This Wild Heerbrugg RDS came up for auction on a Dutch auction site in July 2014. The abbreviation RDS stands for Reduktions Distanzmesser für Senkrechte Latte or 'Reduction Distance-meter for a Vertical Staff'. The RDS is one of the rarer theodolites Wild produced. It was a successful attempt to create a self reading tachymeter that would not simply give the slope distance to the assistant holding the vertical reference staff, but the horizontally reduced distance and the height difference.

The RDS saw the light in 1950 together with the RDH (Reduktions Distanz- und Höhenmesser or 'Reduction Distance- and heightmeter') and uses a dedicated levelling staff (model GVLV) that has an extension rod at the bottom end. The staff is read using the special curved cross-hairs of the instrument (see figure 14), which are superimposed on the field of view by the so-called Hammer-diagram (see figure 17) that is etched on the secondary vertical circle next to the telescope (see figure 11). A geared connection to the telescope rotates the Hammer-diagram-circle at a fourfold rate around the secondary axis in respect to the telescope (three-fold in respect to the instrument). The telescope itself is limited in its movement to about 350 gon (see figure 12).

With the extension rod of the levelling staff set at the instrument height minus 1 metre (i.e. at an instrument height of 1.34m the rod was set at 0.34m) the vertical angle of the telescope is adjusted until the zero-line of the instrument (the bottom one) runs over the 1 metre mark of the staff (see figure 18). The uppermost line now gives the horizontal distance to the staff (41.3 metres), while the height-line (the middle one) shows the factor to multiply its reading by to get the height difference (+0.1 x 21.7 = 2.17m).¹

Alternatively the 1 metre reference of the staff could be used at a height equal to the instrument height above a rounded reference height. Suppose a station height of 8.87 metres and an instrument height if 1.36 metres, the secondary axis of the instrument would be 1.23 metres above 9.00 metres. The extension rod was then made 0.23 metres and all height differences would now be in respect to the 9.00 metres reference height, making the mental exercise much easier.²

Although (or perhaps because of) the RDS was not the first tachymeter to use this method, it was regarded the most modern in 1957.³

Based on the T16 the RDS is not a very accurate theodolite.⁴ The circles are directly read to 1 centigrad (1 arc minute) and can be estimated to 0.2 centigrad (0.1 arc minute).⁵ 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 10). 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.

At 10 metres distance the vertical staff gives an accuracy of approximately 2.5 centimetres, while at 100 metres this is approximately 10 centimetres. With distance d in metres the error in distance measurement comes to 0.0869d+1.3 centimetres.⁶

In distance measurement too the RDS was not the most accurate vertical staff tachymeter. In the early 1960s Kern produced their DK-RV that could read the staff at whole millimetres and estimate tenths (the RDS could only read to whole centimetres and estimate millimetres). As a result of this the distance measurements with the DK-RV are about two to three times more accurate than those taken with the Wild RDS. For even higher accuracies one had to use a tachymeter with a horizontal staff like the Wild RDH. The vertical staff had however the advantage that it was easier to set up, making the surveys with it faster.

The instrument came in its original steel domed case and original screw driver and adjustment tools. Given the serial number 116555 it dates from around 1963, but despite that it still is in near mint condition with only a few minor paint chips missing from the base and ocular. It was once sold by the Belgium firm Van Hopplynus (see figure 3), which was established in 1957, just a few years before they retailed this RDS.