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|Single message by Martin Wynne Topic: Ludgate Hill North|
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Here I have overlaid the 1951 version (transparent) over the 1890s version (white area):
Notice a couple of things.
1. Templot has got them both to the same scale for 4mm/ft. Which may not seem much of an achievement on the face of it, but in fact I'm very pleased that it has worked ok.
The 1890s map has been resampled and georeferenced by the NLS for their "slippy" map on the web site, and the scaling maths is based on the latitude and zoom level using the known circumference of the Earth at the Equator.
The 1951 "Find by place" map has not been georeferenced, it is displayed by the NLS exactly as scanned from the original sheet based on the OS National Grid, which uses a different map projection. I spent a long time looking at different maps on the NLS web site to establish scaling factors for use in Templot.
2. Because of the different map projection, in order to get a match it was necessary to twist the 1951 map. The angle varies according to the location. There are tools on the OS web site to perform such conversions, but in practice trial and error is probably just as good for the old maps. I found that this one needed a twist of -1.7 degrees.
In most cases Templot users will be using one or the other, not both, so this angular difference is of no significance. The finished track plan is likely to need shifting and rotating anyway for convenient printing and to match the railway room.
p.s. The transparent option is useful for aligning different screenshots and maps, but it prevents deep zooming when aligning tracks over them, and makes zooming and panning significantly slower. So having got it in the right place, it is better to switch off the transparency for actual use. Just how much difference it makes depends on the graphics capabilities of the system.
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