Scaling from photos

[DrPR]

A question was posted elsewhere about how to determine the dimensions of objects when you don't have scale drawings.

One way is to do what I call "photoguestimation." This involves finding something in a photo of known dimensions and using it to calculate the dimensions of nearby objects. For example, for ages the US Navy used stanchions for life rails and lines that are 43.25 inches tall. In addition, the spacing between railings and lines were almost always 12" or 15". Life rails and lifelines are everywhere on ships.

Lets say you have an object 2" high in a photo and a nearby stanchion is 0.75" tall.

stanchion height/measured height = object height/measured height

OR

43.25/0.75 = X/2

OR

X = 2 x 43.25/0.75 = 115.333 inches

Take this number with a grain of salt. They usually didn't build things to thirds of an inch. 115.33 is 9.61 feet. It was probably really 9.5 feet high.

Keep in mind that you might not be able to see all of the stanchion. US Navy ships typically had 3" high coamings around the edge of superstructure decks. If you are looking at a side view you probably see only 40.25 inches of the stanchion, so use this number for the height.

Always use a size reference close to the object you want to measure. For example, there may be many stanchions in a picture, but more distant stanchions will appear shorter than those closest to the camera. Don't use the same stanchion as reference for many objects in the photo. Always measure the stanchion closest to the object of interest.

Also be cautious about using dimensions obtained from vertical objects (like stanchions) for guestimating horizontal dimensions. Unless the camera angle is directly perpendicular to the plane of the objects and taken at the same elevation as the objects, horizontal and vertical dimensions WILL be at different scales. This is almost always the case. You need to find something of a known width for a horizontal scale reference. Doors, life rings, distance between stanchions, etc., anything will do for a reference if you know the width of the real thing.

You may be able to find dimensions of common objects in plans or on the Internet. For example, dual 5"/38 gun mounts were used on a lot of ships. All versions of these gun houses are all close to the same dimensions, about 180" wide side-to-side and 120 to 124" tall at the rear (most were 121.25").

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Sometimes when visiting ships I carry home made rulers and blue painters' masking tape (doesn't leave adhesive behind). I print the rulers with six alternating black and white 1" squares followed by a 1" x 6" rectangle, to create a 12" ruler. I tape these things to whatever I want to photograph. Then later, when photoguestimating I have a built-in scale reference in the pictures.

In other situations I have carried a tape measure with me and I measured the widest dimension (or circumference) of an object. Then in photos of the object I have a reference dimension.