Before we get too involved with the Spatial Reference System, let's have a look at some of the basics.

2D Reference System

What exactly is a 'spatial reference system'? The most familiar example of a reference system is the two-dimensional reference system learned in grade school to plot coordinates on a piece of paper.

Referred to as a 'rectangular Cartesian coordinate system', we learned to create such a system by picking a starting point or origin (0) and drawing two lines that intersect each other at right angles through the starting point (the axes). Once we decide on which direction to line up our 'y' and 'x' axes we have defined a 2D reference system. It's that simple.

If we divide the axis to set the 'scale' (one division equals 1 metre) we could then plot 'coordinate pairs' of points that are of interest to us.

Transferring this idea from a piece of paper into the real world, this reference system works well for small areas such as property surveys or engineering projects such as small sections of road construction.

Once the distances involved become too big, we begin to have problems. You see, while the sheet of paper on which we designed our reference system is flat, the surface of the earth is curved.

For small distances, the curvature of the earth has little effect, but once the distances become larger, say 10 km or so, the fact that the earth is a 3 dimensional surface must be taken into account.

3D Reference System

Okay, well, if the earth is a 3 dimensional body, then we should have a 3 dimensional coordinate system right? Adding a 3rd axis (z), which intersects the plane formed by the x and y axis at a right angle, and locating our point of original (the point where all three axes intersect) at the centre of mass of the Earth it would make our system 'geocentric' and would allow us to assigned a 3D, Cartesian coordinate value (x, y and z) to any point on, above, or below the surface of the earth.

Once we've located our point of origin at the earth's centre of mass, we need to line up, or orient, the axes of our reference system.

If we line up the Z-axis parallel to the Earth's axis of rotation (from North pole to South) and if we line up the x/y axes so that positive X-axis intersects the Greenwich meridian (longitude 0°) we've completely defined (with the exception of a few really technical details) our 3D spatial reference system.

Officially, this system is known as the Conventional Terrestrial Reference System (CTRS), sometimes just called the Conventional Terrestrial System (CTS).