Hi Mark,
Great question.
The good news is that many of the plans were scanned at the correct scale. In other words, the scan matches the scale of the original drawing. But not all of them.
It is very possible that different sheets within the same set might have been scanned at different scales. It all depends on how the person or persons who did the original scanning set the scale on the scanner(s) when he or she scanned the original sheet. That person or persons may have changed the setting when scanning different sheets.
The plans are offered as-is, at whatever scale they were originally scanned. This means that each sheet may not necessarily have been scanned at the scale that the original plans were drawn in as stated on the sheet.
Also note that these are scans of original drawings that are in most cases many decades old. Depending on well they were stored over those many decades, the original drawings may be damaged or distorted. The scanning process would capture that distortion, not correct it.
For modeling purposes, each sheet should be checked against any graphic scale that appears on the sheet to determine the scan's actual scale and whether or not they should be rescaled within your own computer to bring them to as true a scale as possible.
Here's one way to do that:For US Navy Booklet of General Plans drawings (BoGP), this can best be done by noting the longitudinal scales that show the "stations". See sample, cropped, closeup scans below.
The length of US ships are often measured using "stations". The images below are closeups of scans that show where stations are located on a typical BoGP drawing, in this case USS
Saratoga CV-3. On BoGP drawings, stations are typically drawn at either 8 feet apart or 4 feet apart. There are always exceptions, but 8 feet and 4 feet are common. We can then use a bit of math and a digital caliper to check to see if our drawing was accurately scanned at the correct scale.
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CV-3 1944 Conning Tower Barbette.jpg [ 116.85 KiB | Viewed 2457 times ]
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CV-3 Uptakes.jpg [ 199.33 KiB | Viewed 2457 times ]
1. For example, let's pick a favorite subject's BoGP drawing, USS
Saratoga CV-3. Let's say that we learned from Wikipedia that the real USS
Saratoga CV-3 was a bit more than 800 feet long. We see that the graphical scale on the full-size scan of the BoGP drawing records the length of the hull with station numbers up to a bit more than 200. 800 feet / 200 stations = 4. That means that the stations are 4 feet apart. We don't need precise measurements yet, we're just trying to determine whether the stations are 8 feet apart or 4 feet apart or some other sensical, round number.
2. US BoGP drawings were commonly, originally drawn in 1/192 scale (1/16 inch = 1 foot) and that's what this drawing states is the original scale, 1/16 inch = 1 foot, in the table at the bottom right corner.
3. Let's pick a usable length of the ship to check, say 20 stations.
4. So, 20 stations at 4 feet apart = 80 feet on the real ship (20 x 4 feet = 80 feet). Next we need to convert 80 feet into a usable 1/192 scale measurement. For scale modeling purposes, let's shoot for measurements in millimeters because the math is easier in millimeters.
5. To get 80 feet in 1/192 scale in millimeters, here's the math: 80' x 12 inches per foot x 25.4 mm per inch divided by our scale of 192 =
127 mm.
6. Using a digital caliper, let's measure 20 stations on the scan of the drawing. If 20 stations measures 127 mm, the scan of the drawing is accurately 1/192 scale.
But let's say that using a digital caliper, 20 stations on the drawing only measures 101.6 mm. No worries, we can correct the scan. Take 127 (the correct number we want) and divide it by 101.6 (the actual number we measured). We get 1.25. That means that the drawing should be enlarged 1.25 times (125%) to be properly scaled to 1/192 scale.
7. Using whatever graphic software you have on your computer or printer, enlarge the scan 125%. Then print it and check it again to make sure 20 stations measures 127 mm.
To check for distortion, check the stations at different points along the graphic scale and make sure they all measure the same 127 mm. In other words, 20 stations at the bow and 20 stations at the stern should both measure 127 mm. If they don't match, the original drawing is distorted. That can be corrected, too, but requires some more mathematical gymnastics.
Once you have the scan at a correct scale, you can resize it to any scale you want.
Hope this helps.