Here are some photo interpretation basics for converting a photo into usable dimensions for scratch building. This technique was used when I was working in Naval Intelligence and interpreting U-2 aerial photography of Cuban missile sites. Stereoscopic imagery was normally used, but for our use, single photos are sufficient.

It is really simple but involves a little math. To find the scale dimension for any height or width of a structure taken from a photograph, first find an object in the photograph that you know the actual 1:1 dimension of. The easiest is a door, which is normally 80” high for residential (or 84" for commercial).

Now measure the actual dimension of the door on the photo (this can be done right from your monitor). With these 2 dimensions you can now come up with a scale factor. Let’s say that the photo dimension of the door is ¼”. Scale factor then = Photo dimension / Actual dimension or:

SF = PD / AD or .25 / 80 = .003125

This scale factor can now be used to find the actual (real life) dimension of ANYTHING in that particular photo. Actual dimension then is: AD = PD / SF

If you want to find the length of the wall that the door is in, measure the length in the photo. Let’s say it measures 7/8”. Actual dimension is then: .875” / .003125 = 280”

All that’s left is converting to your scale by dividing by the scale number. For N scale it is 160. (280” / 160 = 1.75”) So the wall, in N scale, should measure 1 ¾”. For other scales divide by the appropriate scale number (HO = 87).

The most common error when converting dimensions is mixing feet and inches. Keep all your dimensions in inches to make it easier.

Also, a scale factor can only be used on one photo. For another photo you must find another scale factor. If you change resolution, or enlarge photo, you must calculate a new scale factor.

These are just basics and will get you in the ballpark. When you actually draw out your walls or other parts, you can change dimensions to fit an available footprint or modify openings to fit available windows or doors. Just try to stay as prototypical as possible.

NOTE: If you are taking your own photos, take along a yardstick, and place it in your photo. It will make scaling more accurate.

Some of you may already know this, but can be a big help for beginners.