determining dimensions from photographs

Hey, how are you doing? One of the guys (Don Wertz?) at the Tractioneers was talking about that subject and one thing that he said stuck in my mind. Most subjects have items of standard dimensions, especially doors. Using one of them as a base, scale the rest off of that.

thanks Dave. from my days of schooling for a draftsman I remember studying perspective drawings and how to create a perspective drawing of a building of known dimensions. so i should be able to "reverse engineer" it. basically the completed perspective drawing is the photograph. then by reverse engineering it by using the methods used to create a perspective drawing such as construction lines, vanishing points, horizon line, ground line etc......... I can determine the actual size dimensions of the structure. I will still need to get the scale correct and this is were Don Wertz was correct. I will need to find one object in the photograph that I can estimate is true size. in this case the height of a door can be fairly accurately determined.

As Dave mentioned, if you can figure out a dimension of one object on the building, it's pretty easy to scale from there by using the scale conversion formulas.  The drawings for this model barn were scaled off of a prototype photo and is actually pretty close to correct although I mistakenly centered the side windows when they should be below center with their tops at the same height as the front windows.  I used the approximate height of a horse standing next to the prototype in my photo as my scale reference so in my case I considered it to be close enough vs exact.  I had the rough drawings with dimensions put together in an evening.  You could get it a lot closer to the true size if you know a dimension of part of the building or even an object nearby.  

My first thought was rail code if you could see the rails.

I would guess that was very consistent and easy to find out about.

If you are going to take the photos,  make a stick 6 Ft long and maybe mark at one ft intervals.    Get fancy and paint it white first - use maybe a 1x1 inch.   

You prop it next to the building up close and then take your photo.     You can use the dimension off the stick to very closely estimate other dimensions in the photo.    I think this idea was in that article you mention, but I can't remember where the article is.

The other option of course is know dimensions - generally people doors.     these give you good estimates also.   and if no doors, perhaps a man in the photo, figure maybe 6 fit tall roughly.      Of course he would have to be next to the building to be a good measure.

with prospective using the size of known objects to determine the size of other objects can be deceiving.  A parallax photo taken with parallax camera corrects certain  measuring issues with perspective.  

The point is that if you know an objects dimensions such as a door width; that distance must be reduced along the length of another object away from the perspective and increased as you move closer. It's subtle determination that is helped if there is another similar known object dimension further way in the perspective.

I’ve sometimes done this for preparing preliminary studies for work procedures on old structures. It works best on buildings and masonry structures like bridges, because you can reliably expect standard dimensions to recur. 

I scratch build most of my structures and I usually use the doors as a guide, most are three feet wide and 6' 7" high. If they are not, then they are easy to get to for a good measurement. Enclosed are a few  models I have scaled from photos.

ANother method is counting the rows of bricks, or the boards on a structure.  A brick is roughly 7x3.5.  boards run in many sizes, and easily found on the net.  The rows of either will give you very ACCURATE measurements...If it is stucco or cement, you have no choice, and must use windows and doors.  Windows are generally three feet above the connection of the foundation and the floor... But that can vary depending on the structure'... 

By the way, you can snap dimensions in Google  Maps if you zoom in all the way in satellite view, right click and from the drop down select the last item- measure distance. Once you know the plan size, you can scale the elevations.

There is another layer to this. Most of what is constructed for a layout has to be scaled down due to its actual size. I use all the tricks described above to construct house models and then have to come up with the next twist of how to make them fit into the scale of the buildings on the layout. 

Since I make models for various customers, I always tell them the actual calculated size before I go ahead and build it. 

I measure off the photo with a ruler and taking into account the size of windows and doors I can get 1:48 scaling. If you have the proportions then scaling is not hard.