Co tents Introduction Basic principles Plane Table Surveying Chain Surveying levelling Tables
page 1
3 8 11
'9 33
EQUIPMENT
BOAT LEVEL
ALlDADE
o PIN
TRIPOD PLUMB
BOB
Fig 1
PLANE TABLE SURVEY RADIATION METHOD
Factory
------
,,\ MARTINS LANE
A PLAN OF A TYPICAL SITE
Fig 2
ID
CHAIN SURVEYING Chain Surveying has long been established as the easiest an basic form of land mea$urement and is still in common use. 3ased on triangulation and consisting of carrying out the survey in the field and plotting the survey to scale in the drawing office from recorded measurements in the field book. ~t is a job for two people the surveyor and his assistant. There are several possible inaccuracies which may occur '.'hen carrying out a chain survey and great care should be ~aken to avoid the,. The most common being confusing the -~llies, miscounting the links, the chain having knots in ink joints, bent links, the chain not being laid out in a straight line between stations, incorrect bookings, miscalled ·i ensions and booking offsets at wrong points. Practice in ~he field will highlight some of these problems as there is ~o substitute for experience in the use of a chain.
EQUIPMENT The basic equipment for chain surveying is the metric chain 20metres in length with 100 links each .20metres in _e. th. Every tenth link is marked by a brass tag or teller '-i t:-t intermediate tags for every link. The chain is made of E eel wire and great care should be taken when folding so as ~o avoid entanglement and bending the links. When the chain is to be used and laid out it is done by grasping both :a~dles of the chain and throwing it out in one operation and ~ .en straightened out ensuring all links and joints are outst ret ched properly. There are two other chains used these being the Gunter Chain which is 66 feet in total length with 100 links of 7.92 inches, and the Engineers Chain 100 feet in length and :.~E_00 links each a foot long and marked at 10 foot i:r:tervalsv:ith tallies accord.i ng Ly ,
Tapes These are made of steel being the most accurate and being r-e I'e r-r-e d to linen tapes. They are c ornmon.l y in lengths of 10,
20 and 30 metres and are used to measure offsets to the main chci.n line.
11
EQUIPMENT
90 THE METRIC CHAIN
TAPE CHAIN
ARROWS
((
"g 3
RANGING POLES POLE SUPPORT /
.,.
e' 12
ArmlWS .-.•.
0
':c are used to mark the end of each chain line and are
JOOmm to 450rmTl in length and carr-y a red cloth so that they can be easily spotted on the ground.
poles :~ese ere usually 2 metres in length of wood or metal . :n bands alternately in red, white and black.
book _
:::"e_d oak approximately 100mm x 200mm with entries at the back continuing to the front so that the ._:~~c are recorded in the same direction as the Surveyor c.::' -:':::F' along the chain. -C~~~::.~ C.re entered in the book such as hedge"', fence8, ~~~ coundaries, buildings, trees, manholes and poles etc. e afore mentioned are illustrated in Fig.J.
orocedure iE to make a reconnaisance of the site the f,ener~l shape and layout and any other o be surveyed. Then decide on the framework of -.::.----a ion and drive in station pegs A. B. C. as shown in . . :~€ stations to be fixed by taking two tic lines to : ~o that it can be re-located if a return visit is C"~:.~S> If the distance between the stations exceed a ::'e:::-th line in with an intermediate ranging pole as ·f.5. Then by p~oceeding up the chain line from .easure with 90 offsets, features as before u~til you have reached the end of the line A-B, • ::. ";;0" n all the relevant information. This is then 0 -E' ~or B-C and C-A. If the 90 offsets exceed 9 metres -~~.t en an Optical square can be used to give greater r.=-'''''-:'_'''''''~:' or the J:4: 5 Triangle rule employed using a tape. ractical nature of the survey has been carried - ~r_e::ullv checking that all the information required has . ':"noted and loeged the Eurvey can be drawn to scale :!f:..'.'.in[; office.
13
RECONNAISSANCE & SETTING OUT STATIONS
The S· e _.ClllJjAP
,.(~~.
_
Fig 4 14
CHAIN
SURVEYING
intermediate pole
SETTING UP CHAIN LINE
ranging
Fig 5 15
CLINOMETER ~ strument is used for measuring slopes of 3' or is simply illustrated below. By firstly viewing ~nstrument on the horizontal plane the instrument and focussed onto the ranging pole up the ope with the counterweight in the vertical position - ~ ~_evation can be read on the dial, in this case 300• strument is hand held and is most useful for slopes quickly and easily giving instant readings _~s
16
CHAIN
SURVEY
SLOPING SITES To measure "Leg" slopes greater than o 3 , measure convenient horizontal distances, plumb down to ground level to fix point from which to take next measurement.
A
B ~e of slope with CLIlWJ·::ETER ''0(11 Trigonometry AB=AC Cose{.
....---- >--
CLES
--- ---
- -- --- --------- ---
measure slope
c
- - --
---- RANGING ---
OVER HILLS
and B. AssiEtant ~ith pole 2t D lines in surveyor "t C on line AD. Surveyor linef'in pole at D on line sequence ~ith four poles in line.
ft
17
d
b
e
E
B
cain line through A.B.C. set out equal offsets to a.b.c. i~ d.e.Set offsets of length as before to fix D & E. ~~ continuation of main line through F.G. etc.
1Il:dll"\IIWl
G ACROSS A RIVER
'~------~------~------~------~D -----
__
-!:
~_------=rtIVER -
:es at A & B on chain line. AD at Rt. 1s to AB. Pole C na t AC=CD. DE at Rt. 1s to AD, wi t h E.C.B. on straight ::"en AB =DE.
EY OF LAKE OR WOOD - - ~ fraJework of chain lines _= area and triangulate - _ -ies across the corners _:'e'; C:- IN TRAVERSING.
18
I TRODUCTION This book has been set out to illustrate the fundamental basis of land measurement and procedures to adopt when surveying areas of land. The actual practice of carrying out surveys has not changed much over the years with only perhaps the refinement and advanced equipment now in use. Wost of the procedures detailed in this publication are in corr~on practice and will give a good grounding to Architectural, Surveying and Building students. It is emphasised t.at only practice in the field will give a good and thorough understandinG of the methods outlined here and the key being accuracy in performance.
1
LEVELLING
___ ~asic instrument for levelling is the surveyors e '!hichis an adaption of Keplers telescope ernp loyang - __ -ex Lenses which produce a real image and line of _ line of collimation (i.e. principal axis) and will -seus sed in more detail later. The other piece of _~-~~- used in conjunction with the level is the _ ~~-_~c _evelling stave in 4 metre and 5 metre heights. -~ :s are taken using the sight line throueh the telescope -_ '= ::"e'.-el onto the staff wlri ch is collimated in metres and Readings of course will be inverted, this will be ater.
.••-:;,_<::;;:;.=- __
_.::!: =o::"_o'ting statements are accurate for most practical
=_:::'onof transparent material bounded by either plane e:' surfaces. _:==ere~t types in use: (Fig.l.)
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Fig 1 fJL!J,UO COA./c4V£
right angles to the two parallel tangentc: of the urfaces of the lens.
re on the principal axis whose distance from the tRO _ :'sproportional to their radii.
19
- -~~c~~ fro. one transparent material to another - -~~ec~ion. For example, a stick partly immersed in -_ears to be sharply bent at the surface of the water r-or above the water surface. Thus, in passing :e~s the rays of light will be bent or refracted as e -. ';;i th a convex lens the rays of light will ~ _assing ttrough it. co__ ea e lens the rays will diverge on passing through lens is suitable as a burning glass while
through a convex lens will converge on a he principal axis called the "principal focus" - ::'~:::". The distance from this point to the optical -::e :ens is termed the "focal length" of the lens.
ib.IS,
Fig2
~ c ~ ex lens ~ill produce an image on a screen T_is imat;eis termed a "Real Image". ens however causes the light rays to diverge pr-oduced backwards to the principal focus. i".c.gev:ill be seen on a screen at "F" and this is ',-irtualImage". len th of a convex lens is positive (+) length of a concave lens is negative (-) ion of different shaped lenses will produce a ~ocal length related to their individual focal -_,eexpression I
-
+
+
+ •••••••••••
:-C~_ length of the combincation, or, compound lens, " ov:nas the "jiowe r" of the lens.
~!"
I
fJ'
etc.
the focal length of individual lenses.
20
Production of an image An object being viewed through a lens is considered to consist of a number of connected points, each radiating tV10 rays of light; one ray passing through the optical centre of the lens without being refracted, the other, parallel to the principal axis, refracted through the focal point, both intersecting to produce an inverted "real image" (See Fig.3.)
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There is a definite relationship between the focal length f and the object distance V to the image distance U for a lens. This is expressed as: I
f
=
I V
+
I
11
for a convex lens.
and I
f
=
1 V
I U
for a concave lens (where f is negative)
As the image must be produced on the same plane as the crOLS hairs of the telescope of surveying instruments, and the distance from object to lens will vary causing a proportional variation in value U, it is essential to provide adjustment of the lens by a focussing mechanism. A single lens hov:ever possesses a number of faults, the two i portant ones being: (a) Chromatic aberration; v!hite light is split into its co ponent colours on refraction, terr-:ed"dispersion", and due to differing angles of refraction for each colour the image tends to be blurred and surrounded by a halo of colours. This is usually overcome by using two lenses of different material, e.g. one of crown glass and one of flint glass. (b) Snherical berrationj Thic is caused by various rays of liGht fIling on the lens not beine; r-e f'r-ac t ed to pass exac t Ly t hr-ough the SC:.T:ie focal point. ~'l)i s f'au It is controlled by using thin Leris es and restrictine; the object to be viewed to an area close to the princip~l axis of a flc:..t curvature lens.
21
The telescope The surveyors telescope show~ in Fig.4. provides a line of collimation passing through the optical centres of the lenses and the cross hairs. The e epiece magnifies both the real image and the cross hairs in the same proportion as these are both on a common plane. The eyepiece is usually the Ramsden type, consisting of two pIano-convex le~ses mounted a short distance apart in a self-contained case, wh i ch is threaded on the outside to allow it to be screwed f'o rwar-d or back in the telescope body to bring the cross hairs and image into clear focus. This arrangement of lenses reduces spherical aberration. The object glass is usually a compound lens to reduce chromatic aberration.
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Two forms of focussing mechanism are used to bring the image onto the cross-hair plane as the distance of the object from the telescope varies. These are: (a) External focussing: The body of the telescope is made of two concentric close fittinG barrels, the object glass mounted in one and the eyepiece and cross-hairs in the other. A rack and pinion mechanism operated by the "focussing screw" advances or retracts one tube in relation to the other, thus increasing or reducing the distance between object glass and eyepiece/cross-hairs. This method was common on older instruments and is more accurate for tacheometry purposes, but has the disadvantage that wear on the tubes may allow entry of water, dust, etc., and impair the efficiency of the instrument. Cb) Internal focussing: A double concave lens mounted on () frame is fitted inside a one piece telescope body between the object and eyepiece lenses. The frame pocition is adjustable by means of a rack and pinion, as before, which slides the lens tov:ards or away from the object glass.
22
The concave len disperses the light rays from the object glass to greater or lesser degree depending on the position of the lens and thus allows focussing of the image on the cross-hairs. The disadvantage of this method is the loss of brilliance due to the extra lens.
Cross-hairs Originally, spiders yeb was used, but these broke easily and were difficult to replace. Today, very fine lines are etched on a piece of very thin optical glass fastened to a "reticule", forming an interchangeable capsule which fits into a flanged metal ring called the "diaphragm", held in the telescope barrel by four capstan headed screws - which should only be touched when changing or making major adjustments to the cross-hairs.
Parallax This term refers to relative motion between the object and the cross-hairs when the eye is moved to and fro across the eyepiece, and means that the image and cross-hairs are not on the same plane. To eliminate parallax a piece of white paper is held in front of the object lens and theeye-piece moved in or out until the cross-hairs stand out clear and black. The telescope is then focussed on the object and tested for parallax again, the procedure being repeated if necessary until the parallax is eliminated.
The spirit level Cormnonly fastened to the ba.r-r-e I of the telescope on a levellirig instrument, but usually on the top plate of a Theodolite. The more sensitive spirit-levels are barrel shaped curved glass tubes, with the less sensitive only a portion of the surface is curved, when thee'termed nonreversible levels. The glass tube is filled with ether or alcohol with a sm2.11air space left to form a bubble. These fluids are less viscous than water, ~nd have a much lower freezing point, but a greater ey-pansion, so that a level left in very hot sun may burst. The top surface of the tube has graduations etched on it which aid in centralising the bubble in the centre of its run. The sensitiveness of the bubble is defined as the amount the horizontal axis of the tube has to be tilted to cause the bubble to move from one graduation mark to the next, e.g. 1 division )0 sec. means a tilt of )0 sec. of arc above the horizontal v.Ll L cause a "run" of one graduation.
23
The spirit level tu e is e~c osed in a metal case, attached to the body of ne L_ trur.!entby one or two capstan headed screws which alLov t he tube to be adjusted so that it loneitudinal axis is paralle to the principal axis of the Telescope. By this means one can adjust the instrument so that the line of collimation is horizontal, i.e. at right a~gles to the plane of gravity acting at the centre of the instrument: Do not adjust the capstan screws unless making a major correction. The centralising of the bubble may be observed in different ways. On some instruments the tube is viewed in a rnirror 0 hinged to an angle of about 45 • In other cases an internal mirror may be used, or a right angle prism, to give the surveyor an image of the bubble in the eyepiece so that he does no~ have to move round the instrument. Another method presents two half bubbles images in the field of view and these have to coincide to form one complete bubble to bring the telescope level. The circular or "cats eye" bubble is inferior in accuracy to the level tube but gives an approximate level plane for initial "C'uickset" levelling purposes.
Types of levels DU.DV Level: The simplest form of level consists of a bubble tube attached to a telescope which is rigidly fixed to a horizontally rotating centre post and top plate, in its turn connected to a bottom plate by three or four levelling foot screws, which can be adjusted to bring the spirit lev~l and telescope axis horizontal. T~o conditions are essential for accurate work. 1.
The axis of the bubble tube must be parallel to the axis of the telescope, i.e. line of collimation.
2.
Both must be at right angles to vertical axis of the instru ent.
Tilting Level or Quickset Level: An improvement on the Dumpy as the telescope can be pivoted in the vertical plane by means of the tilting screw under the eyepiece end of the telescope. It is only necessary to set the instrument to an approximate horizontal plane by reference to the "cats eye" bubble, the telescope being brought level for each sighting by using the tilting scre~.
)
example
24
Precise Level: This is a development of the now almost obsolete Wye level and the Tilting level. The telescope tube may be tilted and also revolved in its mountings so that b taking the mean of two readings on the same station a true reading may be obtained, this compensating for any error in the collimation of the instrument.
example
Terms used in levelling Bench rark:
A fixed point on the earth's surface whose level above Ordnance datum is known.
Ordnance dat un.e
Back sight:
Foresight: Intermediate
r~ean sea level to which all other levels and bench marks are related.
The first sight taken from a given level position. The last sight taken from a given level position. sight:
HeiGht of Instrument:
Any other sight taken from this level position. (H. of I). The height of the line of collimation above the datum. (i.e. si ht line or principal axis of telescope.)
25
Reduced level:
(R.
=..
poin Change Point:
Calculated height or level of a a 0 e or below the datum.
The point at wlri c h both a foresight and then a backsight are taken (i.e. when the level changes position.)
METHOD Setting up the level 0
1. Open tripod legs to 60 ground.
and press the feet firmly into
2. Note how instrument is packed in the box before
removing and lift out carefully, never by the telescope tube.
3. Screw firmly onto tripod head; never crossing the threads. 4. Roughly level the instrument y adjusting the tripod legs, complete adjustment by means of the footscrews, i.e. bring the bubble to the centre of its run.
plan c:;
~
.
axis at ric;ht ent:
of
Check that there is no play on the axis of the instrument. Turn telescope over a p~ir of footscrews o and level-up. Then rev rse tube through 180 • If the bubble runs off centre bring it halfway back with footscreVls and the balance of the way with the capstan he ded screws on the bubble tube.
26
Corrections Curvature: as the ear t.h is curved, a horizontal sight does not give the true relati e heights of two points over a long sighting distance. See Fig.6.
.
As C is so small cor.:paredto 2R it is ignored and the expression is written thus X2 = correction. 2R 2 2 Distance measured d or i.e. curvature correction Diameter of Earth 2R
Refraction: differin~ atmospheric conditions cause the rays of light entering the telescope to be refracted, i.e. bent, towards the earth's surface to greater or lesser degree, so that on a long sight the reading seen on the staff is not on a true horizontal line but below it and nearer to the earth's surface than it is believed to be. This error due to refraction partly offsets the error due to curvature. As an average figure it is assumed that the error due to refraction is 1/7 that of curvature and in the opposite direction. The combined correction for curvature2and refraction is usually quoted as a deduction of 6 7 x d2R from.the staff readlng.
R
Fig 6
27
For short sights t::& ci::ere:!1ceis so small t11Citit De.' be ignored, but in exact Ieve:l~~ sights should not exceefi a quarter of a mile to a Ol ::"euncertainty caused by these errors. By taking back a.. fore-sights to staff positions at nearly equal distances fro _ t:he level these faults may be consiciered cancelled and are not calculated in normal levelling operations.
Forms of levelling Simple Levelling: in which all levels are obtained with the instrument in one position, or, in other words: (a) there is onl
T
one line of collimation.
(b) the first sight is the only backsight. (c) the last sight is the only Foresight. (d) All other sights are intermediate sights.
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B
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Co pound Levelling: in whi.ch the levels are obtained only by changing the position of the instrument as one follows the undulations of the ground in a long line of levels, or to obtain clear sights which <::.re otherwise obscured by obstacles. Therefore there will be: J
(a) Kore than one line of colliDation. (b) A bo;;.cksie;l1t and foresight for each change in instrument position. (c) Separate groups of intermediate sights reI ted to different instrument positions.
Fig 1 1-70
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28
LAND
SURVEYING
GENERALLY The main function of carrying out a practical land survey is to ascertain as accurately as possible the size and shape of building sites, fields or other areas of land masses. They may have on them ponds, buildings or include other features such as ditches, streams and trees etc. which may need to be recorded and plotted on a survey dravring. This is generally known as land measurement. In addition to this information it will also be important to·know how the site slopes for which you will need to carry out a level survey. Basically it is forming a three dimensional picture of a particular area of land with points of elevation marked on it so that vertical can be taken through showing planes of elevation. There are various methods of gathering this information which will be illustrated and explained in this book. It will be necessary to have knowledge of geometry and a good understanding of mathematics in order to plot and calculate information gathered by the practical survey. Experience and practical knowledge can only be gained by actually carrying out the methods and procedures described in the field.
There are two basic methods of land measurement in building Plane Table Surveying and Chain Surveying, these will be discussed and illustrated more fully in the following pages of this book. Other methods include the use of the Theodolite which is used for measuring horizontal and vertical angles. This is used mainl:y in Civil Engineering wo rk i.e. road construction etc. where greater land masses and distances are involved. Over larger areas of land the curvature of the earth's surface will have to be taken into account and is known as Geor'ieticSurveyine;. It is used in Ordnance Survey work and is a specialised branch of land survey work.
2
BOOKING There are two forms of booking down level readings, Collimation also known as Height of Instrument and Rise and Fall method.
Rise & Fall Method Using Fig.l. to illustrate this method the readings taken by the level are entered into columns of either Rise or Fall i.e. the reading A datum is entered into the back sight column with reading B entered into the foresight column. As B is smaller than A this denotes a Rise and the difference is entered into the Rise column and recorded as such with a reduced level reading entered by adding to a datum figure of 100.00. for level at B. Any intermediate Sights are entered into the appropriate column with reduced levels accordingly. Distances and Remarks Columns are used to record change points and level points etc. This method makes it possible to check each page by adding the Rise column and Fall column, the difference should equal the difference between the backsight and foresight column as illustrated below.
RISE & FALL METHOD SAC!::
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Collimation Method Again using Fig.l. to illustrate this method the succeeding reading is subtracted from the last calculated collimation height i.e. reading at A is height of instrument being 102.00 after datum of 100.00 is added. This will then give a Reduced level at each staff position. To check add the back sights and add the foresights and find the difference. By finding the difference between the level of datum and the last reduced level should equal the difference between the backsight and foresight totals. Always check if possible back to the datum point by taking flying levels this will avoid a return visit to the site.
COLLIMATION METHOD 84a: 3ti/IT :2. eo
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30
SOURCES OF ERROR IN LEVELLING: Incorrect setting-up of instrument. 1.
Bubble off centre when taking reading.
2.
Movement of staff from position when changing level station.
3.
Staff not held vertically.
4.
Parallax - adjust.
5. Instrument knocked or moved during backsight-foresight reading. 6.
Staff not properly extended and locked.
INSTRUlmNT ERROR AND CORRECTION: Ca)
Collimation error;
check before use and equalise sights.
(b)
Under sensitive bubble.
(c)
Errors in staff graduation;
(d)
Loose tripod head.
(e)
Telescope not parallel to bubble tube - Permanent adjustment.
(f)
Telescope not at right angles to the vertical axis Permanent adjustment.
check.
LEVELLING USING A GRID This method of levelling is commonly used and is shown set out in Fig.6. A grid pattern of lOmetre squares is set up over the site which can be pegged out off a chain line and setting up the level at station A and by taking readings in sequence a,b,c and d etc. an accurate way of levelling a site.
31
LEVELLING USING A GRID
r
0
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m
~o.
/ f
e
d
-+----~----~--~~--~----~--~-
MARTINS
lANE
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LAND MEASUREMENT BASIC PRINCIPLES The basis of all land measurement is the use of a base line and fixed points from which measurements can be taken. The following examples are used as basic principles for all surve;ying methods. 1.
RIGHT
A
ANGLED CO-ORDINATES.
xK
z
~
Y
Point A is located or fixed by the distance XZ along the line XY measured at right angles to the line. It is mainly used for measuring boundaries and buildings along a chain line, and is commonly known as taking right angled offsets. 2.
FOCAL CO-ORDINATES.
x
y
Point A is located or fixed by measuring from X-A and Y-A along a known chain line XY. This is quite often used for fixing station points in order to set up another chain i.e. XA.
3
J. ANGULAR CO-ORDINATES.
, 'A '
x
z
/
r
Y
Point A is located or fixed by measuring the angles ol and 'at points Z and R along a chain line XY. Where the angles intersect point A is located. This method is used as lines of intersection in Theodolite, plane table and compass survey work.
4. POLAR CO-ORDINATES.
A
Y Point A is located or fixed by a known point Z along a chain line XY and measuring the distance ZA together with angleo(. A method used in radiating lines on a plane table and locating points generally.
4
AREAS & VOLUMES AREAS
Triangles Area = base x height 2
Corrmonly used in surveying when lengths of sides are often only values S known ,
~
b
2
a
Area = / s( s-a )(s-b )(S-C)
A
b
Lengths of two sides and included known ane;le.
C
a Area =
~
-
222
aesinL
=
c B
_be sin A
Trapezium
5
The parallelogram
D~
a~
~
area =(a x h) Side8 included ~ known but not h. Area = ab Sin A or ab Cos B.
a
A
B
IRREGULAR FIGURES Simpsons rule ~ssume that the bound ries are parabolic in shape and is most Eccurate for normal survey conditions. Can be used for VOLUJlr:ES if areas are substituted for ordinate lengths.
Y7
Odd number of ordinates. Even number of spaces. Area
Are
(width of strip) x [SUP.1 of Ls t and last or(hm,te] (twice sum of other odd ordinates) + (4 times sur, of even ordinates.)
j +
x_
3
[(y ~ '1
+ v7 v ) + 2(" "3 +vv r.; ) + 4(V2+YA~+Y6)J er
/
L!
6
Trapezoidal rule sed when there are any nu _ber of or-d i na t s or height s , -ay be used for volumes if are~s of sections are substituted for ordinate lengths. Area = Interval (half sum of 1st and last ordinate + remaining ordinates.)
Prismoidal formula
Al and A2 areas of ends. Am are
of middle 08ction.
L over all distance between end sections.
7
PLANE TABLE SURVEYING Plane Table Surveying is a ouick method of measuring areas or sections of land. All of the work is carried out on ~ite by the Surveyor. It is not very often used as a practical method of surveying for reasons of general accuracy and weather conditions in this country but it is very useful on more drier climates. There are four main methods a) Radiation, b) Intersection, c) Resection, d) Traversing.
EQUIPMENT To carry out a Plane Table Survey you will need a drawing board on an adjustable tripod with rotating head. A small level i.e. boat level with a compass to orientate the survey. An Alidade sighting straight edge, plumbing fork and plumb bob with stationers pin. Ranging poles, pegs and arrows with steel measuring tape or chain. Some of these items are shown in Fig.1.
RADIATION METHOD To illustrate the method of Plane Table Surveying Fig.2. shows a typical building site to be surveyed. Set up the Table in the centre of the site and level board ensuring that all aspects of the site can be seen. Locate all other points i.e. change of boundary lines, positions of .anholes, gates, poles, trees etc. Then using the Alidade sight onto the fixed points measure the distance from the centre peg marked on the ground. By using a suitable scale draw in the radiated line on the paper fixed securely to the board from the centre pin which should be directly plumbed in over the measuring peg on the ground. When carrying out this process the board should be clocked into position until EiBhtings and measurements can be taken in sequence A,B,C etc. for the whole of the site. If this procedure is carried out carefully then a reproduction of the site will be recorded to sc~le onto the board.
8
