In November 2020 this instrument was donated to the collection. It originates from a survey department of a nearby council and is a nice example of the many inventions made to measure distances using the same geodetic instrument that would measure the horizontal and vertical angles. Most of these instruments would use a graduated vertical staff (a normal levelling staff would do) in combination with fixed stadia-hairs (like the Wild T2), or curved ones that allowed to reduce the measured slope distance to the horizontal (like the Wild RDS). Other instruments used a horizontal graduated staff in combination with a plan parallel plate micrometer (like the Wild RDH) to achieve the same.

What all these instruments had in common, is that they needed a graduated staff to make the distance measurements. This graduated staff formed the basis of a triangle of which the acute angle was measured and converted to a distance in an optical-mechanical way. This implied that an assistant was required to get the staff at the location that needed to be surveyed. With the Zeiss BRT 006 this radically changed. Instead of having the base at the survey location, the base was created at the instrument, just like in the rangefinder that was invented by Barr & Stroud in 1888.

So, thanks to the base now being on the instrument, it became possible to use any target with vertical features (e.g. a lamp post, grass, flowers, trees, brickwork, etc.) to measure a distance to (see figure 8 and figure 9), quite similar to contactless distance measurements that nowadays can be taken electronically using modern total stations like the Leica TRCA1101.

Of course there are limitations, which have all to do with the length of the baseline on the instrument. The Zeiss BRT 006 has a 305 millimetres base that allows distance measurements up to 61 metres. A larger base would have made longer measurements possible, but of course would have made the, already quite wide, instrument rather unwieldy. For larger distances the BRT 006 could be used in combination with the BRT-Latte 60-80m, a dedicated horizontal graduated staff that allows to expand the range up to 180 metres. Of course this takes away the advantage of unassisted distance measurements.

The actual measurement is done by sliding a cart, that holds a penta-prism, along the base of the instrument. Once the two images of the target coincide, a loupe at the cart allows to read off the graduated scale that runs along the basis (see figure 10).

Distances can be read with the Zeiss BRT 006 directly in decimetres and estimated down to centimetres. When observing at non-horizontal vertical angles the slope distances can be automatically reduced, depending on the setting of a small lever at the front of the instrument (see figure 5). Apart from a small correction, that can be read off the body of the instrument and needs to be applied to the distances (see figure 4).

The horizontal and vertical circles are read directly to 50 milligon and estimated down to 5 milligon.

The Zeiss BRT 006 was first produced in 1960¹ and stayed in production until at least 1972². In that year Stöpler (see figure 4), the firm that was the sole agent for this instrument in the Netherlands, still advertised the BRT 006.