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The Need for Surveying
Surveying helps to determine the three-dimensional position of any natural or man-made feature on the earth. These then allow a contoured map to be drawn up giving all details that will allow setting out of new structures on the plat and transferring them to the actual position on the ground. Vertical surveying refers to the methods of deciding elevations and can use sea level as a datum or geodetic positions referred to as an ellipsoid. Precise geodetic leveling helps to establish networks of vertical control points that are used to determine other height positions in the area being surveyed through supplementary methods.
Image source: wikipedia : Geodetic Survey
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Heights and Their Use in Surveying
The geoid is the surface that would coincide with the ocean’s surface if it were always in equilibrium and not subjected to tidal or other forces. The geoid surface is irregular, but the reference ellipsoid is smoother with variations not exceeding 200 meters. The geoid is also a surface to which the gravitational force is always perpendicular and the local horizon tangential to it. A geodetic vertical datum would refer to some zero point and would have no further reference to sea level. As a starting reference a tide gauge may be used. The orthometric height of a point is the distance along a line of force from another point that is on the physical surface of the geoid. A geopotential height gives the potential energy per unit mass at that level. It is a gravity adjusted height.
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Methods to measure heights
The most accurate of methods for measuring heights is the differential method. In this method a leveling instrument is placed in position so that its horizontal axis is exactly at right angles to the force of gravity. Calibrated staffs are then placed at the points whose heights are to be determined and their readings noted. These will give height differences with respect to the horizontal axis of the instrument that serves as a temporary datum. This temporary datum can be further translated to mean sea level readings if readings are also taken on fixed reference points whose heights above or below mean sea level are earlier determined and established. Most towns and cities would have a series of permanent markers that have been surveyed and whose heights have been precisely verified. Quite often, town hall plinths are used, and it is common to refer to this datum as THD or town hall datum. All heights in any survey in that town are then referenced to this datum. This data is very important to decide drainage, road levels, and for deciding other municipal utilities.
Trigonometric leveling involves measuring vertical angles from a known distance from the theodolite that is used to measure the angle. The height is established by triangulation and may not have the accuracy of the previous method. It is a very useful method to determine elevations in mountainous areas.
Differences in atmospheric pressure are used to determine differences in height in the barometric leveling method. Accuracy here is not very good, but the relative height of places quite far apart can easily be gauged, and it is a system that is in use in initial or reconnaissance surveys. Once these have served their purpose in planning or other requirements, differential leveling can then be used for more accurate readings. The instruments used are called altimeters.
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Global Positioning System and Heights
Heights measured by GPS are not as accurate as the horizontal distances obtained through this system. This is due to satellite visibility, signal errors, and other secondary effects caused by tides, diffraction, and uncertainties in alignments. However it is noted that if observation times are extended, accuracy can improve. The main advantage of using GPS systems is that it can combine the global coordinates X,Y and Z at the same time, something that requires separation in the case of other surveys.
Determining of heights is very essential especially in the design of roads, pipelines, and other utilities