Draw matching 1:1 real life lengths on both screen and printing

I wonder what the best approach would be create a drawing with dimensions matching 1:1 with reality. What I mean is that if I draw two lines at a distance of 1 inch from each other, then you could take a ruler and measure exactly 1 inch on the screen where you display them, and/or the printer where you print the drawing.

I'm trying specifically to do this in iOS, and keep it device independent so that the drawing will always match 1:1 whether if displayed on a iPod or iPad and so on.

So far the only solution I came up just takes in account the diagonal resolution of each device (163ppi diagonal on iPod and 132ppi diagonal on iPad), but a test drawing 10 lines spaced at 1 inch from each ot开发者_开发百科her reveals that on the iPad screen 1 inch is a little it short then the real life one.

ideas or suggestions are greatly welcomed.

You have to detect the device, then know the device's PPI (pixels per inch). Then create your artwork to match.