Reprojecting and merging maps with GIS

[woodb3kmaster]

As I mentioned in my current WIP thread, I'm having trouble turning my maps of individual continents into a single world map. Part of the trouble is, arguably, my own making: I used different projections and parameters for each map, in order to minimize distortion on the individual maps. I've tried using ReprojectImage to convert unlabeled versions of my maps to equirectangular, but due to its limited options vis-à-vis projection parameters, I've so far been unable to avoid introducing distortion compared to my original equirectangular sketches of each continent. It appears that the only way to match the original sketches as closely as possible is to use GIS software, hence the thread title - and my need for advice.

I have limited experience with QGIS and MicroDEM from unrelated hobbies, but not in the area of converting between different projections. An earlier experiment in georeferencing a map introduced some unexpected distortion, perhaps due to my choice of datum, or having too many control points. What I've read about ISIS3 makes it look like it might be better suited to the task, but it doesn't run on Windows, so I can't currently run it. Is there an alternative I can try? Or is there a way to properly georeference a map, such that errors and distortion are minimized? What would those of you with experience in GIS do in my position?

[johnvanvliet]

with Qgis the Documentation on the software website is rather good

http://docs.qgis.org/2.14/en/docs/user_manual/
or
http://docs.qgis.org/2.14/en/docs/ge..._introduction/

and tutorials
http://www.qgistutorials.com/en/index.html

A normal work flow would be

1) georeference
http://www.qgistutorials.com/en/docs...ng_basics.html

these
https://www.cartographersguild.com/s...ad.php?t=35724
will import and georeference rather well


2) convert to the same pixels per degree
a full earth map at 8 ppd is 2880x 1440 in simple cylindrical projection

3) merging
-- this is the tricky part
qgis uses gdal and GRASS

you can also use Hugin or nip2 or what i use ISIS3
but isis3 is very specialized software for space missions

[woodb3kmaster]

Thanks, johnvanvliet! Georeferencing turned out to be easier and more accurate this way; apparently I had been using a less precise transformation in my earlier attempts, but the errors became vanishingly small once I switched to Thin Plate Spline. I've now managed to georeference all my heightmaps.

How would you suggest getting all of my maps to the same ppd? I tried doing it in Photoshop but found it harder than I expected, and the composite map I produced still didn't look quite like the one in QGIS (I told the georeferencer to load all the transformed maps into QGIS and saved the resulting project). Is there a utility within QGIS I can use?

EDIT: Figured out how to rescale using GDAL Warp and its -tr switch.