Hydrographic Surveying

                           using s/v Mother of Perl

Ben Smith spends his summer months as captain of the hydrographic boats for the Center of Coastal Mapping / Joint Hydrographic Center at the University of New Hampshire. Being a sailor, charts, currents, and tides (as well weather), are of particular interest. Now, with his exposure to how all this data is collected, processed, and plotted, Ben felt the urge to try his own hand at it using Mother of Perl.

The software which CCOM/JHC uses for gathering and processing the data costs tens of thousands of dollars (US), and the equipment (multibeam sonar, position and attitude sensors) costs hundreds of thousands of dollars. Hydrography of this sort was way beyond the financial reach of Ben and Mother of Perl.

Ben's goal was not to produce IHO (International Hydrographic Organization) compliant surveys. The goals were to learn more about hydrography by aquiring and processing data that might be useful to Mother of Perl and similar cruising boats. To that end, Ben has been developing a combination of inexpensive hardware and freely available software along with Perl scripts to simplify and automate the process. He has created a dinghy based bathymetric (depth) survey system, and used Mother of Perl as a platform for collecting relative tidal information.

The Dinghy-Based Survey System

The survey system consists of a Humminbird WAAS-GPS/Fishfinder feeding data to a Persistor-based ASCII data logger. The sensors are mounted on a frame made from PVC-pipe sections that screw together and are lashed to the gunwals of the dinghy. The whole system is powered by a sealed lead-acid battery. The battery and datalogger are enclosed in a waterprooff Pelican case on which is mounted the console for the fishfinder.

The dinghy is a 12-foot RIB with an articulated extension to the tiller which facilitates operating the equipment at the same time as controlling the dinghy.

The datalogger records the NMEA data from the GPS/fishfinder on a memory card which is later read by the laptop computer onboard Mother of Perl

The Shipside (s/v Mother of Perl) Hardware

Sensors

• Garmin GPS
• KVH gyro-compansated fluxgate compass
• B&G wind sensor
• AIRMAR Dopplar speed through water sensor
• B&G fathometer

Sensors combined using Noland NMEA multiplexer.

All serial data is fed to B&B serial to ethernet converter.

All data logged on a hacked Buffalo Linkstation running IP timestamping and datalogging program written in Perl. (For hacking information, see:  this document and this discussion group.)

Data Processing

1. Converting the raw bathymetric data from the dinghy into position (x,y), depth (z), and time (t) strings.
2. Cleaning the xyzt data of extraneous points that either lie outside of the survey area or time, or are of unlikely depths. (Call this data set the clean-xyzt set.) 
3. Converting and cleaning the ship-side data similarly to steps 1 and 2. (Call this data set the clean-unfiltered-tidal set.)
4. Apply low pass filters on clean-unfiltered-tidal set to arrive at a relative-tidal set.
5. Adjust the clean-xyzt set with the relative-tidal set to arrive at an adjusted-xyz set. (At this point, we no longer need to keep track of t, time. It was used to tie the raw bathymetry to tide.)
6. Interpolate this adjusted-xyz over the area of the survey.
7. Plot the interpolated data set, and apply any contextual graphics

Software

The Steps

1. From Raw Bathymetry Log to xyzt file

Each unique preface, e.g. $INRMC, specifies a data source. The fields are delimited by commas. The last field is check-sum for the record/string.

The perl script for this data does two things:

1. extracts the time and position
2. interpolates the time and position to estimate when and where the depth sounding occured

The scripts relies on the NMEAfields perl module, which Ben has developed over the last five years. This module has not yet been published on CPAN site, but is available, in its current state of development from Ben Smith.

(to be continued)