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I did convert all of the Te Araroa Hiking Maps from PDFs to JPEGs so far. The original maps are 16-64MB PDFs, which take forever to load on my Android Phone. I converted them using Ghostscript and Imagemagic to jpgs similar as seen here with the following settings:

convert.exe -density 200 05_v32_Manawatu_63_69.pdf %03d.jpg

This gives me jpgs up to 4MB, which look quite nice on my phone. I'm wondering if I could make them smaller. I tried pngs, but somehow all the white borders around the maps are black on my maps.

I'm no image expert. Are there other image formats that would be suitable, like a reduced color gif? If you want to give it a try, the smallest set of Te Araroa maps are the Manawatu maps with 16MB, which can be found here.

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maybe webm (google's own format) – cybernard Dec 25 '13 at 4:35
You can reduce the JPG quality by using the -quality option with a value from 1–100, where 1 is the lowest. The default would be 92, so there's a lot to improve in terms of file size. You can also try using GIFs with a reduced color palette of, say, 8 colors and dithering to smooth out color transitions. But that'll just reduce the readability and meaning of the colors on the map, which may not be suitable for your purpose. – slhck Dec 25 '13 at 11:56
up vote 0 down vote accepted

I went with reduction of quality to 80 and dpi reduced to 200: -density 200 -quality 80 Worked out very well. You can find my convert scripts on GitHub

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