SURVEY PRINCIPLES: PRINCIPLE OF CONSISTENCY

Any ‘product’ is only as good as the most poorly executed part of it. It matters not whether that ‘product’
is a washing machine or open heart surgery, a weakness or inconsistency in the endeavor could cause a
catastrophic failure. The same may apply in survey principle, especially with control. For example, say the majority
of control on a construction site is established to a certain designed precision. Later one or two further
control points are less well established, but all the control is assumed to be of the same quality. When
holding-down bolts for a steelwork fabrication are set out from the erroneous control it may require a good
nudge from a JCB to make the later stages of the steelwork fit.
Such is the traditional view of consistency. Modern methods of survey network adjustment allow for
some flexibility in the application of the principle and it is not always necessary for all of a particular
stage of a survey to be of the same quality. If error statistics for the computed control are not to be made
available, then quality can only be assured by consistency in observational technique and method. Such a
quality assurance is therefore only second hand. With positional error statistics the quality of the control
may be assessed point by point. Only least squares adjustments can ensure consistency and then only if
reliability is also assured. Consistency and economy of accuracy usually go hand in hand in the production
of control.

SURVEYING: PRINCIPLE OF ECONOMY AND ACCURACY

Surveys are only ever undertaken for a specific purpose and so should be as accurate as they need to be, but
not more accurate. In spite of modern equipment, automated systems, and statistical data processing the
business of survey is still a manpower intensive one and needs to be kept to an economic minimum. Once
the requirement for a survey or some setting out exists, then part of the specification for the work must
include a statement of the relative and absolute accuracies to be achieved. From this, a specification for
the control survey may be derived and once this specification has been achieved, there is no requirement
for further work.
Whereas control involves working from ‘the whole to the part’ the specification for all survey products
is achieved by working from ‘the part to the whole’. The specification for the control may be derived from
estimation based upon experience using knowledge of survey methods to be applied, the instruments to
be used and the capabilities of the personnel involved. Such a specification defines the expected quality of
the output by defining the quality of the work that goes into the survey. Alternatively a statistical analysis
of the proposed control network may be used and this is the preferable approach. In practice a good
specification will involve a combination of both methods, statistics tempered by experience.