Gnomonic chart projection

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Gnomonic Chart Projection

In nearly all cases in practical chartwork, we will work with a Mercator projection Chart There is other chart projection in use but these are used predominantly for Practical Navigation and Specialist applications not Practical Chartwork Gnomonic Charts of the Polar Regions need not concern us

Likewise Ocean Charts drawn on the Gnomonic projection cannot be used for measuring course and distance, and are only of use in plotting Great Circle tracks which is beyond the scope off this course therefore of no practical value to chartwork  The only application for a Gnomonic projection that may interest us is its use on very large-scale charts such as harbours and pilotage charts These Charts are of such large scale that the distortion is to small to be off significance

As we have already discussed, when using a Mercator projection there will be distortion causing the East West scale to expand with increasing latitude This distortion cannot continue  to infinitude and once we reach the high latitudes where the meridians of longitude converge towards a single pole we have to look for a more suitable method One solution is the Gnomonic projection

Unlike the Mercator chart the Gnomonic chart is a geometrical projection It can be likened to a spider’s web with the polar convergence of the meridians at the centre and the meridians themselves radiating out like the spokes of a wheel The parallels of latitude are drawn as  small circles radiating out from the Poles in decreasing degrees towards the equator The Poles are given an identifier or name if you like of 90 degrees North or South

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