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control network

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the establishment of two - or three dimensional control network is the most fundamental operation in the surveying larg or small areas of land . control networks comprise a series of points or positions which are spatially located for the purpose of topographic surveying .

for the control of supplementary point , or dimensional control on site

the process involved in carrying out the surveying of a

 area , the capture and processing of the field data .

 and the subsequent production of a plan or map 

, will now be outlined briefly.

the first and obvious step is to know the purpose and 

nature of project for which the surveys are required in
 order to assess the accuracy specifications , the type of 

equipment and surveing processes involved 

for example , a major construction project may require

 structures , etc , to be set out to subcentimetre accuracy

 , in which case the control surveys will be required to 

an even greater accuracy .

earthwork volumes may be estimated from the final 

plans , hence contours made need to be plotted at 2 m

 intervals or less groud , thus indicating the accuracy of 

the final process of topographic surveying from the 

supplementary control and implying major control to a

 greater accuracy .

the location of topographic surveying from the 

supplementary control and implying major control to a greater  accuracy  .

 the location of topographic surveying from the 

supplementary control and implying major control to a 

greater accuracy .

 the location of topographic surveying from the 

supplementary control and implying major control to 

greater accuracy . 

the location of topographic data may be done using total 

station . GPS satellites is closely linked to the contour

 interval required and the number of observations 


the duration of the project will affect the design of 

survey stations required for the control point a project of 
limited duration may only require a long , stout wooden 

peg, driven into solid , reliable ground and surrounded 

by a small amount  of  concrete , 

a fine nail in the top defines the geometrical position to

 be located .

the next stage of the process is a detailed 

reconnaissance of the area in order to establish the best 

position for the control points.

initially , data from all possible sources should be 

studied before venturing into the field . such data would 

coprise  existing maps and plans , aerial photographs 

and any previous surveying data of that area . 

longitudinal section may be drawn from the map 

contours to ensure that lines of sight between control 

point are well above ground level and so free of shimmer 

or refraction effects.

 if the surveys are to be connected into the national 

survey of the contry (ordnance survey national grid in the uk ) 

then the position of as many national survey point as 

possible , such as (in the uk) those on the GPS active or

 passive network . should be located . these studies , 

 very often referred to as the , paper survey , should then

 be followed up with a detailed field reconnaissance .

this latter process locates all existing control in the area

 of interest both local and national and establishes the 

final position for all the new cotrol required these  final 

positions should be chosen to ensure clear 

uninterrupted lines of sight and the best observing 

positions the location of these point and the type of 

terrain involved , would then influence the method of 

survey to used to locata their spatial position 
indicates control points A,B ,,,F , in the area to be surveyed it, is required to obtain the coordinate position of each point , this could be done using any of the following methods :

a-intersection or resection
d-GPS satellites

 illustrates possible line of sight , all the horizontal angles shown would be measured to the required accuracy to give the shape of the network , at least one side would need to be measured , say AB , 

to give the or scale or size of the network ,by measuring a check baseline , say ED , and comparing it with its value ,computed through the network ,the scale error could be assessed ,this from of survey is classical triangulation and although forming the basis of the national maps of many countries is now regarded as obsolete because of the need for line of sight between adjacent point , such control would now be done with GPS

if the lengths of all the sides were measured in the same triangular configuration without the angles , this technique would be called,,trilateration,, Although giving excellent control over scale error ,swing errors may occur, for local precise control surveys the modern practice therefore is to use a combination of angles and distance ,

measuring every angle and every distance ,including check sight wherever possible ,would give a very strong network indeed using sophisticated least squares software it is now possible to probably the most favoured simple method of locating the relative coordinate positions of control points in engineering and construction is traversing ,

illustrates the method of traversing to locate the same control point A to F , all the adjacent horizontal angles and distance measured to the accuracies required resulting in much less data needed to obtain coordinate accuracies comparable with the previous methods ,

also illustrated are minor or supplementary control pint a,b,c,d located with lesser accuracy by means of a link traverse , The field data comprised all the angles as shown plus the horizontal distance Aa,ab,bc,cd and DB the rectangular coordinates of all these point would of course ,be relative to the major control

whilst the methods illustrated above would largely supply a two - dimensional cordinate position , GPS satellites could be used to provide a three- dimensional position 

all these methods , including the computational processes ,are dealt with in later chapters of this book