Dr Kamal Abdellatif Abdalla
Planning and Survey Sector
Municipality of Al Ain
United Arab Emirates
In order to place individual surveying projects into a larger spatial context, a geodetic control network is necessary. Such a network consists of a number of points spread across the area under consideration, these points are usually in the form of monuments established and placed in the ground, along with a high-accuracy positional value for each point. Traditionally, there have been completely separate networks for horizontal and vertical control, but some and recent networks combine the two on common monuments. By referencing field measurements to such a network, the resulting data and information from multiple local survey activities can be accurately connected. The accuracy of each activity or project is no higher than the control network to which it is referenced. A control network itself is established by highly precise surveying methods followed by a statistical adjustment to reconcile all of the measurements. For each monument the result is a published positional value along with a stated accuracy level.
Actually, geodetic control is typically separated into two components: horizontal (latitude/longitude) and vertical (elevation). This is because latitude/longitude and elevation are based on completely different concepts and measurement methods. Even today when GPS can provide extremely high-accuracy horizontal results, a more traditional method is required to establish vertical control. Geodetic control surveys are usually performed to establish a basic control network (framework) from which supplemental surveying and mapping work are performed. The required accuracy for a control survey depends primarily on its purpose. Factors that affect accuracy are: type and condition of equipment used, field procedures adopted, and the experience and capabilities of personnel employed. Different sets of detailed standards of accuracy and specifications for geodetic surveys are to be considered to get reliable control points.
Horizontal geodetic control networks can be established by a number of different methods, however GPS has become the most widely used method due to its efficiency and superior results. These networks provide positional information with reference to a mathematical surface called an ellipsoid (horizontal datum) defined to model the size and shape of (all or some part of) the Earth. The high accuracy reference network is usually being established in many countries. It will be based upon GPS positioning and produced highly accurate results that can be the basis for the national datum adjustment. It also provides the surveying community with a network of highly reliable positional coordinates to control their surveys.
Vertical control networks are a series of points on which precise heights, or elevations, have been established. Vertical control stations are typically called Bench Marks (BM) as part of a vertical information network, the benchmark’s elevation is known relative to a vertical datum, usually approximating the mean sea level. The vertical accuracy standard as referenced in the 1998 FGCS standard specifies a linear value (plus or minus) within which the true or theoretical location of the point falls 95 percent of the time.