In the construction of railway tunnels it is usual to work from both ends, and sometimes from intermediate points also. The line of route is laid out on the surface to facilitate observations underground: but if this is impossible the extreme points have to be connected by accurate trigonometrical surveys and exact levels, so that their relative positions are precisely known. The centre line at formation level is then accuratelv set out by theodolites and standard chains, a smaller he adiug being driven in advance of the main tunnel, so that (apart from facilities of construction) in the event of a slight error in meeting the heading from the other end, the directions may be adjusted.