I've found one manual solution using Inkscape, am looking around for ways to automate it.
- Open the PDF in Inkscape (I too had a map like yours). Go with the default import settings.
- Menu > Object > Objects . (and not Layers)
- It opens an objects panel. This is just like layers. We can click on the left columns to toggle visibility, lock it, etc.
- There's one item there, but it has an arrow indicating there might be more. I click that, and it expands to show several sub-items.
- As I click on each one, on the image the different objects get selected. On toggling visibility (closing the eye), each object disappears from the image.
- Thus after hiding all the stuff I didn't want, I go to File > Export PNG image. I had to increase the size and DPI to get it to a good resolution.. the default setting have a small thumbnail.
- I now have the map I needed.
I found a command line way of doing this.
inkscape -z -i g2846 -j -D -d 300 test3.pdf -e 3.png
Reference doc: https://inkscape.org/sk/doc/inkscape-man.html
Explaining the parameters:
-z : no gui, run inkscape in command line only
-i g2846 : Selecting the specific group/layer id to export. I got to know this id/label by the above mentioned manual steps in the Inkscape gui.
-j : hide all other layers etc in the export
-D : Keep the export image's dimension same as the whole drawing/doc, and maintain the extracted object's position. (this is important in the event that the original object is rotated/warped and you want the output no the original, or if you're exracting multiple layers and need to maintain their positions on the canvas)
-d 300 : 300 DPI : the default made the output png too lossy, this setting kept it all good at my end.
test3.pdf : my input pdf
-e 3.png : export as PNG, and filename given.
Unfortunately we can only extract one object/layer at a time for now. There is a bug filed for inkscape requesting to allow multiple layers : Allow several -i (--export-id=ID) options.
[EDIT] Another workaround if you want multiple (but not all) layers:
inkscape command shared above to get individual layers out as:
1.png, 2.png, 3.png
Then, run the following command from imagemagick:
$ convert -page +0+0 1.png \
-page +0+0 2.png \
-page +0+0 3.png \
-layers merge +repage merged.png
That should merge the layers together to