Mapping Question

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Mapping Question

Postby Ashtagon » Wed Nov 04, 2015 12:25 pm

I am trying to generate a series of transformational formulas in Excel to rotate, resize, and otherwise distort map A to something that approximates the projection used in map B, at least close enough that they can then be superimposed on each other and traced by hand to copy the pertinent information. My destination map template is the Robinson projection.

So far, I have a way to calculate first a rotation (measure the XY position on two identifiable points on each map, calculate the angle on each image, and rotate to correct for the difference), and then a resize factor (measure the XY length of a box that covers two identifiable points on each map, calculate the width and height of the box on each image, and resize accordingly; this may involve different vertical and horizontal factors).

However, I have realised that where the map is significantly removed from both the zero latitude or the zero longitude, there is an additional "shearing" distortion. Does anyone know how to calculate a correcting factor for this?
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Re: Mapping Question

Postby Thorf » Thu Nov 05, 2015 12:19 pm

Reprojecting from scratch using mathematical formulae? You're braver than I am. ;)

I would just use a program to reproject and be done with it. G.Projector comes to mind.
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Re: Mapping Question

Postby julius_cleaver » Fri Aug 19, 2016 1:02 am

Well the distortion that occurs the further you get away from your "Origin", anywhere the scale factor is 1:1, is technically known as parallax:

This tends to happen anywhere scale factor moves further away from 1:1, whether the base map or the projection/projected map (in your case new map) is the one that is "smaller" in scale or more distorted.

But Thorf's suggestion is sound...assuming they are equirectangular maps. Are they?
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