Geodetic Surveys entail locating and relating the position of objects on the earth relative to each other, while taking into account the size, shape and gravity of the earth. This type of survey is suited for large areas and long lines and is used to find the precise location of basic points needed for establishing control for other surveys. Horizontal and vertical networks that span the country form the primary spatial reference system used in mapping, boundary demarcation, and other geomatics applications. Practical applications of geodesy include monitoring earth movement and determining the precise location of points on the surface of the earth, for use in satellite tracking and global navigation.
Control Surveys encompases the positioning of survey ground points by using GPS or GNSS as per the specifications for intended uses. The extension of GPS/GNSS geodetic control networks to remote areas, Establishment of Survey Control Networks for offshore and underground surveys among others. Survey control networks are the fundamental foundation to support any project from building construction, deformation monitoring, to the national survey network for legal boundaries, geographical information and mapping. The diagrams below show establishment of horizontal controls using traversing method and vertical controls using levelling method.
Traversing is that type of survey in which a number of connected survey lines form the framework and the directions and lengths of the survey lines are measured with the help of an angle measuring instrument and tape or chain respectively. There are two types of traverse surveying i.e. Closed Traverse and Open Traverse. The closed traverse is suitable for locating the boundaries of lakes, woods, etc and for a survey of large areas. the open traverse is suitable for surveying a long narrow strip of land as required for a road of the canal or the coastline. Traverse networks involve placing survey stations along a line or path of travel, and then using the previously surveyed points as a base for observing the next point. Traverse networks have many advantages, including: The main instruments used for levelling include but not limited to:
Levelling is a process of determining the height of one level relative to another. It is used in surveying to establish the elevation of a point relative to a datum, or to establish a point at a given elevation relative to a datum. The basic concept of leveling involves the measurement of vertical distance relative to a horizontal line of sight. Hence it requires a graduated staff for the vertical measurements and an instrument that provides a horizontal line of sight. There are two accepted methods of booking observations – Rise and Fall method and Height of the Plane of Collimation method (HPC/HOC). Neither method can be said to be more accurate than the other. Rise and Fall does have an additional check on the arithmetical reduction of the observations which makes it more popular on line levelling. The HPC method is used for setting out because one always needs to know the height of the instrument.
Two methods are in general use i.e. the "rise and fall" method and the "height of collimation" method. The latter reduces levels relative to the instrument height. As it has inferior in-built checks it should not be used and will not be covered here. The "rise and fall" methods shall be used for reduction of all site levelling. Reduction shall be carried out on site before packing up to ensure that the levelling has been done correctly.
If the levelling has been done correctly and all arithmetic reductions are correct, the differencesbetween total backsights and foresights, total rises and falls, and starting and finishing R.L.'s should be the same. This difference is the close; and for site inspection purposes it should be within ± 2mm or ± 6mm, depending upon which water-level standard is being followed, ± 3mm or ± 10mm.
All levelling need to be booked in either level books or levelling sheets which shall be retained as permanent records. Level books shall be numbered so that they can be referenced on station history and inspection forms. They should be stored in fire-proof storage as for original record. They should also include an index. Levelling sheets shall be filed in time-sequential order in site files, and also need to be in fire-proof storage as for level books.