NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2026 Jan 4, 17:54 -0800
Colin B, you asked:
"I was curious. Is there any way to practically correct for gravitational deflection of the vertical at sea?"
Yes, in principle, it's no problem. It's just a question of carrying around a global chart. Trouble is, for readability it would be quite large. But 95% of it would be "no correction". One could put together a compressed chart covering just those regions where it matters. That doesn't exist, as far as I am aware, but it could be created without much difficulty.
The attached map is a DOV chart for the tropical southwest Pacific. It's an area about half the size of Australia. You can figure out the location by comparing against a global bathymetry chart (like Google Earth or similar). The colors indicate the size of the DOV correction as given by the key... but not the direction. Apart from a 180° ambiguity (which could be resolved by a mapping convention), the direction is implicit. It's just like reading the direction of steepest slope on a land map showing elevation contours. Up or down slope is always perpendicular to the contours. This DOV map works the same way.
Frank Reed
