Pic interpretation basics for scratch building

[gary60s]

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.

[gary60s]

In some photos, where you have a straight on shot of a wall, but other walls
are in perspective (angled view), then scaling will only work on the
straight on view. If possible, another scale factor should be used for walls
in perspective. If a scale factor can't be calculated, then proportioning
can be used.
 
If in the straight on view you have the width of a window, and that SAME
type of window is in the perspective viewed wall, you can determine the
length of that wall using proportioning.

Other tricks such as knowing the actual length of bricks or siding laps, and
then counting bricks or siding laps, can be used to find other dimensions.

[pnolan48]

Gary's got the basics covered really well. I use similar techniques when I have a good profile of a ship, or an overhead view from Google or other map services. In photos, sometimes I know only the length of the ship, for example, so I can scale everything from that length. With Google, I can also measure the actual length of a ship (or a wall, or a footprint of a building). This is invaluable when someone asks me to model a ship where I cannot find actual dimensions, or when I'm just looking for an interesting project to build and have no idea of the ship's (or structure's) name

In either case, it's a little simple math, as Gary explained. With most drawing software, you can set your ruler increments very finely. While 2 mm to the foot is fine for small dimensions in N scale, I also use 1.95 mm to the foot for larger dimension such as the length of a 400' ship. The 2.5% difference becomes significant on large true-scale models. Later versions of Canvas could display that real foot dimension as I drew (i.e. display "400'); I'm not sure Illustrator will do that, but it's easy enough to enter dimension in the control pallette.