Surveying Instruments - Practical Boundary Surveying - page 55

is about 0.005 feet, less than 1/16 of an inch! The Wild T1A theodolite shown in
Fig. 5.3 reads to 20 seconds of arc. Although these instruments have been largely
replaced by total stations, which can also measure distances, they were made in
Switzerland and are of superb quality.
5.2 Total Stations
Total Stations are theodolites which have built in Electronic Distance
Measuring Devices ( EDM ). These devices emit light signals at varying frequen-
cies. The light signals are reflected back to the instrument from a prism and the
EDM is able to accurately measure the distance between it and the prism. On
some of the earlier total stations, the EDM was mounted above the telescope.
This allowed an EDM to be retrofitted to some older theodolites. With this design,
when the telescope was pointed downwards, the EDM would move forward of the
telescope axis. When the telescope was pointed upwards, the EDM would move
backward from the telescope axis. This meant that distances measured with the
EDM had to be adjusted for the offset. The greater the telescope deviated from
horizontal, the greater the adjustment. A top mounted EDM is shown in Fig. 5.4
where it can be seen just above the telescope eyepiece. All new total stations place
the EDM inside the telescope so the EDM is coaxial with the line of sight, thereby
eliminating the need for offset compensation.
To measure a distance with a total station, the telescope is precisely pointed at
the prism and a button on the total station or data collector is pressed to initiate
the measurement. When a data collector is used to store the data, the informa-
tion is transmitted from the total station to the data collector. Because the prism
is rarely at exactly the same elevation as the total station, the distance measured is
always a slope distance. When the total station sends data to the data collector, it
is therefore necessary for the total station to transmit three pieces of information:
the horizontal angle, the slope distance and the vertical angle. When we discussed
measuring distances with a steel tape it was shown that if a slope distance and
vertical angle were known it would be possible to calculate a horizontal distance.
Most total stations have onboard software which converts the slope distance to
a horizontal distance. Recall that in boundary surveying we are primarily con-
cerned with horizontal distances, not slope distances. With an EDM it is possible
to measure a mile to an accuracy of a few hundredths of a foot or less in a matter
of 1 to 5 seconds. When using an EDM it is necessary to see between the points
being measured—the points must be intervisible. This is usually not a problem
because in order to measure an angle, the points must be visible through the tel-
escope. Nevertheless, in some situations, such as when the wind blows a tree leaf
online, or when a small twig is exactly online, it will be possible to sight the tar-
get but the EDM will not be able to measure the distance until the obstruction is
removed.