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Some of you may already know this, but for those that don't it may be useful.

Here is 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 aerial photography of Soviet missiles in Cuba. 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 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. 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 chosen scale by dividing by your scale ratio. For N its 160. (280” / 160 = 1.75”) So the wall, in N scale, should measure 1 ¾”. For HO its 87.(280" / 87 = 3.218").

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.

Another trick, if you have nothing to compare, is to place a yardstick in your proposed photo.
Gary, good idea to share this good old trick!

I use this technic quite often when I'm doing architectural survey with very little information on hand or no possibilities to visit the site myself. If done correctly, it can achieve quite precise results.

Standard materials can also give us relevant information: brick size is habitually standard, the same with clapboard and other things like window mullions, fascia, etc. Also, most building follow mathematical patterns. Most of the time, dimensions are increment of 2", 3", 4" and any combinaison of them. That is true for modern buildings, that is true for older ones too.

About the doors, most residential doors are 80", but for public buildings, most of the time they are 84". It may be tricky, just make sure you got the right size before starting the complete process.

I also found out even distorted pictures with too much perspective in them can be useful. It only means each sector of the picture will have it's own scale factor. Not the best, but may help to get some more clues.

The best thing is still to take pictures with little perspective, makes the process so much easier to figure out.

Personnaly, I like to draw my scale drawings in millimeter to save ardous conversion process and mistakes. Make things much easier when building the structure than using 1/64, 1/32 and other cryptic increment (don't misunderstand me, I prefer using feet-inch in 1/1). Good old 3.5mm = 1 feet is your friend!

Excellent added information Matt!