I was interested in this display at Rheged. Firstly that the survey of India was done as a series of triangles extended into long lines. Secondly, I had not come across Plane Table surveying. It seems to be a way of plotting a map at the same time as the survey measurements are taken. The table needs to be levelled and it has a plumb bob to help with this. Sight lines to points are measured. If the distance is known, the map can be made radially from a central point. In mountainous areas where horizontal distances are hard to measure you line up the same features seen from 2 known points and where the sight lines intersect, you can plot intermediate points.
