Along the Water's Edge
Better and faster—qualities most all of us want in our work activities and the instruments used to complete them. Technology often makes this possible. Robotic total stations, or RTS, offer a solution for better, faster surveying and data acquisition. If you need to obtain surveying data at a fast rate, like taking a snapshot, you may want to look to RTS technology.
Researchers at the Conrad Blucher Institute for Surveying and Science at Texas A&M University-Corpus Christi (TAMU-CC) needed to collect data very quickly to document a “snapshot” of the morphology of a newly restored beach along Corpus Christi Bay. TAMU-CC, in conjunction with the Texas General Land Office and the City of Corpus Christi, restored the beach along the shoreline of the university’s campus for research and recreation. After construction was completed, we needed to obtain an accurate topographic map describing the subaerial and subaqeous morphology (underwater physical geography) including sandbars, beach face and shoreline position. These “beach profile surveys” include survey lines that begin at a position landward of the beach and extend perpendicular to a baseline offshore to the depth of closure (offshore extent of measurable change in elevation between survey dates). This data will serve as the baseline for the comparison of all future surveys to be conducted seasonally for a minimum of two years.
GPS was not efficient to the vertical accuracy we needed to maintain, so we decided to use a robotic total station (RTS) based on its capability to lock and track the prism as the bottom was scanned. Because we did not want to jeopardize the placement of the remote control unit in the water, a person still operated the total station by only recording data as signaled by the rodperson. We found this to be a rapid method of surveying. Often, surveyors have the opinion that a RTS must operate in a remote control operation to increase productivity. This is not the case as this project will prove.
Highlighting RTS TechnologyWe saw that a RTS was useful based on the ability to quickly acquire data. Surveyors typically use RTS technology to independently operate between the instrument and the rod location. This is usually done using a spread spectrum radio system that communicates what would be seen at the instrument. This is called a remote control unit or RCU, which can send commands back to the instrument such as turn left/right or up/down, search for target, reverse face or turn on flashing line indicators used to put rods on location during a stakeout. The RCU doesn’t have to necessarily be at the rod location; an operator can be in an area close to the rod as long as he is not in the water. We didn’t use this particular concept for our application because it would be difficult for the instrument operator to be aware of the pointing direction of the instrument if realignment was necessary.
We evaluated two types of RTS technology for our application. The first type of RTS works by locking on a target by using image or signal return algorithms. One is considered an active target search. With this type of technology, a surveying pole is equipped with a light emitting diode (LED) that the RTS can receive, track and maintain lock using a servo motor. Therefore, a special pole and target is needed with a battery source. This method can differentiate its light source readily from others. The second type of RTS is passive at the rod but active at the instrument. The RTS may be used with any surveying prism, including small “peanut” prisms for tracking. This technology applies to the target where the instrument emits an infrared beam that is captured through to a beam splitter, which separates the path of the return signal to a CCD chip. The returned signal is then computed using a digital image processing algorithm to find the signal center spot. Mathematically this is unique and robust. Both systems remove manual focusing and manual-optical precision and have been proven to be well-established technology.
We used the passive rod with an active instrument for several reasons: we were in the water and did not want battery systems getting wet; face two measurements could be made; and data collection was easier and met our distance requirements. For this project, we used the Leica TCA 1103 RTS (Leica Geosystems, Norcross, Ga.).
We studied the accuracy of the RTS, and it was noted that an offset or calibration of the Automatic Target Recognition (ATR) was needed. When a measurement using ATR is initiated, the offsets from the reticule to true target center are computed and applied to the measurements. This resulted in angles measured to the accuracy specifications of the instrument. This accuracy was a function of the distance-measuring mode. Regardless of the error, it can be said that any error would be systematic across the entire topographic site typically resulting in less than 2 foot arc or 0.004 m at 400 m distance.
Profile of a BeachUniversity Beach is located adjacent to TAMU-CC along the south shore of Corpus Christi Bay. The beach serves not only as a shoreline restoration project but also as a research field station for studying environmental issues, in particular, coastal processes. A primary component of the proposed research is the comparison of the observed beach evolution with predicted numerical models. The analysis based on our survey with RTS and subsequent surveys will assist in determining if the beach is functioning as anticipated. Therefore, an accurate baseline measurement of topographic and bathymetric features was initiated the week following the completion of beach fill placement and construction of the associated coastal structures.
The baseline data set consists of two individual surveys. The purpose of the first survey is to document the position of the shoreline (water/beach interface). The purpose of the second survey is to document elevations along 20 cross sections of the beach and nearshore region. Future shoreline positions and elevations along cross-sectional reaches can then be measured and compared to this initial data to determine how the beach is evolving (eroding or accreting) in response to wave and current energy. Of particular interest is how the beach will respond to waves that are diffracted at the end of each groin and breakwater.
Shoreline elevation and horizontal positions were measured at 5-25 ft intervals varied to capture shoreline features such as salients (accretionary features that develop in the lee of breakwaters) along the 1,200 ft length of the shoreline. Measurements along the cross-sectional reaches were taken at 5-10 ft intervals to capture nearshore features such as sand bars. The survey area was broken into 20 cross-sectional transects including 12 located within the confines of the coastal structures and an additional four transects to the east and west of the beach.
The purpose of these surveys was to document the morphology of the beach and nearshore. The survey data will define the location and elevation of features such as the beach, sand bars, and salients for comparison with future surveys. Each profile transect incorporates an average of 100 data points.
The data collected during these and subsequent surveys will allow scientists to determine how the dry beach and the submerged portion of the beach change over time. In addition, this data will be applied to calculate volumetric change of the beach and assist in verification of the predicted re-nourishment (placement of additional fill) schedule. Both complete survey sets included the collection of sediment samples (positions manually recorded during the survey) that were analyzed to determine trends in sediment size distribution. This data will assist in determining migration patterns of the sand fill over time.
Evolution in TechnologyIn the beginning of the project, the beach was surveyed once a month and after major storm events. The years following will include surveys conducted seasonally and after major storm events.
Cross-sectional transects for the beach fill project are located at 100 ft intervals along the length of the project. Four of these are long-term profiles and have been monitored yearly for the last five years. These long-term profiles are located at approximately 500 ft intervals for the full length (1.5 mile) of the island and were used to determine the best location for the beach fill project. Additional new transects are located outside of the beach fill project and are placed at 150-200 ft intervals and used to monitor the 500 ft region on each side of the fill area.
Previously, the beach was surveyed with a conventional total station and data collector. The instrument was set up on a bench mark for each beach profile and the reflector had to be centered manually for each position measured. The instrument person was responsible for keeping the rodperson online. After each profile was completed the instrument was moved to the next profile location.
Currently, 13 profiles are measured from two instrument locations. Tripods are set up at each profile location, one high on the curb alongside the road, the other set low near the shoreline.
When a conventional total station was used previously, each beach profile took approximately 80 minutes to complete. The entire survey of six profiles took eight hours. In contrast, using rapid RTS surveying, each profile took approximately 34 minutes. We were able to measure 14 profiles, with an average density of 100 data points in eight hours.
During this project, a measurement was only taken after the rodperson signaled that he was on the bottom and level. In future projects, a survey sled will be used in continuous contact with the bottom. The sled will be towed by boat and will enable the robotic total station to be used in continuous mode. This will allow us to record the sled’s location at a set time interval or a minimum distance of movement interval, minimizing the time needed for the rodperson to move and set up between measurements.
RTS BenefitsEnvironmental observations, in particular those related to coastal processes, represent “snapshots” of information that must be collected quickly to accurately represent the conditions. In this application, the RTS technology allowed a substantial increase in data density and in the number of transects that was possible to collect during a one-day survey period. In the future, RTS will make “event” monitoring during storms more practical and economical as more data can be obtained more quickly and by a smaller crew.
The RTS technology used on the TAMU-CC project decreased the time needed to complete the surveying process by a ratio of 3:5 compared to conventional methods. Rapid surveying with RTS is more accurate and the measurements are more homogeneous because the observer does not fatigue. The process of collecting the data using RTS and conventional equipment generated different sequences. RTS was more automatic with little human error. The conventional instrument method required more training on operation procedures for the students, or multiple students to complete the survey and consistent procedures to generate optimal results. Overall, the benefits of using RTS generated problem-free measurement solutions based on removal of human error of targeting, and increased the speed of the data collection process.