The acquisition of this Wild Heerbrugg T2 theodolite - DI4 Distomat combination was a rather nostalgic moment for me. I was trained as a hydrographic surveyor in the years 1985-1989. After having worked for my first employer for half a year around the Netherlands I was sent to a large breakwater maintenance project in Algeria. In total I spent about 13 months on the project with two to three weeks leave every ten weeks. Apart from the hydrographic surveys for the underwater part of the breakwater, which were done using our survey vessel Denise, the parts above the waterline were done from the breakwater using a T2/DI4 combination very similar to the one shown here. Most likely the Distomat we used was a DI4L (see below) as we also surveyed the baselines of our positioning system Microfix which were that long that we had multiple ambiguities on the distances (see below).

Many a pleasant hour was spent behind this, for the time magnificent, combination. It would take another year before we started to work with total stations, at the time mainly the Sokkia SET 3C.

The T2 is a model "T2 mod", similar to the one on the 1990 Wild T2 mod - Di1000 combination in the collection, but more than a decade older, so for the details about that instrument please visit that page. The only difference with the other T2 mod in my collection is that this one is approximately 15 years older and that it came with a DI4 instead of the more modern DI1000.

The Wild DI4 was the direct successor of the Wild DI3S and again a product of the joint-venture Wild Heerbrugg - SERCEL (Societe d'Etudes Recherches et Constructions Electroniques). With the Wild DI4 an important last step was made in miniaturization of the electronics and at the time (perhaps even up to now) the DI4 was the smallest detachable distance meter (or distancer as they called it themselves) for theodolites. In successive models they only became lighter and more powerful, but in size they remained the same or became slightly larger.

The DI4 was produced between 1978 and 1982 (although it was still advertised in 1983), this particular example dates from 1979 with a 1980 keyboard.¹ It sends a conical light beam, which diverges with 24cm per 100m, from a GaAs diode with a carrier wave frequency of 75kHz to which a 4.87MHz amplitude modulation is applied.² With these frequencies the wavelength of the DI4 is 30.8 metres for its carrier wave and 2000 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 DI4. Although nowadays this seems quite cumbersome, at the time the DI4 saw the light, this was normal and similar to how the DI3S worked.

Distances of up to 2500 metres could be measured using a reflector containing nine prisms, for shorter distances up to 1600m a three-fold reflector was available, while 1000m could be reached with a single reflector.² Using 11 prisms and optimal atmospheric conditions distances up to 3000 metres could be measured.³ For even larger distances the DI4L was available which could measure 2.5 times the distances mentioned above. Measuring time was about 10-12 seconds.³ 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.^2,3 The measured distance could be reduced to a vertical (height difference) or horizontal distance by entering the theodolite's vertical circle reading using the optional numerical keypad (see figure 4).

Distances could automatically be corrected for scale factor and curvature in steps of 2mm/100m from -16mm/100m to +12mm/100m (see figure 6).

The DI4 was available with LED and LCD displays, the one here is the LED version (see figure 8). It came complete with its original case (see figure 11), optional GST1 keyboard, cables, Allen key, quick start guide, battery, counter weight 5 (see figure 3), and charger (not pictured). The battery is at the end of its useful life, all measurements shown here were done using a modern Leica car adapter cable and a period 12V power supply (great that this stuff is backwards compatible!).

Both the T2 and the DI4 once belonged to the Meetkundige Dienst (survey department) of Rijkswaterstaat (Dutch Ministry of Infrastructure and the Environment). The previous owner used it in the 1990s within a different department for bridge surveys. It was during this decade that Rijkswaterstaat decided to stop maintaining survey equipment as they thought it was more appropriate to leave that to commercial parties. When he wanted to return the instrument to the Meetkundige Dienst it was for this reason refused and as a result of that ended up in his attic. Now after two decades gathering dust he decided to clear his attic and that is how the instrument arrived in my collection. Upon arrival it was directly put to the test, showing that it still is fully functional after so many years.