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Equipment | GPS


I am a little embarrased to admit that we have not one, but three GPS receivers on board.

One is a portable Garmin 76S (intended for use when exploring ashore, mapping dive sites by dinghy, in emergencies, etc.) that we also use for input to the 'Bohemian Rhapsody' to view C-map charts and our position - see below. This is a brilliant little unit. Inexpensive, rugged and accurate, with all the basic features you really need.


The second is a Humminbird NS25 receiver that is integrated with a depthsounder (as a backup to the wreckfinder). It has a small chart plotter screen with C-map cartridge capability. This unit has a crude large-scale world chart database, but nothing detailed. It is especially useful for setting depth and drift alarms while at anchor and provides us with our day-to-day positions and crude track while on passage, far from the crunchy bits near shores.


The third is an expensive and elaborate Cetrek chart plotter and GPS receiver system with large monochrome LCD charting screen and C-map capability. This has a seriously hostile user interface (e.g. it takes about 8 button presses to turn on the screen illumination for night use) and I find the LCD contrast rather poor and difficult to read, specially without colour.

Unfortunatley, the C-map cartridges we have are of an older type and do not fit the new Humminbird or Cetrek systems, so they are useless to us now. We do, however, have a CD-ROM with some C-Map charts...

We find it much more convenient to use the Toshiba 'Bohemian Rhapsody' laptop (with its large, colour LCD screen) to view the C-map charts on CD-ROM and plug in the portable Garmin GPS via a USB port to input our position on the chart.

This is a fantastic solution! Bright, clear charts with one's position, heading and speed all integrated directly onto the colour chart! Just remember that the chart datum may have errors considerably larger than the typical GPS error... offsets of 200-400 m are not uncommon, and small, isolated islands may be a mile or more off charted position.

Seen here is a section of our trip round the northern tip of Madagascar, with our position (small cicle with arrow indicating heading), our track (a line of dots), the route (in red) and the chart (in glorious colour). This capability is a great asset for coastal navigation.




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