In June 2016 I found this early Wild Heerbrugg RDH on a Hungarian web site. After a few days negotiations with the owner the tachymeter was sent to the Netherlands and arrived in the collection. A week later I acquired a box with two horizontal staff sets from a befriended collector (see figure 23). An original Wild 21a tripod was already in the collection making the Wild RDH set complete (see figure 24).

The abbreviation RDH stands for Reduktions Distanz- und Höhenmesser (reduction distance and height measurer) also known as the Reduktions Distanzmesser für waagrechte Latte or 'Reduction Distance-meter for a Horizontal Staff'. The RDH is one of the rarer theodolites Wild produced, even rarer than the RDS. It was a successful attempt to create a self reading tachymeter that would not simply give the slope distance to the assistant holding the horizontal reference staff, but the horizontally reduced distance and the height difference as well.

The RDH saw the light in 1950 together with the RDS (Reduktions Distanzmesser für Senkrechte Latte or 'Reduction Distance measurer for vertical staff') and uses a dedicated horizontal staff that has metric division and three verniers (see figure 4 and figure 26). The staff is read using the double objective of the instrument (see figure 15), which consists of a plane parallel plate micrometer (upper halve) and a Boskovic and Bosshardt double optical wedge system (lower halve). The optical wedges, a similar system as used in the variable Fall or Rise 'slope' attachment of the Cowley level, are driven by a geared connection on the instrument's secondary axis, while the plane parallel plate is driven by a micrometer knob on top of the telescope (see figure 19).

When the horizontal staff is properly levelled the verniers along the upper half of the staff are optically displaced in a horizontal direction with respect to the divisions on the lower half (the view through the telescope is inverted, see figure 21 and figure 22, so the observer will see it the other way around). The micrometer allows to coincide the vernier to a whole 2 centimetre mark, after which the millimetres can be read from the micrometer drum using a collimator lens in the visor above the ocular (see figure 11 and figure 12). A knob on the side of the telescope allows to switch between horizontal distance and height measurements. When operated it gives the optical wedges a further 90 degrees rotation and changes the engraved correction scale on the secondary axis (see figure 13 and figure 14). This correction scale gives a correction in centimetres for the measured horizontal distance and height differences at steep vertical angles and needs to be subtracted from them.

Alternatively the RDH could be used as a normal theodolite by rotating the toothed collar of the ocular. By doing so a geared connection raises a shutter behind the lower objective as a result of which only the upper objective is used for the observations. It also changes the shape of the ocular from a slit aperture to a full circular opening (see figure 9 and figure 10), The slit aperture is to ensure the images of the two objectives are properly superimposed onto each other when the instrument is used as a tacheometer.¹ In the theodolite mode the RDH has a stadia correction of -0.5 metres.

Based on the T16 the RDH is not a very accurate theodolite. The circles are directly read to 1 arc minute and can be estimated to 0.1 arc minute.² Setting the horizontal circle is done using a triangular lever at the side of the instrument instead of the usual capped knob of the Wild T2 (see figure 20). The lever may work easier, but has the disadvantage that is too easy to disturb the instrument's horizontal orientation as one only needs to lower the lever partially in order to disengage the lock.

The plate level has a sensitivity of 30 arc seconds per 2 millimetre run, the accuracy of the vial for the vertical circle is not given.

With 5 metres the minimum focus distance is quite large. At 100 metres distance the distance to the horizontal staff can be read with an accuracy of approximately 1-2 centimetres, which is 5 to 10 times as accurate as the Wild RDS.³ The vertical staff of the latter had however the advantage that it was easier to set up, making the surveys with it faster.

The instrument came in its original steel case in which were the original dust brush, adjustment pin, oil cannister, and two circular vials for the horizontal staff. In total just over 1000 RDHs have been produced since 1950. Given the serial number 30042 the RDH dates from around 1952.⁴ It was one of the instruments produced in the second batch since Wild started the RDH production. This RDH was number 33 of the second batch of 50 instruments (see figure 18), making it the 83^rd RDH that saw the light at Heerbrugg, Switzerland.

The instruments of this second batch had a few improvements over the first 50 RDHs. The first model RDH had a circle setting knob as known from the Wild T2. In this second series it was replaced by a circle clamp similar in design as found on the Wild RDS (see figure 20). The circle reading telescopes had their magnification increased to 75x, making it easier to estimate the circles to a 0.1 arc minute (6") level. Also the layout of the apertures through which the scales are read changed.⁵ Initially they were rectangular, while now they had the shape of a plano-convex lens cross-section (see figure 27). On the vertical circles the stops for the distance and height switch now protrude from the housing, making it easier the adjust them. Finally the second series RDH was packed in a metal container (see figure 5 and figure 6) whereas the first series came in a wooden box.

Despite its high age this RDH still is in very good condition with only some paint chips missing. It was sold or maintained by the Hungarian firm Parragh Finommechanika in Budapest in 1960 (see figure 7). The former owner used it for cross-section measurements, surveying of water control structures, for baseline measurements, and for small network surveys.

The box with two horizontal staffs (see figure 23) came from a Dutch collector and has been used by a large surveying firm in the Netherlands. An original Wild 21a tripod was already in the collection, by which the set is now amazingly complete (see figure 3). A set of two horizontal staffs allowed to speed up the survey as one assistant could position his staff while the other assistant held the staff that was observed by the surveyor.