Whilst this technique obviously has many applications, it was encountered in the process of recording significate rocks within a trench otherwise full of rocks so is written to demonstrate this scenario.
You will need two surveyors tapes, preferably of different colours as that makes it much easier to record and minimises errors (proven by experimentation!). Secure each tape at a corner of the trench. At the start of the recording, we had a white tape and a yellow tape, so labelled the trench corners as W (for white) and Y (for yellow).
Taking care to keep the tapes horizontal (not parallel to the ground as we were on a crazy slope) bring the loose end of the tapes together at a corner of a rock. Note the readings on both tapes. Take readings at all interesting points of the perimeter of the rock, as these will describe the outline when you come to draw it.
Once your rock has been described as a series of Y and W readings, it is time to draw! Drawing as soon as possible enables you to see if it has come out at all reasonably-shaped. Remember here that you are interpreting where the edge of a rock is, and the scale at which it is to be drawn will not allow much accuracy of depiction, so even when you do this carefully it is perfectly possible to end up with something preposterous.
You will need to have prepared your drawing with a scale outline of the trench (in our case we used 1:20 as a scale) with your Y and W points (or whatever you want to call them) marked on the drawing. You will need a pair of compasses with which to mark out the distance from Y or W – put the point of the compass against the scale rule and measure out the appropriate reading. This gives you the distance scaled to match the drawing. If you dislike using the scale rule constantly, it is much easier to draw out the scale as a reference on your drawing. We drew the 1, 2 and 3 meter points, and then another metre with finer subdivisions so that you can put the point of the compass at the correct number of metres away and then adjust for the sub-metre fraction. Which is harder to explain than to do. If you want 2.24 metres, you put the point of the compass at the 2 mark and then add on the 0.24. This avoids the need to mark in graduations for every metre, and stops you wearing a hole in the zero point!
Using the compasses, draw arcs of the appropriate length from the appropriate corners and where they cross is the point on the rock that you measured (hopefully!). Repeat for all the points you recorded and then freehand draw the appropriate outline. For the more interesting rock shapes this was aided by a sneaky vertical photo of said rock.