Range Determination Using Target Images

An inexpensive and simple technique was developed for the survey of inaccessible objects, in which the image of the target is formed at an accessible location by means of an optical beam splitter. The three dimensions of the inaccessible object are determined from measurements conducted at the location of the object image and observed through the transparent component of the beam splitter. A simple prototype was built for tall vertical structures, horizontal or inclined roofs, and objects at potentially dangerous locations. With this prototype, the image may be formed at a convenient location for conducting the measurements, and cross sections can be determined at different planes. The beam splitter of the prototype is a 50/50 reflectance/transmittance half‐silvered mirror. The technique was used to subdivide a distance between two inaccessible targets, and also to map an ancient dome for restoration purposes. The time saved with the proposed technique in comparison with indirect theodolite surveying is considerable. The accuracy obtained matches the accuracy of optical distance‐measurement techniques, and can be largely increased by using prism‐binocular field glasses.