The instrument that Danfrie describes is similar to a simple theodolite (see catalogue no. 45), but with a semicircular scale replacing the full circle, and he describes its use for vertical as well as horizontal angles. (Though oddly, and presumably mistakenly, in the illustration opposite, the instrument is given a circular scale.)
Danfrie explains how to survey a fortified town from a distance, using a measured baseline. He points out that this can be done from as far away as the eye can manage and out of danger from cannon-fire.