200245 Inappropriate Reliance on GPS

Inappropriate Reliance on GPS
Report No. 200245

We were approaching a small port at night, using the newly supplied and corrected British Admiralty chart (published 1847, latest edition 1927) for the passage through the reefs and rocky islets into the lagoon. The intention was to approach the entrance of the buoyed channel where a pilot would board. The 2/O had plotted the position of the buoys and channel limits into the basic radar mapping programme, linked to the GPS. I was aware that he rather resented my insistence on fixing the ship's position primarily using visual lighthouse bearings and radar distances. That was until he saw that the radar map graphics placed the channel clearly across the nearest rocky patch, even when account was taken of the half mile correction to WGS datum.

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Mariners should use GPS in ocean navigation to check the Astronomical position, but in coastal navigation they must be very prudent with the GPS position.