Select the map using the rectangular selection tool from Acrobat.
Copy/paste your selection in an image editing software (GIMP, Photoshop...)
Now's the tricky part: you have to crop the map in order to be exactly aligned with the grid on roll20.
To do this, count the number of full 5ft squares on the map (for example: 52 squares X 37 squares). You then have to crop the excess on the border.
If you do it right, you can import the map on roll20 and slowly stretch it. You should get to a point where the grid drawn on the map and the actual grid from roll20 are aligned, so it's possible to move tokens around with no problem.
Once you get the hang of it, it takes 5-10 minutes or so per map.
I would like to share my files with you, but those maps are not open-licensed.
Bumping this thread, I'm trying to do the same thing but am using a MacBook Pro, does anyone have any tips?
I also use Roll20 to run my current Kingmaker campaign. I use a few "tricks".
One: get the quality of the map to be as sharp as possible (to better see the grid lines).
Two: Turn Roll20's grid lines on to a strong and bright colour (on the strongest opacity in the Page Settings) so that they are easy to see (I personally use the colour red).
Three: Set the Page's width and length to at least 60 squares each, just to give you enough room to stretch that map image without having to constantly widen the page's width and length).
Four: Stretch the map out (make sure you are on the Map Layer setting when you carry over the map image from your Library to the map screen).
Five: Zoom in so that you can clear see a handful of the squares from the map itself. Stretch the map in the directions as needed. I personally strive to get one set of the measurements correct first before moving on to the other. For example, I start off with the height of the squares. I make sure that the top and bottom lines of the map image's grid align with the top and bottom lines of Roll20's grid lines. The trick is to make sure that all (or at least the majority) of the lines matches and I generally go from top of the map to the bottom of the map as I stretch the top of the map first, then the bottom of the map. When stretching, make sure you hold the Alt button; otherwise the map image snaps to the grid lines of the Page.
When I get the top and bottom lines (or horizontal lines, if you will) of the map's grid lines to match those in Roll20, I move to aligning the left and right lines (or vertical lines, if you will) of the map.
Honestly, the first handful of times I tried getting the extracted maps to line up perfectly on Roll20 took me a long, long time. But the more I did it, the better I got at it. Nowadays, I can probably get most maps done in under 12 minutes. The bigger the map, the harder it is.
Sometimes though, some maps are simply going to be impossible to get 100% accuracy because too many liberties were taken with the map illustration to begin with (i.e., the scales are not uniform throughout the entire illustration). In those cases, don't use the grid map of Roll20. Instead, use the grid lines of the map itself and let your players know that instead of relying on the automatic measuring capabilities of Roll20, for that particular map everyone has to use their eyeballs and count the squares, :)
Good luck!
CB out.
I use the maps extracted from the PDFs in 3D Virtual Tabletop. They do require a bit of messing about to get it to align nicely.
Once nice thing with copy-pasting the maps from the PDFs is that it only copies the image layer, not the text overlays, so you get the map without the encounter numbers and other text - makes for a very nice player map.
After that, I load the map up in The Gimp and crop it to a whole number of squares (as Chuckbab describes). Then I count the number of squares the map is across and down, and resize the image to be an exact multiple of that many squares, to ensure that the squares are a whole number of pixels across. If necessary, I don't preserve the aspect ratio, to ensure that the squares are actually square.
After pre-processing the map image like that, it's usually pretty painless to align it with the grid in 3DVT, and would presumably make it easier in other systems like Roll20 too.
I'm doing Skull & Shackles now, and I can never get the maps to line up perfectly with the Roll20 grid. I finally went with a different solution ... I just turn off the Roll20 grid when I import a Paizo map and I tell my players "These squares are five feet." They can usually figure it out from there.
This. Near as I can figure, Paizo's maps utilize squares that are not of a consistent size with each other. In other words, I can get the map on roll 20 set up so that some of the squares align perfectly with the grid, but then other parts of the map will be very misaligned. So I did the same and turned off the grid when I imported the maps.
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This recent blog post may be of interest. Paizo has announced that they're updating their digital maps for ease of use on VTT platforms like Roll20. I don't know if the changes discussed in the blog post will address any of the issues discussed in this thread, but I thought I'd point it out.