NavList:

Paul Dolkas, in a comment in the thread about anomalous dip, you wrote:
"The reason I am asking is that I have always wondered about the practicality of an electronic sextant using a very precise solid state inclinometer, rather than having to manually align the celestial target with the horizon or some surrogate (e.g. bubble). If the local gravity field is that “lumpy”, then this would limit its accuracy."

Aha, but that is exactly what a chart like this is for. You calculate your position using extremely accurate celestial observations, presumably using some solid-state or inertial system to get the local vertical, then you take that position and do a lookup in a map like this "global map of celestial DOV" that I posted earlier or an equivalent dataset and you read out the correction. It's no worse that correcting an altitude for refraction by looking up the observed altitude. The deflection of the vertical tells us exactly how far we have to move our celestial fix. I prepared this latest map, not for practical use, but to show those regions where we might need to consider such corrections. For example, sailing from almost anywhere on the east coast of the US into the western Atlantic (outside the tropics), it's clear from the map that the deflection of the vertical is less than a couple of tenths of a minute of arc except in two areas: the halo around Bermuda, which I've already described, where the correction is as large as three-quarters of a nautical mile, and also while crossing the edge of the continental shelf where the correction is sometimes as large as half a mile. If we actually wanted to use this data in practice, we would also need a map showing the compass direction at each point in which the correction should be applied. For ocean islands, it's not usually difficult: the correction is towards the contours around the island (the celestial fix would place us three-quarters of a mile too far out to sea). Alternatively, we could carry maps/datasets of xi and eta, which are the traditional displacements of the vertical in the N/S and E/W directions respectively. The DOV values in the maps I've prepared in the past couple of days are just the square root of the sum of the squares of xi and eta at each point.

Note that these maps were generated from the EGM2008 dataset which incorporated some data from recent satellite missions, but there is more in the pipeline.

-FER

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