Pixels per inch question - will decrease image size increase resolution?

[swiss]

I think this is a dumb question, so bear with me...

Say I am importing a map that is 100 PPI and 30 inches by 30 inches

I want to increase resolution (for printing) to 300 PPI without resampling

If I increase the resolution in Photoshop to 300 PPI, the image size decreases to 10 inches by 10 inches, right? So have I successfully created a 300 PPI image, albeit one 1/3 the size?

[Greg]

The key thing to bear in mind with all this isn't the dpi/ppi, but the pixel resolution of the image. For example, your image would be 3000x3000pxls and that will remain constant no matter what you set the dpi. Yes, at 300dpi, it would be scaled to 10" by 10" and at 100dpi 30"x30", but in actuality it's really the same 3000x3000pxl image.

DPI in itself isn't hugely important, as like you did there, it can easily be changed and is easily reversible. The key thing to remember is that 300dpi is your standard printing resolution, so I always find it's a good idea to set up a new document with this as the default setting. Then when you choose a size, it will adjust it to the 300dpi setting, otherwise, it will always be 1/3 the size you pick for a 100dpi setup.

[waldronate]

You must resize the image if you want to change the PPI while maintaining the overall image dimensions.

100 pixels per inch (PPI) multiplied by 30 inches is 3000 pixels.
300 pixels per inch (PPI) multiplied by 30 inches is 9000 pixels.
100 pixels per inch (PPI) multiplied by 10 inches is 3000 pixels.

[swiss]

So is it fair to say that if an image is less than 300 PPI, you can easily make it 300 PPI, but it will just be a smaller image when printed. Correct?

[Greg]

Yup, that sounds exactly right to me, Swiss.