Historical information

Although it’s unknown when these two photos were taken, the South African Pattern Mk IV Slotted Template Machine was in service from 1936 to 1952. The machine was used to cut slots and holes in an acetate film medium. This template was then used to fit several runs of aerial photography to ground control points at a desired scale with a substitution of the templates in the assembly. Radial slots on the templates from central holes representing the photograph’s principal point (centre) were placed over hollow metal studs mounted over control points on the scaled base sheet. The studs represented the intersection of radial lines through common points of detail of overlapping photos. A pricker was used to transfer from the photo-points to the templates. The slotted template methodology was later used as an essential aero-triangulation exercise in advance courses conducted at the School of Military Survey.

Physical description

These three photographs are of a slotted template machine used in aero-triangulation.
The first two photographs were printed on photographic paper and are part of the Army Survey Regiment’s Collection. The photographs were scanned at 300 dpi. The third photograph was captured from a collection using a smart phone.

.1) and .2) - Photo, black & white, 1936-1952. South African Pattern Mk IV Slotted Template Machine.
.3) - Photo, black & white, 1936-1952. WO2 Bob Pratt operating the Slotted Template Machine.

Inscriptions & markings

.1 and .2 –annotated on back “Slotted Template”