I can't recall a particular article outlining the process of obtaining dimensions from a photo, but here is what I do:
- start with photos that are as square to the object to be measured as possible (anything not at a 90 degree angle to the lens is going to involve more complex math - not my hobby)
- find an object shown in the photo that has a known dimension
- precisely measure its visible dimensions - so for example, imagine a broadside of a boxcar. Assume you know the wheel diameter is 33" When you measure the wheel diameter in the photo you find it is 1/4".
- the ratio between the 33" wheel and its 1/4" size in the photo gives you a factor.
- So (assuming everything else in the photo is to the same scale), a 1" long object in the photo is 132" long in real life; i.e. 33" divided by 1/4".
- Looking at it another way, if 1" on the photo = 132 real inches, multiple any measurement on the photo by 132 to get the proto dimension.
Most photos are not precisely square. Reality is that each 33" wheel on the real car may show in the photo as slightly larger or smaller than the others - so for example, one might be .235" diameter, another .24", the one you first measured .25" and the fourth .255". Each serves as the basis to convert other measurements on the drawing. so, for example,
1st wheel is .235" in the photo; that equals 33" in real life, so 1 inch on the photo is equal to 33" divided by .235, or 140.43;
2nd wheel is .24" in the photo; that equals 33" in real life, so 1 inch on the photo is equal to 33" divided by .24, or 137.5;
3rd wheel is .25" in the photo; that equals 33" in real life, so 1 inch on the photo is equal to 33" divided by .25, or 132;
4th wheel is .255" in the photo; that equals 33" in real life, so 1 inch on the photo is equal to 33" divided by .255, or 129.4;
(Assuming I didn't create a typo)
What you might deduce from the different wheel dimensions taken from the photo is that the factor based on a wheel that is closest to the feature you are measuring is going to provide a better basis for your conversion than a factor based on a wheel that is further away.
For long measurements (e.g. side sill length), you might do well to average the factors converted for each wheel and use the average factor.
When I scale a model from photos, I open an excel spread sheet and start to put in scale factors from known objects. I will then use each of those factors (in a separate column) to calculate dimensions for everything in the photo.
Sometimes dimensions using one factor jump out at you. So the width of the side sill with each factor may come out at 9 3/8", 9 13/16", 10" and 10 7/16". I'd use the 10" dimension. Other times you have to make a best guess as none of the conversions results in a sensible dimension.
Another tip - I import the drawing into a drawing program, enlarge it to 400% or more, and draw lines over top of the photo, following the shapes of the parts I am going to measure. My software includes a scale so I can measure the length of the lines I draw. By enlarging the photo, I draw more accurate lines and get more precise measurements off of it.
At some point you'll also start to think about how many decimal places you care about. I always do over-kill and then round, but with a bit of thought I'm sure I could save myself the trouble.
From: "Scott" <repairman87@...>
Sent: Saturday, January 12, 2013 8:02 PM
Subject: [STMFC] Determining car dimensions from photographs.
Hello everybody hope your new year is going well.