Eureka! (as they say)
Starting to build a structure and taking measurements off a photo, it all of a sudden struck me that there was a much easier way to do this.
I had planned to measure elements on the photo (actually a laser print of a scanned photo), figure out the scaling factor to HO, then convert all the measurements to scale dimensions, using my scale ruler. The measurements from the print were from an architect's rule, using a finely-divided scale. So what I ended up with was a mind-numbing table of "actual" (from the print) and scale measurements, all in decimals from my calculator that had to be converted, rounded off, etc. Ugh.
But the solution is so simple and beautiful, it's hard to believe I didn't think of it years ago. And I'm sure it's not original with me. Just thought I'd share it here.
Lacking actual illustrations, I'll try to describe it, using these things called, um, words:
This method requires a photo of the thing to be modeled, obviously. (See below for guidelines on this.) The photo should be slightly larger than the scale being modeled, but not too much larger: somewhere between 1.1x and 1.5x or so. It will become obvious why this is so.
On the photo, you need to identify at least one element that you know the actual size of. If one plans ahead, one can actually measure something at the time the photo is taken. In my case, since this is a building, I used one of the doors and assumed it is pretty close to standard height (6'8", or about 80 inches).
The only step involving measurements is this first one to establish the scale; after that, the beauty of this method is that no measurements are needed. No numbers.
Draw two intersecting perpendicular lines. Make sure the vertical line (near the left edge of the sheet) is long enough for the largest element IN THE PICTURE that you want to scale. On an 8-1/2 x 11" sheet, the short edge was just tall enough for my building.
On the vertical line, transfer the size (from the picture) of the object of known size, using a pair of dividers. No need to measure, just carefully transfer the size.
Draw a sloping line from this point on the vertical line to intersect the horizontal line, somewhere near the right edge of the sheet. Doesn't matter exactly where. The longer this line, the easier.
Now you need to draw a second vertical line to the right of the first one. The place you draw this is where the distance between the sloping line and the horizontal line is exactly equal to the SCALE SIZE of the object of known size (80 HO inches in my case). Measure this carefully. You need to use a T-square (or drafting machine, in my case), or use graph paper to make sure that both vertical lines are really perpendicular to the horizontal line.
You have now established the scaling factor. There's no need to measure it, compute ratios, none of that. The drawing does the computation for you!
Now you can put in the sizes of all the features you want to scale on the first, leftmost vertical line. Widths and heights of doors and windows, wall heights, etc. For each mark on the vertical line, draw a sloping line that intersects at the SAME POINT AS THE ORIGINAL SLOPING LINE. The place where this sloping line intersects the SECOND (scale) line is the scale height of the object.
Be sure to identify the objects you're scaling. I marked the endpoints on the left line with letters, then made a list of letters and what they refer to. If two things on the picture are very close to the same size, you can just lump them together to avoid drawing too many lines. (Maybe make a note that the scale size should be bumped up or down a notch.)
The ASCII drawing below shows this scheme (view in fixed-width font):
| | | | | | | | |\ - | - - - size of thing on picture | \ | | \ | | \ | | \ | | \| | \ - - - scale size of thing | |\ | | \ | | \ | | \ | | \ | | \ - all sloping lines intersect here
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You can think of this as the poor man's pantograph ...
NOTES:
The slope of the line in the drawing above is way too steep. Line should have a very shallow slope; this will minimize scaling errors.
The photo you use should be carefully planned. Try to take it as head-on as possible, to minimize parallax errors. In my case, since it is a
4-story building, I'm introducing a "fudge factor" for the size of things near the top, since they're receding away from my vantage point and are therefore larger than they appear.The size of the printed photo is important, as explained above. Basically, you don't want the left line sizes (from the photo) to be too much larger than the resulting scale sizes. Certainly not 2x larger. This will make this manageable on a sheet of ordinary-size (letter/A4) paper.
Have fun!