USACE Field Research Facility
Field Research Facility
Coastal & Hydraulics Laboratory
LARC
Historical Survey Methods & Instruments

The first 10 years of 4 line surveys have been carefully quality controlled and published in two reports including Howd & Birkemeier (1987) "Beach and nearshore survey data: 1981-1984 CERC Field Research Facility" TR CERC-87-9, and Lee and Birkemeier (1993) "Beach and nearshore survey data: 1985-1991 CERC Field Research Facility" TR CERC-93-3. Both reports are available from the U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS. The following discussion is from Lee and Birkemeier (1993).

Instruments used for the Collection of Data

Two basic tools are needed to accomplish accurate surveying and data processing:

  • The Coastal Research Amphibious Buggy (CRAB)
  • and an instrument to determine the CRAB's location and elevation.
    From 1981-1991, two instruments were used:
       - the Zeiss Elta-2s Electronic Survey System and
       - the GeodimeterAuto-Tracking Survey System.
       - the Trimble 4000SE GPS survey system (starting in June 1996).

    Elta-2s Electronic Survey System

    The Zeiss system incorporates in one compact unit a first- order electronic theodolite, distance meter, microprocessor, rechargeable power supply, and an interchangeable solid state memory module. When optically aimed at a reflecting prism on the CRAB, the instrument uses a collimated infrared beam to measure distance and the electronic theodolite to measure both horizontal and vertical angles.

    The microprocessor then uses these measurements plus the coordinates of the instrument and the height of the CRAB to compute X, Y, and Z Cartesian coordinates of the ground point under the CRAB (corrected for earth curvature).

    The Zeiss system required about 10 sec to aim, shoot, and record each survey point. The survey of each profile line took about 40 to 50 min, collecting an average of 36 data points. Because the actual coordinates of each point were displayed, the CRAB could easily be kept on line to within 1.5 m (5 ft) or better through radio communications between the CRAB driver and the instrument operator. Once the survey was complete, the solid state memory was removed from the Zeiss and the data were transferred through a Zeiss interface to a desktop computer.

    A unique feature of the Zeiss system is its ability to accept and record an additional piece of information (up to 7 digits) with every survey point. This feature was used to manually enter the angular tilt of the CRAB, which was measured with two orthogonal tilt meters. Up to 14 deg of tilt were recorded on steep portions of the beach. Tilt angle less than 3 deg was ignored. The survey data were adjusted during processing to account for the tilt.

    Geodimeter Auto-Tracking Survey System

    From May 1990 to December 1991, an auto-tracking survey system, the Geodimeter 140T, was used to enhance the speed of the surveys The Geodimeter 140T consists of an electronic theodolite, distance meter, tracker, joystick, and cables. The system is designed to track the position of a moving object with the aid of a servo unit, which permits automatic motorized rotation of the instrument in the horizontal and vertical directions.

    The angle and distance measuring unit is both mechanically and electronically connected to a top-mounted tracker unit. The initial aiming of the instrument is controlled by a joystick. Once locked onto the prism array mounted on the CRAB, the Geodimeter 140T continues to follow it. Hence, the CRAB is able to run continuously.

    With the auto-tracking ability of the Geodimeter 140T, it took, on average, just 25 min to survey one profile line. Position information was obtained every 2-3 m along the survey transect. The average number of data points for a survey of a profile line was 236. Therefore, the important advantages of the auto-tracking system were its ability to detect small features that were not observed in the Zeiss surveys, as well as its higher survey speed.

    The data were collected by a PC connected to the system, using the RG7 program (for " Read Geodimeter" ) developed by Mr. Michael W. Leffler. After a survey, the collected data were processed using ISRP.

    Errors due to the tilt of the CRAB were removed using an iterative technique in the analysis. The measured slope was used as a first guess to adjust the data points both horizontally and vertically. A new slope was then determined and a second pass, if required, was made. On a 10-deg slope, the CRAB tilt can result in a +20-cm vertical adjustment to the data.

  • LARC
    Errors in Data-Collection Methods
    Survey errors usually arise from three sources: operational errors, instrument errors, and external errors. Operational errors include operator errors and limitations of the surveying procedure. Instrument errors result from limitations of instruments or devices with which measurements are taken. External errors arise from variations in natural phenomena such as temperature, humidity, wind, and gravity. These errors, along with how they were identified and removed, are discussed below.

    Operational errors
    Operational errors of the Zeiss survey system resulted from improper leveling of the instrument, mis-aiming the instrument at the center of the prism cluster while taking the measurement, an error in positioning the instrument in coordinate space (particularly in elevation), and movement of the tripod during the survey. Improper leveling of the instrument affected all the points measured and was not always easy to detect. Incorrectly aiming the instrument also resulted in an error but only on individual points. This kind of error occurred when the instrument was triggered to begin a reading prior to properly aiming at the prisms. This resulted in sampling the azimuth and zenith angles at the time of triggering. These incorrect angles were then used with the distance to the prisms to calculate the CRAB location. Errors affecting single points were usually easy to detect and remove. Errors from mis-positioning the instrument and from movement of the tripod were eliminated after August 1985 when two permanent fixed-instrument mounts were established on the pier and on the roof of the facility.

    A different type of operational error resulted when topographically important points, such as the bar crest or trough, were missed. Survey points were selected based on the timed travel of the CRAB, with more points taken close to shore where the profile shape is more complex. While it was possible for the Zeiss operator to follow the vertical movement of the CRAB as it moved, small features and some peaks of significant features were sometimes missed or inadequately defined.

    Use of the Geodimeter 140T greatly reduced operational errors. The auto-tracking system eliminated errors caused by missing topographically important points and errors caused by improperly aiming at the prisms.

    Instrument errors
    The Zeiss has not been subject to systematic instrument errors, except for requiring occasional servicing. However, the Geodimeter has a number of idiosyncracies that were not fully understood during the early Geodimeter surveys.

    One of the instrument errors of the Geodimeter results from the separation of the tracking unit and the angle measurement unit. For accurate vertical measurements, the two units must be parallel. Instead of attempting to fine adjust the parallelism, the angular error was computed and applied to the measured vertical angle in the collection software. Unfortunately, the angle can change during a survey, apparently resulting from temperature changes. Although checks of the vertical angle correction were made, they were not always made frequently enough to fully remove the error. The associated vertical error increases with distance from the instrument. The vertical angle change was usually less than 30 sec. Over a distance of 1,000 m, a 30-sec angle error can result in a maximum elevation error of 16 cm.

    Another error associated with the vertical angle correction resulted from the instrument shifting out of level caused by the slight shifting of the pier and the FRF building, probably by differential heating. Some of this movement is automatically compensated for by the Geodimeter and it is designed to stop acquiring data if the instrument tilts outside the range of the compensator. However, this applies only for the horizontal compensation, not for the vertical. Consequently, even if tilted, the instrument continued to collect erroneous vertical data with no indication, except for the bubble level on the instrument being off-center. Because the operator watched a PC screen, and not the instrument, a tilted instrument could go undetected. From the beginning, this error was minimized by sheltering the Geodimeter with an umbrella or by setting it up in a "dome shelter" located on the roof of the FRF building. Frequently sighting and re-sighting a prism of known location was also used as a setup check and for computing corrections. Unfortunately, during the first year of use, some of the out-of-level errors were wrongly corrected for by re-computing the vertical angle correction. This unfortunately added to the error. Once the tilt error was fully understood, a program of frequent level checks was instituted.

    The other instrument error resulted from oscillations of the telescope caused by an improperly adjusted tracker amplitude as it searches to locate the center of the prism. This resulted in jagged data with an oscillatory amplitude of a few centimeters. Although the data appeared to follow the true profile shape somewhat, it was difficult to remove the oscillations since they were not centered about the true profile. A final problem was interference from sunlight, which could overwhelm the tracker power. This problem affected early morning surveys of lines 188 and 190. The problem was eliminated by surveying the northern profile lines, 58 and 62, first.

    External errors
    Electronic survey instruments are sensitive to atmospheric and climatic variation since they use the speed of light to determine distance and optical aiming to measure the angles. The instruments allow for rough adjustment for these variables. During the summer months heat shimmer and the temperature gradients near the land-sea interface may also affect the accuracy of the angular measurements. The authors have no quantitative feel for these sorts of errors other than they are usually negligible relative to the other types of errors. These errors may explain some systematic offshore changes amounting to several centimeters during the summer months that do not appear to be related to the dominant processes.

    An additional, but un-quantified source of error resulted from the thermal expansion and contraction of the CRAB frame and its liquid-filled tires. Although the CRAB's height was routinely measured on land, there was no way to measure its height fully submerged. No adjustment to the data has been made to account for this effect. The variation is suspected to be on the order of 3 to 6 cm (0.1 to 0.2 ft) between summer and winter extremes, but it occurs gradually. This amount of variation, combined with the slight uncertainty of the over 300 different instrument setups, results in a survey noise level that obscures small bed-level changes along the offshore reaches of the profiles.

    Error Identification and Correction

    Example of typical survey errors and their effects

    Examples of typical nearshore survey errors and their impact are shown in the figure above. Errors in the data were most easily recognized through comparison plots. All data were compared to at least the previous survey of the profile. Usually the data were error free. If not, this comparison showed where possible errors occurred. The suspect points were then inspected more carefully. A decision was made as to whether the point or points were in error or represented real changes. The figure above shows the utility of comparison. Questionable data, where no clear error could be discerned, were noted and compared to the next survey as well. This provided a double-check. Errors were also identified using the measured location of the reference prism checked during the instrument setup and throughout the survey.

    Corrections to the data consist of two primary types: deletion of points and subtraction or addition of biases. Data points that were obviously erroneous were removed from the data. These erroneous data points usually resulted from incorrect targeting of the Zeiss to the prism cluster, improper adjustment of the Geodimeter tracker amplitude, or the use of points that were unaccountably, but obviously, wrong. Biases were either constant or gradual (distance dependent) due to improper leveling of the instrument. Vertical errors due to leveling became increasingly evident with increasing distance.

    Most often, constant vertical offsets were the result of either improper stationing of the instrument (setting its elevation incorrectly), or rarely, improper entry of the elevation of the prism cluster on the CRAB. These errors could be traced back to the data through the recorded setup procedure. If there was no evidence of a mistake in stationing the instruments or entry of the prism height, removal of the suspected bias was dependent on three factors. The bias had to extend over the entire profile (past the normal closure point); the bias was restricted to only one or two of the four profile lines; and there had to be no reason to expect evidence of profile activity at depth. For instance, if the measured profile showed significant erosion at a depth below the extreme profile closure depth during a period of below normal wave activity, errors were strongly suspected. However, the bias was removed from the data only after a second survey of the profile confirmed that an error had been made.

    Gradual biases, or rotations of the data, which resulted from calibration or leveling errors rather than stationing errors, were more difficult to discern in the data. If the shift in the data could be directly attributed to a mis-calibration of the internal level compensation, or to a leveling error during or after setup of the instrument, then the rotation needed to correct the data was determined by re-calibration of the level.

    All changes made to the data, with the exception of the tilt adjustment of the CRAB, were recorded in processing and data collection logbooks and were coded into the data file.

    LARC

    Accuracy of the Survey Systems
    The stated range of distance measurement of the Zeiss is 2 km with a triple prism assembly as used on the CRAB. The distance accuracy is 2 cm in the rapid measurement mode. The angle accuracy of the Zeiss is 0.6 sec, of which vertical accuracy is 1.5 cm at 1,000 m. Actual accuracy in a repetitive survey program is less, due to the errors described above. The accuracy of the Zeiss survey system is shown in the figure below which plots 10 repetitive surveys of a profile line collected over a 2-day period under near ideal conditions. While there is movement of the nearshore bar during the period shown, of greater interest is the stability of the offshore zone. Seaward of 220 m, the average range in elevation was 5.0 cm. The standard deviation of the 10 elevations for a given distance was usually less than 2.0 cm.


    Results of 10 repetitive surveys of single profile line using the CRAB/Zeiss system.

    The stated operating range of the Geodimeter is 5.5 km with a single circular prism array. The distance accuracy is 10 mm in the tracking mode. The angle accuracy is 3 sec in the tracking mode and its vertical accuracy is 3.0 cm at 1,000 m. The figure below shows the depth change and the standard deviation of six different offshore distances for four repetitive profile surveys using the CRAB/Geodimeter survey system. The range of elevation changes varies from 0.1 to 3.8 cm indicating that the system accuracy is somewhat better than the Zeiss.


    Results of four repetitive surveys of profile line 135 using the CRAB/Geodimeter system

    One way to estimate the operational accuracy of the surveys is to measure the bottom variation offshore where the bottom is relatively stable. A running mean filter was used to filter out the real sea bottom change from the survey noise. The figure above shows the depth of each survey and the five-consecutive-survey running mean at 800 m offshore. The standard deviation of the residuals, which is the variance of the elevation to the five-survey running mean, is 2.1 cm and 2.7 cm for the Zeiss surveys and the Geodimeter surveys, respectively.


    Depth change at 800 m offshore, five-survey running mean, and their variance on profile line 62