Philips AP navigator GPS coordinates error

[Sjk1000]

Hi all,
We have an admittedly ancient Philips AP Navigator that was on the boat when we bought it. We simply use it for GPS ar the nav table, although it looks like it was a pretty competent anchor alarm and waypoint manager in its day. It has an error in its GPS coords that place us about 400m SE of our actual location. It's not a massive error so it's still usable for offshore. Any idea how to correct this? The settings show that it's set to Datum 36 which could be the issue, but the max value is 49.
Thanks, Steve

 

[pvb]

36 is the 1935 OS Survey of Britain! You need to set it to 01 which is the default WGS84 setting.

 

[AntarcticPilot]

36 is the 1935 OS Survey of Britain! You need to set it to 01 which is the default WGS84 setting.

+1. This is almost certainly the problem. Incidentally, even if used with an oldish chart that is on the OSGB36 datum, you'd probably find offsets in places. That's because OSGB36 is a survey based datum. That means that the OS took triangulation data from a vast array of trig points, and then computed the position of each survey point. In 1936, this took over a year using electromechanical desk calculators! The result is an array of points whose position is defined WRT the rest of the network. However, it is NOT an ellipsoidal datum, which is the only kind that a GPS, especially an elderly GPS, can use. The difference between the survey position and the position on the equivalent ellipsoidal datum (which is what the GPS would provide) can vary by 100m or so. There are also a few (very rare!) survey point where the position computed from one direction differs from that computed from another. You're unlikely to find them in a chart, though!