Small-boat Survey Goes Big

The recent arrival in Bridgetown of the massive Cunard flagship Queen Mary II has caused a minor parking problem. With a length of 345m and a draft of 9.7m she gives cause to investigate the further dredging of an extended docking area for use as an auxiliary haven for smaller cruise ships. A recent hydrographic survey with modern portable hydrographic survey equipment was used to chart depths for the feasibility of extending dredging to the area currently used as a grain terminal.

The survey-company HHF employed for the project is involved in survey work among some twenty islands within the Caribbean. A technical requirement clearly identified early in the development of their business was the need to use truly portable hydrographic survey equipment. Their kit needs to be easily transported by light passenger aircraft to sites in the islands; it also needs to make use of local ships of opportunity rather than transporting a dedicated survey craft to site. The system used for the Bridgetown survey comprised a shallow-draft boat Poco Loco and a portable survey system based on a Bluetooth echo sounder, PocketPC data logger and conventional dual frequency GPS. The advantage of using the portable survey system and the shallow-draft boat was the ability to easily manoeuvre within berth areas, particularly useful as the survey quickly detected areas of dredging missed by the contractor.

Positioning Equipment
Positioning and vertical control for the survey was provided by the dual-frequency Ashtech Z Surveyor equipment used in a static base station and rover configuration. As line of sight and radio communication proved difficult at this port location without moving ships, a post-processed solution was adopted in preference to using RTK positioning. A new Transverse Mercator Projection Grid was developed and used for survey control and mapping in Barbados. This software provides a high degree of rigour in the transformation and calibration process of survey data. During the survey there was a brief experiment with a new sub-metre DGPS using the North American WAAS correction signal; due to the survey location being on the periphery of the coverage footprint, it was found the equipment was more accurate with the correction disabled!

Bluetooth Echo Sounder
The echo sounder used was the recently introduced SonarMite portable device manufactured by Ohmex. The instrument uses portable computer equipment integrated with new measurement technologies by Bluetooth communications. The use of wireless technology enables the instrument to be waterproof and used in a hostile environment while the more sensitive features can be located in a ‘computer-friendly’ environment up to 50m away from the point of measurement. The instrument uses 'smart', low-power, integrated transducer technology, in addition to bottom-tracking algorithms using DSP techniques; the system also outputs a quality value associated with every depth measurement made. The connecting cable to the transducer carries only low DC voltage and digital data, avoiding the EMC problems associated with conventional high-power analogue devices.

PocketPC Data Logger
Survey data was collected using the Trimble Recon device as a data logger, this is a hand-held PocketPC designed for the harsh realities of mobile survey fieldwork. Compact and rugged, it exceeds military specifications for drop, vibration, immersion and operating temperatures. It comes with the standard PocketPC software, giving all the benefits of an industry-standard, open platform for mobile devices. The battery capacity provided the ability to work all day in the field without having to worry about data loss; a spare dry-cell battery pod was carried so that in the event of low power the battery pack could be easily swapped if needed. The survey data was stored on an internal Flash memory that protects data from resets or power loss. The Recon unit was connected to the GPS rover using a standard serial cable, and to the SonarMite echo sounder using a Bluetooth communications card fitted into one of its two Compact Flash slots. The standard D9 serial port fitted to the Recon proved invaluable, as the GPS rover expected to see a RTS/CTS hardwire handshake prior to sending its data, a requirement that causes problems when using equipment supporting only a basic three-wire serial connection.

Transflective Display
The technology used in the Recon display is a combination of ‘transmissive’ and ‘reflective’ LCD technology. With a transmissive device an electronic substrate emits a fluorescent white ‘backlight’ passing through the LCD to enable it to be seen by a viewer; this is the form used by most PC displays, making them ideal for viewing indoors or under low light levels. A reflective display enables light (such as daylight) to pass into the LCD and be reflected back from a passive substrate. Ideal for outdoor use, these devices consume very little power and their visibility performance is in proportion to the available light. By using a combination of these two technologies the new types of computer designed for field use can be used in most light conditions. One flaw observed during the Bridgetown survey was connected to the tendency for the backlight to be either on or off. Whilst following hydro survey lines it was found that at the critical moment of swinging the boat around the display went from sunlight to shade, making it difficult to view the current depth parameter. It was also noted that it was difficult to detect whether or not the backlight was currently on in strong sunlight conditions. Apart from increased power consumption, this also resulted in a ‘flickering’ effect on the display. One suggestion made to the manufacturer was that they consider a simple light sensor to vary the backlight according to ambient light.

SonarMite Software
The software used on the Recon for the survey was the PocketPC version of the SonarMite program. This software performed the basic functions of capturing, displaying and saving data from both the echo sounder and the GPS. All displayed data was colour coded by depth, so both the tracking plot and the echo datagram provide real feedback on depth and survey coverage. Using the interface specified by Microsoft most functions and settings are achieved by a few menu commands or by tapping of the screen: left tap to scroll left, right tap to scroll right etc. One feature used in the software was to aid the typical quality control function of a hydrographic survey in performing a ‘Barcheck’. This routine, intended to check the calibration and performance of the echo sounder, was simplified using the event-marking facilities. The user can raise and lower the barcheck calibration plate as a numeric check, also moving the plate in and out of the echo sounder beam as a functional test using the event marks for later reference. Waypoints can be either loaded from a file of XY coordinates, providing a simple reference frame to help follow survey lines on screen, or as a waypoint event stored at the current location to provide a marker or return reference point.

Conclusions
The survey proved using modern portable equipment has many benefits over earlier-generation equipment. Being connected by Bluetooth gives the advantages of seamless integration between field data collected using PocketPC and processing software running on Windows XP. The results of the Bridgetown survey have proved satisfactory in every way and are currently being reviewed as the basis of the civil engineering works required to extend the harbour. The detection and removal of under-dredged material has avoided potential damage to the propulsion system of a cruise ship and consequent damages.