acontrol network is the framework of survey stations whose coordinats have been precisely determined and are often considered definitive. the station are reference monuments, to which other survey work of a lesser quality is related . by its nature , a control survey needs to be precise , complete and reliable and it must be possible to show that these qualities have been achieved .
this is done by using equipment of proven precision to show that these qualities have been achieved. this is done by using equipment of proven precision with methods that satisfy the principles and data processing that not only coputes the correct values but give numerical measures of their precision and reliability.
since care needs to be taken over the provision of control , then it must be planned to ensure that it achieves the numerically stated objectives of precision and reliability. it must also be complete as it will be needed for all related and dependent survey work . other survey works that may use the control will usually be less precise but greater quantity . examples are setting out for earhworks on aconstruction site,detail surveys of agreenfield site or of an as built development and monitoring many point on a the practice of using a control framework as a basis for further survey operations is often called working from the whole to the part , if it become necessary to work outsithde the control framwork then it must be extended to cover the increased area of operation . failure to do so will degrade accuracy of later survey work even if the quality of survey observations is maintained.
for operations other than setting out, it is not strictly necessary to obseve the control before other survey work . the observations may be concurrent or even consecutive. however , the control survey must be fully coputed before any other work is made depend upon it.