In June 2017 I acquired this Wild Heerbrugg combination of a Wild T2 theodolite with a Wild Di3S Distomat Electronic Distance Measurer (EDM). The theodolite is very similar to the 1962 Wild T2 in my collection, but is equipped with the more modern turning knob type locking mechanism on the tribrach whereas the 1962 version still has a sliding knob.

The reason for this acquisition lies in the EDM. The Di3S and its direct forerunner Di3 were the first EDMs that could be fully integrated with a theodolite, this unlike the earlier Di10 which had a separate control-box that had to be placed on the ground.

Although the controller and the optics are still two separate units (the orange parts in figure 1), earlier EDMs had a controller that large that it had to stand on the ground. Size reduction of electronic components allowed to get all electronics in a small tray like box (see figure 8), allowing to mount the controller of the Di3S on top of the theodolite's tribrach and then to mount the theodolite on top of that (see figure 9). Finally the optical unit of the Di3S was mounted on top of the telescope using a to which it can be attached using a dedicated adapter and a forced centring clamping mechanism (see figure 5 and figure 6). It has to be noted that, although they are similar units, the heads and controllers of the Di3 and Di3S cannot be interchanged as that will cause serious damage to their electronics.¹

The central black part of the controller is nothing more than a column on which the theodolite stands. The downside of this way of integrating an EDM into a theodolite is that is raises the secondary axis of the theodolite by 0.125m. This means that when using this set-up in a traverse with forced centring that at each station should be corrected with this value to avoid errors in the vertical. In later years electronic components were further reduced in size leading to single unit EDMs like the ones on the 1980 SAT AGA-Minilir, on the 1984 Kern E1, and on the 1990 Wild T2 mod - Di1000.

The Wild DISTOMATs were the result of a joint venture between Wild Heerbrugg and the France based SERCEL (Societe d'Etudes Recherches et Constructions Electroniques). The latter firm would later become renowned for their dGPS and RTK-GPS equipment, like receiver models NR103 and NR203, and their dGPS reference station NDS100.

The Di3S was produced between 1974 and 1978, this particular example dates from 1976. It sends light from a GaAs diode with a carrier wave frequency of 74.927kHz to which a 7.5MHz amplitude modulation is applied. With these frequencies the wavelength of the Di3S is 40 metres for its carrier wave and 4000 metres for its modulation wave. As distance is determined by the total travelling time towards the target and back to the transmitter, phase induced ambiguity will occur at 20 and 2000 metres.² This means that the surveyor needs to know by an order of 2km how far his target is away from him and add the number of 2kms to the result supplied by the Di3S. Although nowadays this seems quite cumbersome, at the time the Di3S saw the light, this was already an improvement to previous systems by a factor 2.

Distances of up to 1600 metres could be measured using a reflector containing three prisms, for larger distances a nine-fold reflector was available. The accuracy of this early EDM is surprisingly high: 0.005m (1σ, 68%) + 5ppm (0.005m per km), which is as accurate as the somewhat younger 1980 Wild TC1.³ The measured distance could be reduced to a vertical (height difference) or horizontal distance by entering the theodolite's vertical circle reading using the numerical keypad.

In a similar manner the to the horizontal reduced distance can be used to calculate cartesian coordinates relative to the theodolite's position by entering the horizontal circle reading (see figure 11). Due to hardware limitations the calculated difference in X had to be inverted for horizontal angles above 180 degrees (200 grad). Although this all seems quite cumbersome, especially when compared to more modern instruments like the 1999 Leica TCRA 1101, in those days this was a huge step forwards, both in capabilities and in time reduction. This however came at a firm price: with fl21,057.- ex. VAT the Di3S was more than twice as expensive as a T2 (fl9,120.-)!⁴

This particular combination of instruments arrived in the collection together. Although both had been used by the survey department of Oranjewoud, the Di3S optics did not properly fit the T2 when it arrived, nor was there a travel case for the Di3S. The adapter was in place on the telescope of the T2, but neither of the two counterweights (it came with numbers 4 and 5, see figure 3) could be installed. I had to add a higher mounting block to get enough height to mount the Di3S. The height of the optics above the telescope is now 52mm, whereas it should be 38mm according to the manual,⁵ so it seems that my addition is not correct and that I need another counterweight or another adapter to correct this.