Coordinates
 Coordinate Systems
 A Coordinate Datum

A Coordinate Datum

A datum is a reference system which is used to describe the formation of a surface and the relationship of isolated points within a given area to one another. The shape of the entire datum will depend upon the type of system used to define the points within it. The size of a datum is entirely abitrary and is determined by the end use of the data which defines the datum.

In this instance we are considering datums which are described using either rectangular cartesian coordinates or polar coordinates. There are other methods used to describe datums but these will not be explored in Plane Surveying.

The datum for the coordinate systems may be arbitrary, that is, not located with respect to any other system. A good example is the survey of a beach profile or the foundations of a new bridge where either site is so far removed from any other survey (and need never be connected to it) that the origin (of either rectangular or polar coordinates) is selected absolutely arbitrarily. This may be done for convenience but has the disadvantage that to connect several such surveys together may require a large amount of field work and office computations.

A local datum is one that is chosen for a particular environment. An example of a local datum would be a construction site for a new city building or the survey of a catchment area to monitor erosion. The datum is selected so that the site can be conveniently covered by the coordinates and may be used repeatedly. There may be several ecosystems covered by the one coordinate system. It would be normal for the north (or Y) axis to have a known orientation with respect to true geographical north or to a lesser degree, magnetic north. This is not mandatory or even necessary but reflects the more permanent nature of the datum.

A local datum may be established within another larger existing datum. In this case, the local datum is surveyed using a different scale to the existing datum so as to convey more detail regarding the area of interest.

State or National grids have existed on Australia for a long time; the current system is called The Australian Map Grid (AMG). This grid covers all of Australia in 6° zones. The zones are 6° in Longitude and have a central meridian (north axis) at which map distortions are a minimum. Zone to zone transformations are possible as are computations from one side of the continent to the other. All mapping done at a state or national level will be portrayed using AMG (from scales of 1:1,000,000 to 1:500).

Negative coordinates should be avoided at any cost; this may be achieved be placing the coordinate origin to the south and west of the area to be surveyed (these directions are relative to the coordinate origin and not necessarily geographical north). This will ensure that all the coordinates reside in the first quadrant.

Here the term coordinates is used to mean plane coordinates, there is one other major coordinate system in use that is based on a spherical datum. The system is that of latitude and longitude. Latitude and longitude are angular measures and involve a considerably higher level of mathematics in computations. Fortunately spherical datums need not be considered in Plane Surveying.

Surveyors also use geocentric coordinates based on the centre of the earth, once again we can conveniently ignore them acknowledging however that the mathematics of geocentric coordinates is very similar to those of three dimensional plane coordinates.

This page completes the 'Coordinates and Cordinate Systems' section of Choose a Topic. Use the next button to select another section.