I am in the process of creating some exhibits for a client using very high resolution aerial photography. The problem is, the sources images are extremely resource intensive, and it slows me down considerably. I would like to create some lower resolution photos of these areas to use for this project.
My question is, what is the minimum resolution I can use without sacrificing any detail? My prints will be at 1" = 100’ (or 1:1200 scale). Should I try to use a theoretical DPI for my scaling, so that assuming a target DPI of, say, 600 means that I should use a resolution of about 2 inches per pixel (100’/600 = 0.167’ = 2")? Or is there a better method for determining it?
It all comes down to the resolution of the printer.
If the printer is a continuous-tone (eg - dye sub) device, then you never need more than scaled output resolution. for example, if you are printing a 1’x1’ print at 300 ppi, there is no need to have an image of greater than 12*300 = 3600x3600 pixel resolution. If the printer is half-tone or diffusion-dithered (eg - inkjet or laser), the actual spatial resolution is significantly lower. I did an experiment one time with an HP color plotter (600 dpi, diffusion-dithered) and found that a 50 ppi image was indistinguishable from a 600 ppi image once printed. This is because each pixel in the image becomes many printer dots on output.
I’m a bit confused over your calculations. 1"= 100’ is not high resolution at all (I realize this is your printed output). Most of the nice detailed photos when a plane flies over a house for commercial photo sales (I work with this type) are about 4 or 5 times that. A 40’ wide house measures 2" wide on the print.
But 600 dpi is very high. I scan in photos at 600 dpi only when they are perfectly focused and I am going to print an 8x10 or larger at glossy magazine quality. 300 is good for most photos, 200 for less critical.
As I recall, the highest resolution satellite photos released to the public, like Google maps, are 15" resolution (15" per pixel). They are a bit better in cities because they use aircraft at 1500 ft, not satellites.
My photo resolution is about 1 cm per pixel. I am trying to determine the minimum resolution I should use to not lose any detail when printed at 1":100’.
There are a couple considerations for determining the best DPI. The first question I would ask is how large is your final output medium going to be. Large posters don’t need as high of resolution because they are going to be viewed from a distance. Rule of thumb, don’t go below 180DPI or over 300DPI, unless you have a computer that can handle it.
These are 11x17 inch alignment sheets for a pipeline. Were my DPI calculations above pretty good? In calculating DPI for an image, is the dot analogous to a pixel? If so, I assume that if I wanted to hit a target of 300 DPI for an image printed at 1" = 100’, each pixel then needs to be 1/3 of a foot, so 4" per pixel resolution. This would drastically reduce my file sizes and definitely speed up my drafting process.
I pulled the trigger and re-processed my imagery for output at 4" pixel size, and the results look great! Size has been reduced by an order of magnitude, also, so things are running much more smoothly.