CONTROL NETWORK IN SURVEYING

A control network is the framework of survey stations whose coordinates have been precisely determined
and are often considered definitive. The stations are the 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, with methods that satisfy the principles and data processing that not only computes the
correct values but gives 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 of greater quantity. Examples are setting out for earthworks on a construction
site, detail surveys of a greenfield site or of an as-built development and monitoring many points on a
structure suspected of undergoing deformation.
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 becomes necessary to work outside the control framework then it must
be extended to cover the increased area of operations. Failure to do so will degrade the 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 observe the control before other
survey work. The observations may be concurrent or even consecutive. However, the control survey must
be fully computed before any other work is made to depend upon it.