Civil Engineering Reference
In-Depth Information
A reflectorless total station does not require a prism to reflect the light back
to the EDM.
Reflectorless EDMs are often used for locating buildings and other features which
cannot be conveniently measured using a prism. For example, the top of a tower or
other inaccessible feature is easily measured with reflectorless total station. With a
conventional EDM it would be necessary to place a prism on or next to the object
being measured. For many features, this would be inconvenient or impossible.
Boundary surveys are often required because a neighbor is disputing a boundary
location. In cases where it is necessary to measure objects on a property where the
owner is hostile or uncooperative, a reflectorless EDM often allows the measure-
ments to be made without the necessity of obtaining a court order or injunction
to enter upon the property. Even if a surveyor is allowed on abutting property by
statute, reasonable notice to the property owner is normally required. Sending reg-
istered letters takes time and the letter may not be accepted. With a reflectorless
EDM it is only necessary to set up the total station on a location near but not on
the inaccessible property and take measurements to the features which need to be
located. Not all objects can be located this way but many can.
Some objects, such as the corner of a house may be difficult to measure accu-
rately with a prism. We will discuss prism offsets later in this chapter. For now it is
sufficient to know that it is a distance correction which must be applied to distances
measured using a prism. The offset for a particular prism is based on the hinge
point of the prism or the plumb line of the prism holder. The plumb line is not
always at the back of the prism so if a distance needs to be measured to the corner
of a building, the distance will be incorrect because the plumb line of the prism
will not be at the corner of the building. The reflectorless total station solves this
problem because the light will be reflected from the exact corner of the building.
Reflectorless total stations do require extra caution on the part of the instrument
person. The EDM doesn't care what it is measuring to. As long as the target is
within range it will give a distance. This can't happen with a prism because the
EDM will only measure to the prism. An example may make the problem clearer.
Suppose it is necessary to measure to the outside corner of a building 200 feet
away. There is a fence 3 feet behind the building. We want to measure to the exact
corner of the building. The building has vertical corner boards which give it a very
sharp corner. If the instrument person is not very careful sighting the building cor-
ner and the cross hairs of the total station are slightly outside the corner, the EDM
might measure the distance to the fence, not the building. The error would prob-
ably not be obvious to the instrument person because they would be unlikely to
visually tell the difference between 200 feet and 203 feet. If the building was 100
feet away and the fence was 200 feet away the error would be obvious, providing
that the instrument person actually noticed the distance measurement. This is an
easy omission to make when using a data collector because the instrument person
doesn't need to write a distance into a ield topic so the error is likely to go unno-
