Perhaps more correctly, it's called 'Boning-In', and requires at least one rod to do it properly. But despite sounding all carnal and dirty, there's nothing exciting about it at all, although getting dirty is just about assured. In fact, it's a tried-and-tested method of getting a trench excavated to the correct depth said to date back to construction sites in Sumeria and Ancient Egypt.
A drainage pipe, for example, be it for sewage or for just dull old ditchwater, typically needs to be laid to a fall, running downhill, and on an even gradient. It hasn't got to go up and down like a country walk, but smooth and even and flat, although not in the horizontally level sense because then it won't be conducive to flow at all. In essence, the pipes must be laid on a single straight line which is the shortest distance between two points in three-dimensional space, with the upstream end higher than its opposite number downstream. Indeed, a 'bone' is an Old English word for a slope or gradient.
In order to achieve this goal, the trench bottom must be equally accurately excavated, such that the bottom of the trench is parallel to the intended vertical alignment of the pipes. To effect this without either sophisticated electronic equipment or being permanently1 welded to the eye-socket of one's surveying equipment, it is convenient to set up a boning system, which the trench-gang can operate without excessively vigilant supervision.
First of all, construct a boning rod. This is an object shaped like the letter T, not unlike an outsized set-square which comprises a vertical staff (perhaps a piece of 50mm x 12mm wood) and a flat plank-like piece (also 50mm x 12mm) being the horizontal part. The two should be secured perpendicularly, perhaps with some bracing. Normally, the horizontal part should be in the order of a metre long, and the vertical part should be about a metre or so longer than the deepest part of the trench.
Then, establish two site rails, one at the upstream end of the trench and one at the downstream end of the trench. Each site rail will consist of a similar flat plank-like piece set by means of accurate survey and typically some nifty carpentry in a vertical position such that the distance from the top of the site rail to the intended trench bottom is equal to the length of the boning-rod staff from the upperside of its respective horizontal piece. Moreover, but equally importantly, each site rail should be established so that the horizontal section of the rail is perpendicular to and preferably intersects with at least some of the intended trench. For longer trenches, say greater than 50m, intermediate site rails may need to be established.
The Ins And Outs Of Boning
Start digging. As the trench deepens, a chainman (a surveyor's assistant) or other suitably trained and equipped fellow should hold the boning rod (or 'traveller') dead vertical in normal T-orientation so that the staff touches the bottom of the trench and the horizontal section is perpendicular to the axis of the trench.
A second member of personnel, perhaps a supervisor, will align by eye the top of the two site-rails by standing beyond one of the trench ends. If the trench is not deep enough the horizontal flat of the boning rod will stand proud of this sight-line. If the trench is too deep, the horizontal flat boning rod will be below the sight-line, and will need a bit of filling in. If the trench is dug just right, the top of the boning-rod will align nicely with the site-line between the two site-rails. Job's a good'un.
Keep boning until the trench is well and truly dug.
Work safely. Be properly trained. If you do not know the relevant health and safety requirements, you should take advice from the relevant statutory safety body, for example the Health and Safety Executive in the UK, before commencing any excavation.