Experience has taught me not to blindly trust drawings prepared by others. You never know quite how accurate they are, which bits were precisely measured and which bits were estimated. There's are also drafting errors that can creep in.
One of the benefits of making a 3D mock up to start with is that you can look at it from various angles to see if it looks right compared to prototype photos. Here I have three good photos of the signal box which cover all sides. You're not always that lucky!
In this case, as I was measuring up the rear wall, something just didn't look right with the rear recesses. The lintels on the drawing scaled about 6" tall, which looked too thin compared to the more beefy version in the photo. Additionally, the blank wall above the recesses looked taller in the photo than on the drawing.
This is where my O Level technical drawing lessons come in useful...
The vanishing point of the rear wall horizontals was established. The base of the building worked out based on the height of the upper storey door being transferred to the locking room door below. The top and bottom of the building were then projected to the right from the vanishing point and the point found where the distance between them was the 111mm (height of wall from the drawing). You can then project all the horizontal lines in the photo onto this vertical line and find out what the heights should be...
The results were that the lintels were indeed too thin and the recesses were drawn too far up the wall. Happily most of the other verticals checked out nicely, which is always reassuring, and confirmed that I had the vanishing point and total height correct.
