DPI vs. PPI - What's the difference?

[Carnifex]

72 dpi works as long as the map file created was made at your correct dimension and scaled at 72 dpi when you export the file.

If you convert a higher or lower resolution file to 72 dpi or rescale your map to 20 x 30 when it wasn't that in the first place - then 72 dpi is not what you want - it will look crappy.

Using high resolution like 300 dpi is required if your map file has to be rescaled to fit the dimensions and resolution you ask.

In other words, if you don't have to rescale 72 dpi is fine, if you do have to rescale start with a higher resolution...

I hope that wasn't too complicated?

I really recommend at least 150 dpi for printing and 300 if you want the best quality. It all depends on the quality of the print. In offset printing you should know the resolution of the raster (lpi (lines per inch)) - and you should have lpi x 2 in dpi.

Also scaling down is usually not a problem (say from 600 to 150 dpi).



Here's a little mini-guide to scaling things in Photoshop:
If you want to change only the dpi (to 200 dpi) but keep the pixel size:
Image > Image Size (or alt+ctrl+I)
Make sure "Scale styles" and "constrain proportions" are checked. "Resample image" should NOT be checked.
Type "200" in the resolution box and click OK.

If you want to scale the image and increase both pixel size and resolution (to 200 dpi):
Image > Image Size (or alt+ctrl+I)
Make sure "Scale styles", "constrain proportions" AND "Resample image" are checked.
Type "200" in the resolution box and click OK.

Does it make sense?

Be careful when you increase the number of pixels of a picture - you'll always loose quality.