NavList:

Here's an exercise I throw at folks, after the fundamentals of a current-induced running fix are covered. The beginning of the "art" part of traditional navigation, in other words. In my world an estimated position (EP) is found by applying set/drift to a dead reckoning (DR) position, where set/drift is the total of all external forces on the vessel. Decreasing order of positional accuracy: FIX, R-FIX, EP, DR. Your world may vary, but the problem is the same: are you heading into danger?

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In the following boat log, "A" is a short lighthouse on a small island, where danger is anything within one mile of lighthouse. You want to pass by the north side of the lighthouse. "OK w/EP" means the GPS fix coincides with the EP for that time.

Use Lat:Lon = 1:1 for your plotting sheet. Scale 3/4 inch = 1 mi (3/4 inch is about 2 cm). Find 2330 R-FIX. Need to change course? You have five minutes to decide, after shooting (and logging) the 2330 LOP.

BOAT LOG
2100 C = 000T. S = 6 Kts. Hazy.
(2130 DR N00d 11.0m E000d 09.0m.)
2130 GPS N00d 10.5m E000d 10.0m. OK w/EP.
2200 GPS N00d 13.0m E000d 11.0m. OK w/EP.
2230 GPS N00d 15.0m E000d 12.0m.
2240 GPS died at 2231, won't restart. Power to unit OK.
2258 Land ho. Lighthouse A. Hazy and foggy.
2300 LOP 060T A (N00d 20.0m E000d 17m). Good shot.
2330 LOP 100T A. Good shot.
2330 R-FIX = ????

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Post your answers (and your reasoning, please). No electronic calculators allowed.

Joe

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