Feature: Surveying Goes Underground
Professional Surveyor Magazine - May 2007
Typically, surveyors have been unable to measure subsurface points with a total station above the surface. Even if they can see through the haze in mine shafts and tunnels, optical measurement is possible only with the instrument at the bottom of the shaft. Mostly, it is done combining theodolite measurements or laser plumbing with depth measurement using a measuring tape. And optical measurement from the bottom up the shaft often causes problems because of water dropping down.
Another complex problem is the initial surveying of sewers and pipelines with geodetic accuracies. The measuring systems normally used for utility network plans, like hidden point poles, sewer TVs, locating radio systems, or 3D laser scanners, cannot determine the position and course of a sewer in three dimensions.
But all this is changing. A modular total station attachment system dubbed ArgusTAT (Total station Attachment with Telescope) attaches to a total station and offers a solution for both measurement tasks. It enables plumbing down and orientation transfer in vertical and slightly inclined shafts. The system combines the ArgusTA with a mechanical telescope to supplement existing sewer surveying technologies or link between them. This system was developed and patented by the German company Argus GeoTech GmbH.
With the ArgusTA, the horizontal beam of the total station is deflected by a mirror system consisting of three orthogonal mirrors to a vertical beam running coaxial with the vertical axis of the total station An orthogonal mirror, like that used in the Argus-Eye (see our story on that in the April issue), consists of two single-surface mirrored glass plates and enables rectangular deflection with an accuracy of less than one mgon (1000 mgon equals 1 gon; 100 gon equals 90 degrees). Three orthogonal mirrors are placed in a deflection unit, which connects the handle of the total station with the rotatable middle part of a high-precision bearing. If the horizontal circle of the total station rotates, the deflection unit and middle part of the bearing do the same. A clamp device connects the upper and the lower parts of the high-precision bearing, so the clutches of the total station keep their position while rotating the horizontal circle. The clamp device limits the horizontal rotation range to 330 gon.
For fastening the ArgusTA to the total station, the handle of the total station has to be exchanged with a similar handle that carries a tiltable bracket and attaches to the deflection unit. A micrometer screw gives fine adjustment of this attachment. Orientation measurements to the reference points should be taken before connecting the bracket to the ArgusTA.
With an ArgusTA, you can use any total station for nadir plumbing or oblique plumbing down. If you tilt the vertical circle of the total station to a position V = 100 gon ± a, the beam that comes out of the ArgusTA tilts with the amount ± a out of the vertical axis. Ranging through the ArgusTA allows polar measurement of a target at the bottom of a shaft if the target can be pointed. The additional range variable of the ArgusTA, which depends on the vertical angle, is taken into consideration while computing the coordinates.
After reaching a 90-centimeter (cm) optical path, the aperture is limited to 3.7 cm by the size of the third orthogonal mirror, resulting in a tilt range of the vertical angle of ± 1 gon. This equals a 6- meter (m) base length at a depth of 200 m. Plumbing tasks in deeper shafts can be solved easily and supported by the magnification of the total station telescope, which has a magnification factor equivalent to optical precision plummets. The prototype was built for work with a TCR 1103 or similar instrument from Leica Geosystems. The deflection unit has to be fitted for other types of instruments to handle the height of the tilt axis and the handle connection.
Regardless of your total station type, the ArgusTAT can increase its versatility. The ArgusTA module enables you to use the total station for not only nadir plumbing, oblique plumbing, depth measurement, and vertical orientation transfer in deep mining shafts, well shafts, and tunnel shafts, but also in high-rise buildings, elevator shafts, and retaining walls. Another example of a possible application is the definition of oblique 3D coordinate systems for a shipyard drydock.
Measuring within a retaining wall demonstrates the advantage of optical plumbing with an ArgusTA. Not all plumbing shafts in a retaining wall have plumbing wires, for reasons of economy. The free shafts can be used for periodical optical control measurements. This optical plumbing can easily be done with an ArgusTA after positioning and orientation within the reference net of the building instead of the usual mechanical plumbing combined with alignment measurements on top of the wall. Points on top of and inside the retaining wall can be observed simultaneously with high accuracy by taking optical measurements using mobile equipment. The costs are low compared with the installation of mechanical plummets.
The second component in the ArgusTAT system is a mechanical telescope that hangs vertically on the tripod. Its lowest tube carries a fourth orthogonal mirror, and the telescope can be driven up and down by a motorized wire rope hoist to vary the depth of that mirror. This orthogonal mirror deflects the beam that comes vertically out of the ArgusTA again horizontally. The vertical axis of the telescope should be aligned with the vertical axis of the total station due to the gimbal mount of the telescope and the clamped connection with the middle part of the bearing. The gimbal mount also enables the alignment of the optical and mechanical axes if the tripod plate doesn't stay exactly horizontal. The use of a special tripod guarantees the needed freedom of movement. If the horizontal circle of the total station moves, the deflection unit and telescope follow it synchronously. Normally, the horizontal drive of the total station should be strong enough for the rotation. In exceptional cases, the sliding clutch of the horizontal drive has to be adjusted harder.
If the ArgusTAT is set up over an open manhole, the telescope can be driven down to the depth of a sewer or duct to be measured. This can be up to 8 m deep from the top of the shaft. Turning the horizontal circle changes the viewing direction inside the shaft, so the sewer becomes visible inside the telescope, and the horizontal angle can be measured with the total station. The guidance of the mechanical telescope cannot be manufactured exactly enough to keep the deflected beam parallel to the original beam while driving the telescope up or down. That's why the top tube has an integrated horizontal drive that allows defined rotations of the telescope against the ArgusTA. After focusing the crosshair inside the lowest tube, this crosshair can be adjusted parallel to the crosshair of the total station. Most observers reach an accuracy of less than or equal to 50 mgon for this optical orientation transfer.
Now the distance measurement of the total station can be used to determine the driven depth of the telescope and measure subsurface points. A reflection plate inside the lowest tube can be clapped up by remote control for measuring the depth of the telescope. The reflection plate has to be clapped back after ranging to it. Now object points can be measured reflectorlessly, or you can range to reflective tapes or triple prisms. With reflectorless ranging using the Leica TCR 1103, ranges of 30 m have been achieved with non-cooperative targets and over 300 m ranging to reflective tapes and triple prisms. Up to 80 m was reached with reflective tapes and up to 500 m with triple prisms using the infrared EDM.
The geometry of the ArgusTAT allows limited inclination of the ray of sight out of the horizontal plane. The tilt range is limited by the aperture of the lowest orthogonal mirror (16 cm) if the telescope is driven down and the size of the third orthogonal mirror (3.7 cm) if the telescope is driven up. Depending on the driven depth, the tilt range lies between ± 0,6 gon and ± 1 gon. That means you can see very deep into sewers or pipelines, and they can have a slope of up to two percent. If the slope is higher, the field of view is shorter. The direction of inclination at the total station (up or down) is the same as after passing the lowest orthogonal mirror. Due to the construction of the total station attachment system, you can't turn the total station together with the deflection unit in a full circle. The clamped connection between the upper and lower parts of the console limits the horizontal rotation range to 330 gon, so it is useful to place the blind angle in a direction without a target while setting up the instrument.
Designed for Sewers
The combination of both modules should prove conducive to sewer surveying. Unlike the usual measuring systems, the ArgusTAT enables precise three-dimensional measurement of position and run of a subsurface duct or pipeline. You can track the position and height of a sewer inspection system equipped with a lighted triple prism. If a wagon, a small tractor used for inspecting pipelines, runs inside the duct, you can track the target as long as it is visible from the shaft or from the other end of the duct. First survey of sewers and ducts is possible this way as well as deformation measurement or precise locating of junctions and cave-in positions if the path of the duct is unknown. A decided advantage: the measurement happens above ground, avoiding danger to the health of the observer and field staff.
While the ArgusTAT is designed primarily for sewer surveying, other possible applications can be found where cavities difficult to access have to be measured. For example, orientation transfer and staking out of construction axes in multi-story buildings would be possible. This is also conceivable for tunnels or shaft buildings a maximum 8 m under the surface. The present accuracy is already sufficient for such purposes if the horizontal distances don't amount to much more than 10 m. Then you get a cross deviation of less than 1 cm. This accuracy is sufficient for many tasks in architectural surveying and staking out.
Also, in the fields of archaeology and mining, the principle can be used to survey subsurface hollows or caverns. For example, the staking out of subsurface orientation targets has successfully been done for a 3D laser scanner, which afterwards was used to scan the geometry of a tomb raider hole under the grave of Emperor Otto I. within the cathedral of Magdeburg. Furthermore, dumpsite monitoring is conceivable, optionally in combination with an inertial guidance system running through a piping system. The ArgusTAT can define the absolute start direction for the inertial guidance system.
About the Author
Matthias FuhrlandMatthias Fuhrland is part owner and product manager of Argus GeoTech GmbH in Schonebeck, Germany. He is also a graduate surveying engineer finishing his doctoral thesis at the University of Dresden.
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