Right, first read this Wiki bit on how a Total Station works: https://en.wikipedia...i/Total_station
The two datum points, STNA and STNB were fixed, probably using the GPS phase measurement system on a high end Total Station (some don't have GPS, some do). Once you have a known fixed datum (and using the road is a good fixed datum - you may find there is still a nail driven into the tarmac where the datum points are) you can determine the height and position (so the absolute X, Y and Z coordinates, relative to Ordnance Datum) of any point on the site. The points on the site that were measured were located by the use of a staff or tripod with a retroreflector which the Total Station uses to measure azimuth and elevation angle plus slope range. Knowing those numbers allows the absolute coordinates in all three axes, relative to Ordnance Datum, to be calculated.
In practice no calculation is done now, the Total Station does all the sums and spews out exact coordinates, usually in a form that can be directly read into a CAD program (the one we used fed data straight into AutoCAD for me, for example).
More than one fixed reference datum point is usually required because of the need for the Total Station to "see" every part of the site. Most sites will have obstacles that make using a single reference difficult, plus having two or more fixed references allows error correction, often using the least squares method (no need to go into this but it's a way of minimising the total loop closure error in a long chain of interdependent measurements).
In our case we had the benefit of an Ordnance Survey reference nail (the nails with round washers that you may see on roads and pavements, which are Ordnance Survey spot height references, the black dots on a map) right at the corner of the lane where the edge of our drive was going to be. We knew the precise location and height of this already, from the Ordnance Survey vector data set that I purchased to draw up the site plan, so we could reference everything from that nail (still can, as it's still just on the edge of our drive entrance, at 81.48m above Ordnance Datum, which is in Newlyn Harbour, in Cornwall).
If you have a fixed baseline as you do (the line between STNA and STNB) you can determine the position of anything on the site with just a level and tape, by triangulation. On a sloping site this is a complete pain, as you have to keep correcting for slope distance, which is why using a Total Station is such a joy; it does all the sums for you and spits out the precise (to within about +/- 1.5mm) position and height of any spot on site it can "see".
Edited by jsharris, 18 January 2016 - 11:25 AM.