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Why to use Plane Coordinates for Surveying?

Why to use Plane Coordinates for Surveying


"In geometry, a coordinate system is a system which uses one or more numbers, or coordinates, to uniquely determine the position of a point or other geometric element". (Wikipedia)


Different Coordinate Systems
Surveyors are using different co-ordinate system for different application.  Recent period, every body wants to match the geometry data with Google Earth.  Here, we want to discuss about 'why surveyors follow the plane coordinate system'

Plane Coordinate System VS UTM 

In order to make the coordinate system usable for engineering projects, the horizontal relationships should be defined as two dimensional on one (mapping) plane. To make the coordinate system usable and to simplify linear measurement, the coordinate system should be rectangular so that equal values measured from a datum axis form a parallel line with that axis. Parallel lines to each of the two axes form a "grid" and the intersection of those lines are rectangular "grid coordinates". This type of coordinates is called Cartesian coordinate systems.

In UTM System, grids are curved line & non-parallel/not uniformly. The following table summarizes the differences between plane and ellipsoidal (WGS 84 or UTM) coordinates



If we adopt the UTM coordinate system for Highway or any other projects, traverse closure accuracy will be 1:2500 or lesser. This will not meet the IRC Standards. (as per IRC, 1:10000 / 1:20000 accuracy required)



Since the state plane coordinate systems are developed directly from geodetic values, the use of those systems require the further reduction of the ellipsoid values to grid values. The reduction from ground to the state plane is a simple two-stage process. Reduction from ground to the ellipsoid is called the "elevation factor" and reduction from the ellipsoid to the state plane grid is called the "scale factor”. The scale factor in New Jersey is set to 0.9999 at the central meridian and it increases as a function of easterly or westerly distance of the point from the central meridian. The maximum value of the scale factor is 1.0001.

If the highway projects are surveyed (or) designed on UTM or any other ellipsoid based coordinate system, it will not match while the contractor setting out the design at site. Even the station points will not match because of the scale factor (or) elevation factor. For refer with the google earth or any other free GIS Data sources, drawings can be moved/rotated/scaled. But for Design, we have to follow the Plane ground system.


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