• Join Aa.
• From B, C and D on the line, draw lines Bb, Cc, Dd perpendicular to AA1 to intersect line Aa.
• Join AA1 on the drawing.
• From B on the drawing, set out a line equal in length to Bb and parallel to AA1 to point B2. Fix points C2 and D2 in a similar manner.
• Join A, B2, C2, D2, and the resultant figure will be the adjusted traverse.