Straightening scanned plans and working out their pixel scale

Looks like I do have a bit of time to do one or two more blogs…
 
When you scan a plan from a book, 99.9 times out of a 100 they are not perfectly straight.
Using a paint program it is a relatively simple process to fix this.
It is also a good idea to work out the pixel scale for the image.
This can then be used to get unlisted dimensions from plans by using a pixel tool in a 2D paint program.
First load up your image.
My paint tool (the rather ancient Picture Publisher 8) has a pointer tool that lets me mark out a rectangle on an image without drawing anything.

plan1

 

The status bar shows the start coordinates and the width and height of the box.
 

plan2

Most locomotive drawings show the rail top as a line. Using one end of this line as a start point I draw the rectangle to the other end of the rail top line.
In the example below (red rectangle) it shows I have a height of 25 and a width of 2965.
 
 
I then start up the most useful application in Windows – the calculator. 
Make sure it is switched to Scientific view.
 

plan3

 
Divide the height by the width:
25 / 2965 = 0.0084317032040472175379426644182125
 
Then click Inv and press Tan
Inv Tan  =  0.48308955972084815380076833108458
This is the number of degrees that the image must be rotated by.
 
I then select the rotate by arbitrary amount option in my paint program and enter 0.483 degrees (anti-clockwise)
  

Now we have a horizontal plan, I pick a known dimension from the drawing, In this case the 32′ wheelbase.
 
plan4
This is 1532 pixels wide.
This translates to 1523/32  =  47.59 pixels per foot
or the other way up (32 * 12) / 1523 = 0.2521 inches per pixel
and multiply by 2.54 to get to centimetres per pixel = 0.6404
 
So now, if you measure 10 pixels on the drawing you know it is 10 * 0.2521 = 25.2 inches or 64cm
 
I usually mark this information directly on the drawing that way it never gets lost – provided you remember to save the altered image file.

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