1. Chapter 7 – Electronic Distance Measurement (EDM) and Total Station
Electronic Distance Measurement (EDM)
EDM was first introduced in the 1950s.
EDM instruments use infrared light or laser light or Microwave.
EDM with microwave
Use a receiver/transmitter at both ends of the measured line.
Have an upper measuring range of 50km.
Are now largely outdated and replaced by global positioning system (GPS) techniques.
EDM with infrared and laser systems
Utilize a transmitter at one ends of the measured line and a reflecting prism at the
other end.
Some laser EDM instruments measure short distances (100 to 350m) without
reflecting prism.
They reflect the light directly off the feature, e.g. building wall, being measured.
Long range : 10 to 20 km
Medium range: 3 to 10 km
Short range : 0.5 to 3 km
EDM can be mounted on the telescope of most theodolites.
2. Prisms
Prisms are used with EDM instruments to reflect the transmitted signal.
A single reflector is a cube corner prism that has the characteristic of reflecting light rays
precisely back to the emitting EDM instrument.
Prisms can be mounted on a tripod or bi-pod.
The height of the prism is often set to equal the height of the instrument otherwise
consideration has to be given to the difference between the prism height and the instrument
height.
3. It is important that prisms mounted on poles be permitted to tilt up and down so that they can
be made perpendicular to EDM signals that are being sent from much higher or lower
positions.
4. Total Station
Total station can measure and record horizontal and vertical angles together with slope
distances.
The microprocessors in the total stations can perform different mathematical operations:
• Averaging multiple angle measurements
• Averaging multiple distance measurements
• Determining horizontal and vertical distances
• Determining remote object elevations (heights of sighted features)
• Determining distances between remote points
• Performing atmospheric and instrument corrections
Angle accuracy from 0.5 to 5 seconds
Distance ranges from 1.6 km to 3 km.
Data can be transferred from the total station to the computer either via USB hub or storage
card.
6. Missing Line Measurement Program
This program enables the surveyor to determine the horizontal and slope distances between
any two sighted points, as well as the directions of the lines joining those sighted points.
Resection
This technique permits the surveyor to set up the total station at any convenient position and
then determine the coordinates and elevation of that instrument position by sighting
previously coordinated reference stations.
Azimuth determination
When the coordinates of the instrument station and a back-sight reference station have been
entered into the instrument processor, the azimuth of a line joining any sighted points can be
readily displayed and recorded.
7. Remote object elevation
The surveyor can determine the heights of inaccessible points (e.g. electricity conductors,
bridge components), while using a total station that is already set up, simply by sighting the
pole-mounted prism as it is being held directly under the object.
When the object itself is then sighted, the object height can be promptly displayed (the prism
height must first be entered into the total station).
Offset Measurement – Hidden object offsets
When an object is hidden from the total station, a measurement can be taken to the prism held
in view of the total station, and then the offset distance can be measured.
The angle (usually 90°) or direction to the hidden object along with the measured distance are
entered into the total station, enabling it to compute the position of the hidden object.
8. Offset Measurement – Object center offsets
To locate a solid object’s center position, the turn angles to the prism are measured as it is
being held on each side of the object, equidistant from the object’s center.
Setting-out positions
After the station numbers, coordinates, and elevations of the layout points have been
uploaded into the total station (or entered manually), the layout/setting-out software enables
the surveyor to locate any layout point simply by entering that point’s station number when
prompted by the layout software.
The instrument’s display shows the left/right, forward/back, and up/down movements needed
to place the prism in each of the desired position locations. This capability is a great aid in
property and construction layouts.
9. Area computation
The processor determines the coordinates of each station and then it uses those coordinates to
compute the area.
