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I want to preview a map that I'm generating before sending it off to the (expensive) printer. Unfortunately, the tool used to make it (TileMill) has no concept of paper sizes, DPI or any units. It just lets you specify a certain number of pixels.

So, what's a process (and tools) I could use to view it at close to 1:1 scale of the final process?

I'm thinking something like:

  1. Look up length of desired print size (A2): x1 (59.4cm)
  2. Measure width of screen (it's landscape): x2 (33.3cm)
  3. View the image on screen, zooming to x1/x2 (178.4%).

Is this right, or am I missing something?

And if this right, what tool on OS X can zoom to such precise levels?

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  • If I remember right Tilemill gives raster (pixel) images. Normally, for things designed to be viewed at desk distance, high quality print is 300 PPI (pixels per inch) - you can use this to work out how many pixels your map needs for your paper size. – user568458 Jun 28 '13 at 16:09
  • TileMill apparently also exports svg and pdf: mapbox.com/tilemill/docs/manual/exporting – horatio Jun 28 '13 at 17:42
  • @user568458: yes, I can work out mathematically how many pixels are needed, but that doesn't tell me how big a given feature will be (or let me judge visually whether that's big enough). I think. – Steve Bennett Jun 28 '13 at 23:02
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You want to export the map as SVG or PDF which are Vector formats. These are scalable** without loss of quality. This way you will not need to specify pixel dimensions at all, leaving the rasterization to the Printer and their plate-making RIP.

As to the question of a 1:1 size preview: you merely need to zoom in until you can hold a real-world ruler to the virtual scale bar of your map and have the scale bar be the correct real-world size on screen. Be aware that the print density is going to be higher than the screen pixel density (1.5-3x denser) so you will only get a simulation of the printed item. If you want a true comparison, ask the printer for a test proof. You can save money on a proof by selecting a representative sample section of the finished item exported with the settings you plan on using for the final job. A 5x5 proof will be a lot cheaper than a 50x50

** regarding scaling: if you are using raster images within the vector (e.g. textures), these may not look as good when scaled upwards in size

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  • Ah, thanks - I was indeed missing something: different pixel densities. Unfortunately exporting as SVG or PDF isn't an option as TileMill (well, Mapnik) has several bugs in its export in those formats. – Steve Bennett Jun 28 '13 at 23:00

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