The following is a quote from a geocache page:
The theodolite gave you accurate direction and vertical angle. The chain gave distance. The chain is like a 100m long thin steel band (older ones where chains), and was dragged along the ground as distance was measured. For accurate measurement the temperature and pressure (how tight the chain was held at one end), were calculated into the chains reading. Each chain had to be standardized at a set temperature and pressure. The range behind this cache was one such range, last used in the early nineteen eighties. The base consisted of concrete pillars with a wooden rail in between. Metal plugs were inserted into the tops of the pillars at intervals of 00, 50, 66 and 100 feet distances. When a standard chain was laid on the ground, the thermometer registered the heat of the ground and not necessarily the heat of the chain. By building piers, the chain when tested would be suspended in the atmosphere. Then by suspending the thermometer from a tripod placed close to the chain, a reading would be obtained which would accurately show the temperature of the chain - the thermometer and chain being suspended in the atmosphere under identical conditions. On flat surfaces the chain would be held suspended, a fish type scale at one end indicating pressure the chain was held at. When chaining downhill on the incline, the lower chainman would often hold the chain up over his head to get it level and then use a drop plumbob (no string) and let it spear into the ground at his feet. Wouldn’t the chainmen have loved a GPS!
As we travel long distances over good roads I marvel at the work done by the early surveyors in really tough conditions.