There's a map on NLS that I'd like to load as a background map, but it's just to big to fit on my screen at a zoom level that's usable.
What's the best way of taking a number of background map images and joining them together?
To complicate matters they aren't "slippy" maps, and span three different sheets (one of them is just the corner of a sheet).
Is this related to your previous topic at:
We need a bit more information, otherwise my reply will turn into a long ramble of ifs and buts.
Which maps and date?
Are you on Windows or Wine/CrossOver/Linux?
Does the map need rotating to get a better fit on the screen?
What model gauge and scale?
The old OS County Series map sheets predate the invention of the OS Grid in 1936. They use a different map projection, and don't align properly with each other on the sheet edges.
For the Georeferenced
slippy maps the NLS resample the scans on the global georeferencing scheme. In the process some straight lines become gently curved, some gently curved lines become straight, and the whole map is rotated slightly. These effects become more noticeable as you move a long way from the OS datum in the middle of the UK.
For the "Find by place"
maps the NLS publish the original single sheet scans unmodified.
The basic process is simple:
0. set your model gauge and scale. Don't forget this bit
1. make the first screenshot. Import it as a picture shape at the correct scaled size. On Windows, Templot can do all this for you easily.
(On Wine it needs some manual calculations.)
2. do the same for the second screenshot.
3. make it transparent.
4. zoom in and manually shift it over the first one into alignment with it.
5. switch off the transparency and zoom out.
6. repeat the process for as many further screenshots as needed.
7. zoom out and use the crop/combine
function to get them all into a single picture shape.
8. if necessary use the twist
function to rotate it to the best screen fit. In Templot this is a very slow process and will add a lot of unwanted white space in the corners. But it will get there in the end on any reasonably specified system, and retain the correct scaling.
9. use the crop/combine
function again to remove the unwanted white space.
10. if desired, copy the image out of the picture shape into your graphics editor. Change the contrast, brightness, grey scale, negative image, or whatever to make a comfortable image for working over (without changing the image size in any way). Copy and paste it back into the picture shape.
11. save the background shapes.
If you can say which maps, etc., I will make some screenshots showing the above.
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