I often use a digital or dial caliper and a calculator as much as scale rule. This method is really handy for scaling from both plans and pictures. The nice thing is it works no matter what the scale is and you don't need to know what scale the plans or the photo is. The only thing you need to know is one dimension of the object on the plans or photo.
Let's say I have a photo of an engine and I want to know where to put details on my HO engine based on that photo. I have no idea what scale the scale of the photo is. Or I have plans of an engine that were printed off the internet. Because of the variations in the image size and how my printer printed the image, I can't be sure of the scale of the plans. No problem.
I know the engine had truck wheelbase of 9 ft. Since I will be working in inches, I convert that to inches, 108 inches. In HO scale that dimension is 108/87.1 = 1.24 in. I measure the truck wheelbase using the dial calipers and it is 0.95 in. I divide the HO dimension by the photo dimension: 1.24/0.95 = 1.31. 1.31 is my conversion factor. Any dimension I measure on the photo or plan, I multiply by 1.31 to get the HO dimension in inches. The fuel tank measures 1.45 in long on the photo. 1.31 x 1.45 = 1.90 in. The fuel tank on my HO model will be 1.90 in long. The grab irons on the photo/plan are spaced .22 in apart, 1.31 x 0.22 = 0.29, the grab irons on the HO model will be 0.29 in apart.
If you are modeling in a different scale, just substitute that scale back when figuring out the conversion factor. The truck wheel base is 108 inches, N scale is 1/160. 108/160 = 0.675. The wheelbase on the photo measures 0.95. 0.675/0.95 = 0.71 conversion factor. The fuel tank on the photo measures 1.45 in long, 1.45 x 0.71 = 1.03 in long on my N scale model.
Let's say I have a photo of an engine and I want to know where to put details on my HO engine based on that photo. I have no idea what scale the scale of the photo is. Or I have plans of an engine that were printed off the internet. Because of the variations in the image size and how my printer printed the image, I can't be sure of the scale of the plans. No problem.
I know the engine had truck wheelbase of 9 ft. Since I will be working in inches, I convert that to inches, 108 inches. In HO scale that dimension is 108/87.1 = 1.24 in. I measure the truck wheelbase using the dial calipers and it is 0.95 in. I divide the HO dimension by the photo dimension: 1.24/0.95 = 1.31. 1.31 is my conversion factor. Any dimension I measure on the photo or plan, I multiply by 1.31 to get the HO dimension in inches. The fuel tank measures 1.45 in long on the photo. 1.31 x 1.45 = 1.90 in. The fuel tank on my HO model will be 1.90 in long. The grab irons on the photo/plan are spaced .22 in apart, 1.31 x 0.22 = 0.29, the grab irons on the HO model will be 0.29 in apart.
If you are modeling in a different scale, just substitute that scale back when figuring out the conversion factor. The truck wheel base is 108 inches, N scale is 1/160. 108/160 = 0.675. The wheelbase on the photo measures 0.95. 0.675/0.95 = 0.71 conversion factor. The fuel tank on the photo measures 1.45 in long, 1.45 x 0.71 = 1.03 in long on my N scale model.
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