Finding image resolution in Adobe Illustrator

We often get asked about the resolution of spatial images and the affects of scaling them in Adobe Illustrator CS4.

When scaling an image in Adobe Illustrator (using the Scale tool or the MAPublisher Register Image function) it will not resample the image (the image still has the same number of pixels). So effectively scaling down an image increases the image PPI (pixels per inch). In terms of geographic image resolution (size of a pixel in world unit), it does not change. The image resolution is dependent on the source of collection.

To view the PPI of an image, make sure the Adobe Illustrator Control tool bar is enabled (Window menu > Control).

If an image is reduced by 50% then the PPI is multiplied by 2. The placed image below shows a PPI of 72.

After scaling the image to 50%, the PPI increases to 144. When you use MAPublisher Register Image, it will always perform a uniform scaling (same horizontal and vertical percentage). Your PPI will vary depending on the scale and size of your artboard.

How does this affect your workflow? It is important to consider the medium of final distribution for the image so that you can determine an image resolution to use. As a general guide:

• Commercial printing - requires high resolution of 150 to 300 ppi (or more), depending on the press (dpi) and screen frequency (lpi) you’re using.
• Desktop printing - requires images within the range of 72 ppi (for photographs printed on a 300 ppi printer) to 150 ppi (for photographs printed on devices up to 1000 ppi).
• Web publishing - Approximately 96 ppi for Windows-based images and 72 ppi for Mac-based images. Image size varies, depending on whether you want a user to zoom into an image to see more detail. But generally, online publishing generally requires images with pixel dimensions that fit most monitors.