4. Evolutionary sidebar

Early attempts to integrate EDMs with theodolites resulted in some pretty interesting (and bizarre) hybrid instruments.

The first affordable EDMs were stand alone and could only measure distances - no zenith angles. A typical procedure is shown in Figure F-9:

(1) measure a zenith angle with a theodolite,

(2) remove the theodolite from the tripod and mount the EDM (often using the same tribrach to maintain the same setup) and measure the slope distance, and, finally

(3) manually reduce the slope distance to horizontal.

Figure F-9 Swapping Instruments

As EDMs became more affordable and smaller, other integration methods appeared.

EDMs were placed in yokes mounted to a theodolite's standards, Figure F-10(a). The vertical angle would be measured with the theodolite, and recorded or manually entered into the EDM. The slope distance would be measured with the EDM. Slope would be reduced to horizontal either manually or by the EDM if it could accept angle input.

(a) Standards Mount	(b) Convergence
Figure F-10 Standards Mount EDM

The advantage of this mounting method was that the EDM's measuring center was always vertically above the same point - it did not change position as it was elevated or depressed to sight the reflector.

The disadvantage was that if the zenith angle was measured to the center of the reflector an offset error was introduced because the signal path and line of sight weren't coincident, Figure F-10(b)

Another mounting method placed the EDM on top of, Figure F-11, and later wrapped around the theodolite telescope. Measurement and slope reduction was similar to that of a yoke-mount EDM.This method had the same disadvantage as the yoke mount plus two additional ones:

(1) It shifted the measuring center of the EDM as the zenith angle changed (necessitating more computations), and,

(2) it stressed the telescope mount and lock which were not designed for the additional eccentric weight.

Figure F-11 Telescope Mount EDM

It wasn't until the digital theodolite was developed that the EDM could be seamlessly integrated with an angle measuring device: the Total Station. This has become the primary instrument for most surveyors and represents the latest evolutionary step of the EDM. For the rest of this chapter, we will discuss distance measurement with a TSI.