Photo interpretation for scratch building

gary60s Dec 31, 2015

  1. gary60s

    gary60s TrainBoard Member

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    If you want to scratch build a structure and have photos but don't have a plan, you can start a plan by taking dimensions from photos.

    Here are some photo interpretation basics for converting a photo into usable dimensions for scratch building. This technique was used when I was working in Naval Intelligence and interpreting aerial photography (don’t ask where). Stereoscopic imagery was normally used, but for our use, single photos are sufficient.

    It is really simple but involves a little math. To find the N scale dimension for any heighth or width of a structure taken from a photograph, first find an object in the photograph that you know the actual 1:1 dimension of. The easiest is a door, which is normally 80” high.

    Now measure the actual dimension of the door on the photo (this can be done right from your monitor). With these 2 dimensions you can now come up with a scale factor. Let’s say that the photo dimension of the door is ¼”. Scale factor then = Photo dimension / Actual dimension or:
    SF = PD / AD or .25 / 80 = .003125

    This scale factor can now be used to find the actual (real life) dimension of ANYTHING in that particular photo. Actual dimension then is: AD = PD / SF

    If you want to find the length of the wall that the door is in, measure the length in the photo. Let’s say it measures 7/8”. Actual dimension is then: .875” / .003125 = 280”

    All that’s left is converting to N scale by dividing by 160. (280” / 160 = 1.75”) So the wall, in N scale, should measure 1 ¾”.

    The most common error when converting dimensions is mixing feet and inches. Keep all your dimensions in inches to make it easier.

    Also, a scale factor can only be used on one photo. For another photo, or different view, you must find another scale factor.

    These are just basics and will get you in the ballpark. When you actually draw out your walls or other parts, you can change dimensions to fit an available footprint or modify openings to fit available windows or doors. Just try to stay as prototypical as possible.

    NOTE: If you are taking your own photos, take along a yardstick, and place it in your photo. It will make scaling more accurate.
     
    Rocket Jones likes this.
  2. r_i_straw

    r_i_straw Staff Member

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    Some times you have to derive different scale factors for different areas of a photo if the view is at an angle or there is a lot of depth of field. The water tower in this photo would have a different scale factor than the tall brick building behind it.
    [​IMG]
     
    Rocket Jones and gary60s like this.
  3. Inkaneer

    Inkaneer TrainBoard Member

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    The best method to scratch build specific structures that you photograph is to use 'story sticks'. Insert the story sticks into the picture and you have a built in calibration. I use two 'story sticks', one is three feet long the other is one foot. I cover the entire stick in white tape then divide the stick lengthwise so that there is a top section then a bottom section. I add alternate sections in red. The top is alternating red/white/red while the corresponding bottom is white/red/white. The longer stick is divided into sections exactly one foot long. The smaller stick is divided in half with one half measuring six inches and the other half divided into 1 inch sections. When I take a picture of a house I place the sticks against the house so they are visible in the picture. Take the photo and the calibration is forever in the picture. The sticks are light (I use thin lattice 1/4" wood) that are easy to carry. Saves a lot of time later on when you are trying to determine the size of that window, door, etc.
     
  4. gary60s

    gary60s TrainBoard Member

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    I guess you missed the Note at the end of my post.
     
  5. Inkaneer

    Inkaneer TrainBoard Member

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    No, I saw it. But the story sticks are much better. A yardstick may work for up close photos but for taking a picture of a side of a building, not so much. In addition, the story sticks with their alternating red/white/red coloration really standout whereas the yardstick does not. I tried the yardstick method before going to the story sticks which were recommended to me by someone I chanced upon meeting while taking photos of a structure. He allowed me to take photos with both my yardstick and his 'story sticks'. Since then I have used the story sticks exclusively.
     
  6. SP_fan_1951

    SP_fan_1951 TrainBoard Member

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    An old-fashioned folding carpenters rule painted in alternating light and dark colors makes a good pocket-sized story stick.
     

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