This is meant as a companion guide for DARKNESS' Das Book table and map collection mod. It will teach you how to use your periscope markings to quickly and accurately determine the distance between you and your target, using only methods that would have been available at the time.

Why do you need this, you ask? After all, it is much simpler to just use the stadimeter, or in fact just the map tools. Well, the answer is simple: stadimetric range-finding was not available with German periscopes of that era, much less satellite imaging.

So using these methods requires you to suspend your disbelief immensely. If you're okay with this, more power to you. If you prefer a more historical method, this guide is written for you.

The calculation of the distance is based on trigonometry

At no point during this method do you need to know this formula, but it doesn't hurt being aware where the numbers come from. You could use this formula as-is if you want, but having it in the form of a table is much more convenient.

This method requires you to know the height of the target. The accuracy of the distance measurement is 1:1 linked to how accurate your knowledge of that height is.

In UBOAT, you are equipped with the Identification booklet. Use this to identify the target by its distinguishing features.
These can be the mast count, bow or aft shapes, superstructure position, cranes, ...

Once you have correctly identified the vessel, the identification booklet tells you what you need to know.


The example in this guide will be a NA-1 coastal vessel. As you can see in the booklet, it has a mast height of 20.7 meters.


There is a point to make about the historicity of such an accurate identification booklet, but that is not a discussion for this time. If you refuse to use the ID booklet, you can use an estimated mast height, and the accuracy of your result will be limited by the accuracy of your estimation. For most applications, that will be enough.

The table in question will be the following:



The three areas of this table are

• The top-most row (red). This contains the mast height you base your distance calculation on in meters, and parts the following columns by the magnification levels.
  
• The left-most column (blue). This is in units of the vertical markings of your periscope. It is in units of 1/16° for the realistic periscope setting.
  
• The center. This will be the distance between you and the target in hectometers (units of 100 meters).

The procedure is the following:

• Identify the target, and note its mast height. Here, it's a NA-1 coastal ship with a mast height of 20.7 meter. Find that height, or the one that's closest, in the top most column. Here, that would be the column with the height of 20 meters
  
• Measure the vertical height of the vessel using your markings. In this example, I'm getting to about 28 vertical units using the 6x magnification. Find that value on the left-most column, or the value closest to it. Here, that would be 30 vertical markings
  
• Navigate to the intersection of these two. In our example, that leads us to a distance of 24 hectometers (2.4 kilometers) before interpolation.
  
• Interpolate to the actual distance. An educated guess (a little more, a little less) will do. Let's say between 2.5 km and 2.8 km and call it a day.
  A potentially more accurate interpolation could look like this: 10 vertical marks are 730 m. That means 2 vertical marks are about 150 meters. Add that to our value from the intersection, and add 5% from the difference in mast height, and you arrive at 2.67 km.

For testing purposes, that specific example was done with map contacts enabled. So let's use the map tools to check whether this table is of any use.

Here is a screenshot of the same moment when the measurement using the periscope was carried out:



As you can see, the actual distance appears to be 2.64 km. Let's go with the result from our accurate interpolation, which was 2.67 km. That is a difference of only 30 meters, or about 1% in relative terms. Isn't that amazing?

Of course, this is an example under perfect conditions, during which I paused and took the time to align the periscope. Also, your interpolation might have been anywhere between 2.5 and 2.8 kilometers, that is an error range of about 5%. Expect the result in a more real scenario to be in about that range.

This guide showed you how to accurately measure the distance to your target using the periscope markings, and the height of the target. Use this measurement in your firing solution, or to track the path of your target.

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3419126557

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