Size Versus Resolution

Resolution of scanned images is measured in Dots Per Inch (DPI). This number informs the user how many pixels, or squares, there are per inch. Higher DPIs translate into better resolution (the ability to see detail on the scanned image) but they also mean larger file sizes. Since DPI describes area, file size grows exponentially with increases in resolution.  

For a 9x9" black and white aerial photograph at a map scale of 1:12,000, the following table gives an idea of the trade-off between file size and resolution. 

 

DPI  (click for sample)	File Size	1 pixel side equals how many inches on the ground
100x100	757 K	120 inches
200x200	3 Meg	60 inches
300x300	6.8 Meg	40 inches
400x400	11.8 Meg	30 inches

 

 

Therefore, a 1:12,000 image scanned at 400 DPI will be 11.8 Megabytes but you should be able to discern items on the photograph which are greater than 30 inches in the real world.  For example, if you were interested in looking at shrubs in a photograph, you would want to scan at a resolution greater than 100 DPI because most shrubs are smaller than 120 inches across.   There is also a trade-off between different image formats and the software that can use them.