Hey, it’s been a while since I shared some tips for making maps for games!
Over on Pinterest, where I collect image inspirations for role-playing games, I’ve been seeing a lot of lovely vintage floor plans for manors and houses. It’s great material for your Call of Cthulhu or Good Society game. However, they’re not always available at a scale that lends itself to making nice game maps. Here is a simple workaround.
Most electronic image formats like .jpg, .png, and so forth are raster formats where each point in the image is coded as (x, y) coordinates of a very tight grid. These are very good for photos.
When you scale (enlarge) a raster image to a size larger than its original definition, the points are further from one another. The software you are using (e.g., PhotoShop, GNU Image Manipulation Program aka GIMP) does its best to interpolate, fill in between the original points but you still get a blurry image.
Some art software (e.g., Illustrator, CorelDraw, Inkscape) let you create images using vector format instead, such as .svg, where the information is stored as vertex points and the lines that connect them by angle, mathematical formula, characteristics, and distance. These are very good for drawings.
When you rescale vector graphics, you change the distance between two points and the mathematical formulae describing the lines between each connected points are recalculated. The lines remain smooth.
Enlarging scans of blueprints produces noisy, lo-fi images, as you can see above. However, since blueprints are line drawings, they convert very well to vector graphics, which scale very well.
So I could open the original illustration in a vector graphics editor (in my case, Inkscape — free and Open Source), clip each floor plan separately, trace the lines on a new vector layer, then save this as my new drawing, ready to be scaled however I might like.
But wait! This is not the Middle Ages, it’s even simpler to ask the software to trace the raster bitmap for me and turn it into a vector layer! (In Inkscape this means using Path > Trace Bitmap, and pasting this onto a new layer or as a new image.)
Then I can also clear anything I don’t need to show on my map during the game, such as the dimensions and labels.
After scaling cleanly to the dimensions I want to print, I can then furnish my floor plans in a raster graphics editor. Here, I opened the .svg file with GIMP and dropped images from the Game Props Furniture art pack by David Hemenway on Roll20, just to create an example. (The Game Props Furniture 2 pack would be even better for this house.) I did a pretty crappy job of orienting the shadows and I didn’t add floor textures, I just wanted an example I could show here. Interior decoration is the most time-consuming phase of map-making!
You can see each step in full dimensions below:
Original raster image 
Vectorized image scaled up 
Vectorized image cleaned-up 
Scaled-up and cleaned-up floor plan, furnished
The Reef 