| Analysis and Software
Survey Data Analysis
The software used to process data collected with the LARC survey system is a
custom-built FORTRAN routine that combines the GPS and echosounder data in order to remove
the motion of the LARC due to the waves and the changing tide level. The survey software
also adjusts the measurements to the changes in speed of sound over the survey area.
The final product is a data file with horizontal coordinates in the State-Plane
system, and a Z coordinate relative to the local vertical datum.
The analysis software accomplishes a number of tasks:
- Separate and clean up GPS and echosounder data streams;
- Adjust echosounder data for the speed of sound profile;
- Correct the echosounder data for data collection clock drift;
- Separate topographic (over land) from bathymetric (in water) points;
- Best-match the GPS and echosounder data in time;
- Interpolate an echosounder reading for each GPS point;
- Combine the echosounder and GPS data
- Clean and smooth the final elevation data;
- Produce X, Y, Z file.
To the right is an example of the raw data which is collected from the GPS receiver
(top), which determines the elevation of the LARC relative to a standard elevation, and
the fathometer (bottom), which uses sound pulses to map the ocean bottom.
The GPS reading is sampled at 1 Hz and the fathometer is sampled at 10 Hz.
Note the influence of the waves on the elevation readings. The objective of this process
is to interpolate an echosounder reading at the exact moment that a GPS X,Y,Z position is acquired.
The first step in the process is to separate the GPS and echosounder data and to filter out spurious
echosounder data caused by bubbles in the surf zone or from the echosounder transducer coming out of
the water in larger waves. This filtering is shown to the right. Original data are shown as blue
circles and the final, edited data are shown as the solid black line.
The Echosounder data are then adjusted for the speed of sound using either a constant
value or the most recent sound speed profile.
Speed of sound data is gathered by casting a CTD at multiple locations
throughout the survey area since the fathometer readings are dependent upon the
speed of sound which can be affected by different water conditions.
In order to remove the wave motion, it is imperative that both the GPS and echosounder data
be accurately time-stamped. The GPS data stream is precisely stamped with the GPS clock time.
However, the echosounder data are stamped with the real-time clock of the collection PC.
Because PC clocks drift (for our computer it can be 0.25 sec in 10 min), the drift must be
removed. Since GPS points are stamped by both GPS and PC clocks, the drift of the PC clock
is computed and used to retime the echosounder data.
Land points are separated from in water points by first computing the average and standard
deviation for all GPS elevations collected when fathometer data indicated that the LARC was
floating. The elevation of above water points are based on the height above the ground of
the GPS antenna on the LARC.
Most critical to the process is correct alignment, in time, of the echosounder and GPS data.
This is accomplished by dynamically adjusting the time (or "latency") of the GPS data,
relative to the echosounder data, until a best fit is obtained. As can be seen on the figure
to the right, the computation of the latency is quite sensitive, with hundredths of a second having
an impact. The green line with a latency of 1.36 sec has the least remaining wave action and best fit.
A segment of properly aligned GPS and echosounder data are shown to the left.
Once aligned, the echosounder points nearest to each GPS point are used to interpolate a
depth coincident with the GPS point, and a final elevation is computed.
Since not all LARC motion is removed by this process, the final data can be optionally
smoothed with a 3-pt median smooth, a moving average, or a combination of the two.
Individual cross-sections may receive additional manual editing to remove questionable data
points which survived the automatic checks. A sample resulting final cross-section plot is
The final, processed data from the LARC survey system.