Any point on the earth’s surface can be defined by three numbers: the x, y and z coordinates. The x value is given in the east-west direction and the y coordinate in the north-south direction. The z coordinate refers to height. They are all distances measured from some or other datum point which would have the value of zero for the x and y coordinates. This is very often the case for small local surveys where there isn’t a need to tie them to the national grid. The datum for height could be a survey peg or beacon with a height arbitrarily set at zero or 100 metres or any other convenient level. For larger surveys, the datum in respect of height could be either mean sea level (MSL) or low water ordinary spring tide (LWOST).