Using a Total Station
For those who may wish a
more detailed explanation of the use of the total station, the following
description may be helpful.
more detailed explanation of the use of the total station, the following
description may be helpful.
A total station (Fig. 5)
is a combination electronic transit and electronic distance measuring device
(EDM). With this device, as with a transit and tape, one may determine angles
and distances from the instrument to points to be surveyed. With the aid of
trigonometry, the angles and distances may be used to calculate the actual
positions (x, y, and z or northing, easting and elevation) of surveyed points
in absolute terms.
is a combination electronic transit and electronic distance measuring device
(EDM). With this device, as with a transit and tape, one may determine angles
and distances from the instrument to points to be surveyed. With the aid of
trigonometry, the angles and distances may be used to calculate the actual
positions (x, y, and z or northing, easting and elevation) of surveyed points
in absolute terms.
A standard transit is
basically a telescope with cross-hairs for sighting a target; the telescope is
attached to scales for measuring the angle of rotation of the telescope
(normally relative to north as 0 degrees) and the angle of inclination of the telescope
(relative to the horizontal as 0 degrees). After rotating the telescope to aim
at a target, one may read the angle of rotation and the angle of inclination
from a scale. The electronic transit provides a digital read-out of those
angles instead of a scale; it is both more accurate and less prone to errors
arising from interpolating between marks on the scale or from mis-recording.
The readout is also continuous; so angles can be checked at any time.
basically a telescope with cross-hairs for sighting a target; the telescope is
attached to scales for measuring the angle of rotation of the telescope
(normally relative to north as 0 degrees) and the angle of inclination of the telescope
(relative to the horizontal as 0 degrees). After rotating the telescope to aim
at a target, one may read the angle of rotation and the angle of inclination
from a scale. The electronic transit provides a digital read-out of those
angles instead of a scale; it is both more accurate and less prone to errors
arising from interpolating between marks on the scale or from mis-recording.
The readout is also continuous; so angles can be checked at any time.
The other part of a
total station, the electronic distance measuring device or EDM, measures the
distance from the instrument to its target. The EDM sends out an infrared beam
which is reflected back to the unit, and the unit uses timing measurements to
calculate the distance traveled by the beam. With few exceptions, the EDM
requires that the target be highly reflective, and a reflecting prism is
normally used as the target. The reflecting prism (Figs. 5 and 6) is a
cylindrical device about the diameter of a soft-drink can and about 10 cm. in
height; at one end is a glass covering plate and at the other is a truncated
cone with a threaded extension. It is normally screwed into a target/bracket on
the top of a pole; the pointed tip of the pole is placed on the points to be
surveyed.
total station, the electronic distance measuring device or EDM, measures the
distance from the instrument to its target. The EDM sends out an infrared beam
which is reflected back to the unit, and the unit uses timing measurements to
calculate the distance traveled by the beam. With few exceptions, the EDM
requires that the target be highly reflective, and a reflecting prism is
normally used as the target. The reflecting prism (Figs. 5 and 6) is a
cylindrical device about the diameter of a soft-drink can and about 10 cm. in
height; at one end is a glass covering plate and at the other is a truncated
cone with a threaded extension. It is normally screwed into a target/bracket on
the top of a pole; the pointed tip of the pole is placed on the points to be
surveyed.
The total station also
includes a simple calculator to figure the locations of points sighted. The
calculator can perform the trigonometric functions needed, staring with the
angles and distance, to calculate the location of any point sighted.
includes a simple calculator to figure the locations of points sighted. The
calculator can perform the trigonometric functions needed, staring with the
angles and distance, to calculate the location of any point sighted.
Many total stations also
include data recorders. The raw data (angles and distances) and/or the
coordinates of points sighted are recorded, along with some additional
information (usually codes to aid in relating the coordinates to the points
surveyed). The data thus recorded can be directly downloaded to a computer at a
later time. The use of a data recorder further reduces the potential for error
and eliminates the need for a person to record the data in the field.
include data recorders. The raw data (angles and distances) and/or the
coordinates of points sighted are recorded, along with some additional
information (usually codes to aid in relating the coordinates to the points
surveyed). The data thus recorded can be directly downloaded to a computer at a
later time. The use of a data recorder further reduces the potential for error
and eliminates the need for a person to record the data in the field.
The determination of
angles and distance are essentially separate actions. One aims the telescope
with great care first; this is the part of the process with real potential for
human error. When the telescope has been aimed, the angles are determined. Only
then does one initiate the reading of the distance to the target by the EDM.
That takes only a few seconds; the calculations are performed immediately.
angles and distance are essentially separate actions. One aims the telescope
with great care first; this is the part of the process with real potential for
human error. When the telescope has been aimed, the angles are determined. Only
then does one initiate the reading of the distance to the target by the EDM.
That takes only a few seconds; the calculations are performed immediately.
The total station is
mounted on a tripod and leveled before use. Meanwhile, the prism is mounted on
a pole of known height; the mounting bracket includes aids for aiming the instrument.
The prism is mounted so that its reflection point is aligned with the center of
the pole on which it has been mounted. Although the tip of the pole is placed
on the point to be surveyed, the instrument must be aimed at the prism. So it
will calculate the position of the prism, not the point to be surveyed. Since
the prism is directly above the tip, the height of the pole may be subtracted
to determine the location of the point. That may be done automatically. (The
pole must be held upright, and a bubble level is attached to give the worker
holding the pole a check. It is not as easy as one might expect to hold the
pole upright, particularly if there is any wind; as a result, multiple readings
may be required. Because of that problem, the sighting method chosen at Pompeii
was, if possible, not to begin by sighting on the prism itself but on the tip
of the pole where it touched the ground. The angle from north would then be
fixed and unaffected by the movement of the pole. Then the aim of the telescope
could be raised to the level of the prism, adjusting only the angle of
inclination.)
mounted on a tripod and leveled before use. Meanwhile, the prism is mounted on
a pole of known height; the mounting bracket includes aids for aiming the instrument.
The prism is mounted so that its reflection point is aligned with the center of
the pole on which it has been mounted. Although the tip of the pole is placed
on the point to be surveyed, the instrument must be aimed at the prism. So it
will calculate the position of the prism, not the point to be surveyed. Since
the prism is directly above the tip, the height of the pole may be subtracted
to determine the location of the point. That may be done automatically. (The
pole must be held upright, and a bubble level is attached to give the worker
holding the pole a check. It is not as easy as one might expect to hold the
pole upright, particularly if there is any wind; as a result, multiple readings
may be required. Because of that problem, the sighting method chosen at Pompeii
was, if possible, not to begin by sighting on the prism itself but on the tip
of the pole where it touched the ground. The angle from north would then be
fixed and unaffected by the movement of the pole. Then the aim of the telescope
could be raised to the level of the prism, adjusting only the angle of
inclination.)
In Pompeii a Topcon
total station was used,* and we quickly learned a few features of the
equipment. (Mr. Eiteljorg had driven to Charlottesville to learn the
idiosyncrasies of the instrument in May, but a malfunctioning battery cut the
session short, and a few “simple” or “trivial” processes
turned out to be neither simple nor trivial without practice.) For instance,
leveling the total station is more difficult than we had realized (and spongy
soil is devastating, since the instrument is naturally unstable if its support
is), and it depends upon accurate adjustment of the bubble level built into the
instrument. We also learned that datum points were more difficult to select
than expected, since they had to be repeatable; that is, we had to be able to
find them again and again with absolute accuracy – this year and next.
total station was used,* and we quickly learned a few features of the
equipment. (Mr. Eiteljorg had driven to Charlottesville to learn the
idiosyncrasies of the instrument in May, but a malfunctioning battery cut the
session short, and a few “simple” or “trivial” processes
turned out to be neither simple nor trivial without practice.) For instance,
leveling the total station is more difficult than we had realized (and spongy
soil is devastating, since the instrument is naturally unstable if its support
is), and it depends upon accurate adjustment of the bubble level built into the
instrument. We also learned that datum points were more difficult to select
than expected, since they had to be repeatable; that is, we had to be able to
find them again and again with absolute accuracy – this year and next.
When the instrument is
set up and turned on, it sets itself to be pointing to zero degrees (north)
when power is first supplied. The user must then re-set the instrument to zero
degrees when it is actually pointing north; we learned that there is no
secondary battery for back-up. When the battery dies, the instrument must be
re-set for zero degrees.
set up and turned on, it sets itself to be pointing to zero degrees (north)
when power is first supplied. The user must then re-set the instrument to zero
degrees when it is actually pointing north; we learned that there is no
secondary battery for back-up. When the battery dies, the instrument must be
re-set for zero degrees.
Fortunately, these
lessons came in the first day or two, and we had no more surprises. (One
problem was on-going, however. There are two adjustment knobs for rotating
within the horizontal plane. One rotates the telescope to make a sighting, with
the readout of angles displaying changes. The other, however, permits the user
to rotate the entire instrument and to keep the current angle unchanged during
the process. That effectively re-orients the zero or north setting. That can be
very helpful when setting up or re-setting the instrument, but, of course, it
can be devastating if one makes that adjustment by mistake and thereby changes
the north setting. This particular instrument was designed in such a way that
it was too easy to re-set the instrument when one only wanted to make a
sighting.)
lessons came in the first day or two, and we had no more surprises. (One
problem was on-going, however. There are two adjustment knobs for rotating
within the horizontal plane. One rotates the telescope to make a sighting, with
the readout of angles displaying changes. The other, however, permits the user
to rotate the entire instrument and to keep the current angle unchanged during
the process. That effectively re-orients the zero or north setting. That can be
very helpful when setting up or re-setting the instrument, but, of course, it
can be devastating if one makes that adjustment by mistake and thereby changes
the north setting. This particular instrument was designed in such a way that
it was too easy to re-set the instrument when one only wanted to make a
sighting.)
Since we were dealing
with standing architecture, the prism pole was often inadequate for our work.
The pole is designed to be placed on the survey point in a vertical position;
it cannot be placed on a point on the face of a wall. In fact, a prism pole can
rarely be placed against the face of a wall because of the bulk of the prism,
the pole, and the target to which the prism is attached. We devised two
alternate methods for dealing with points on a wall. One involved the use of reflecting
tape instead of the prism. Since we were working at such short range, bicycle
reflecting tape would reflect the infrared beam well enough to permit the EDM
to make a reading. It was a bit slower than using the prism, but it worked.
(Bicycle reflectors worked, but their back surfaces were not in the same plane
as their reflecting surfaces; so the measurements they generated were from a
point too near the face of the reflector by a few millimeters.)
with standing architecture, the prism pole was often inadequate for our work.
The pole is designed to be placed on the survey point in a vertical position;
it cannot be placed on a point on the face of a wall. In fact, a prism pole can
rarely be placed against the face of a wall because of the bulk of the prism,
the pole, and the target to which the prism is attached. We devised two
alternate methods for dealing with points on a wall. One involved the use of reflecting
tape instead of the prism. Since we were working at such short range, bicycle
reflecting tape would reflect the infrared beam well enough to permit the EDM
to make a reading. It was a bit slower than using the prism, but it worked.
(Bicycle reflectors worked, but their back surfaces were not in the same plane
as their reflecting surfaces; so the measurements they generated were from a
point too near the face of the reflector by a few millimeters.)
The other method for
dealing with points on a wall involved the use of the prism without its pole
and target. We could simply position the prism against the point on the wall to
be surveyed and take the shot. However, the prism is designed to work on the
pole – to give a reading to the center of the pole rather than the back of the
prism. In this case, that meant that the prism gave a reading some 13 mm.
behind the backmost point of the prism housing. We fashioned a shim with a 13
mm. thickness to attach to the back of one of the prisms (fortunately, we had
two prisms). Then the prism could be placed against the point in question and a
reading made. The only problem – and the reason reflecting tape was sometimes
preferred – was that the prism could not always be placed in a corner and
sometimes could not be placed correctly while continuing to face the transit
and EDM for reflecting the infrared beam.
dealing with points on a wall involved the use of the prism without its pole
and target. We could simply position the prism against the point on the wall to
be surveyed and take the shot. However, the prism is designed to work on the
pole – to give a reading to the center of the pole rather than the back of the
prism. In this case, that meant that the prism gave a reading some 13 mm.
behind the backmost point of the prism housing. We fashioned a shim with a 13
mm. thickness to attach to the back of one of the prisms (fortunately, we had
two prisms). Then the prism could be placed against the point in question and a
reading made. The only problem – and the reason reflecting tape was sometimes
preferred – was that the prism could not always be placed in a corner and
sometimes could not be placed correctly while continuing to face the transit
and EDM for reflecting the infrared beam.
When using reflecting
tape or a prism without a pole, the tape or prism hides the point to be
surveyed. So we aimed the telescope at the point to be surveyed before
interposing either tape or prism and maintained the aim of the instrument while
putting the tape or prism in position. That reduced the possible error for
angular measurement. In the case of the prism, after it was put into position,
the transit operator would direct the person holding the prism so that it was
aimed directly back at the instrument. (That would have required a
walkie-talkie had we been working in a larger area.)
tape or a prism without a pole, the tape or prism hides the point to be
surveyed. So we aimed the telescope at the point to be surveyed before
interposing either tape or prism and maintained the aim of the instrument while
putting the tape or prism in position. That reduced the possible error for
angular measurement. In the case of the prism, after it was put into position,
the transit operator would direct the person holding the prism so that it was
aimed directly back at the instrument. (That would have required a
walkie-talkie had we been working in a larger area.)
The survey information
was recorded by hand, and the data were then entered into the AutoCAD model. We
were able to use the data directly, no matter where the machine had been set up
for a given session, thanks to an AutoCAD feature called the user coordinate
system. Using that AutoCAD feature, each set of data could be entered accurately,
regardless of the transit set-up point. (It is unnecessary to describe that
process here, but a complete description is available from CSA.)
was recorded by hand, and the data were then entered into the AutoCAD model. We
were able to use the data directly, no matter where the machine had been set up
for a given session, thanks to an AutoCAD feature called the user coordinate
system. Using that AutoCAD feature, each set of data could be entered accurately,
regardless of the transit set-up point. (It is unnecessary to describe that
process here, but a complete description is available from CSA.)
This process is not
necessary if a data collector with the most modern of capabilities is
available. The data collector can automatically orient all new points to a pre-existing
set of survey coordinates. But the process we developed worked well and easily,
and it gave us a check of our own accuracy as we manipulated the model and
created the alternate user coordinate systems. We also put it into practice in
a way designed to make it obvious to the user if he was not entering the data
correctly. More important, we can use equipment with various levels of
sophistication.
necessary if a data collector with the most modern of capabilities is
available. The data collector can automatically orient all new points to a pre-existing
set of survey coordinates. But the process we developed worked well and easily,
and it gave us a check of our own accuracy as we manipulated the model and
created the alternate user coordinate systems. We also put it into practice in
a way designed to make it obvious to the user if he was not entering the data
correctly. More important, we can use equipment with various levels of
sophistication.
*The instrument used
measures to within 5 seconds for vertical and horizontal angles. The electronic
distance measuring device (EDM) measures to within 5 mm. and 3 parts per
million; so the error will be no more than than the sum of 5 mm. and 3 parts
per million of the measured distance from instrument to prism. Instruments are
available which measure to tighter tolerances, but for short-range work such as
we were doing at Pompeii — nothing we measured was more than 100 m. from the
instrument and most of the work was within 25 m. – the accuracy of the transit
and EDM were more than sufficient. The EDM error at 100 m. would be no more
than 5 mm. (3 parts per million at 100 m. adds less than a mm. to the maximum
error). At 100 m. an error of 5 seconds in an angular reading would make only a
2 mm. error in position; at 40 m., the angular error drops below 1 mm. For the
vast majority of the work, then, the maximum theoretical error was the error of
the EDM, 5 mm. Of course, human error may add to machine error.
measures to within 5 seconds for vertical and horizontal angles. The electronic
distance measuring device (EDM) measures to within 5 mm. and 3 parts per
million; so the error will be no more than than the sum of 5 mm. and 3 parts
per million of the measured distance from instrument to prism. Instruments are
available which measure to tighter tolerances, but for short-range work such as
we were doing at Pompeii — nothing we measured was more than 100 m. from the
instrument and most of the work was within 25 m. – the accuracy of the transit
and EDM were more than sufficient. The EDM error at 100 m. would be no more
than 5 mm. (3 parts per million at 100 m. adds less than a mm. to the maximum
error). At 100 m. an error of 5 seconds in an angular reading would make only a
2 mm. error in position; at 40 m., the angular error drops below 1 mm. For the
vast majority of the work, then, the maximum theoretical error was the error of
the EDM, 5 mm. Of course, human error may add to machine error.
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