Anyone doing WWII U-boat research eventually runs into the problem of having to deal with a grid reference from a KTB and a latitude and longitude from a merchant ship or warship document. The German system was meant to disguise the true latitude and longitude of a given position and required a map with the exact location of the squares and sub-squares which of course wasn’t readily available to the Allies.
Well, there is a new (free) web based WWII German Naval Grid Reference Converter called “Naval Grid Calculator” at: http://grid.nylle.de
that will convert a grid reference like DN8452 (or just DN, or just DN8 etc.) into latitude and longitude and display it on a Google map. It will also convert latitude and longitude (you have multiple choices as to how you enter the latitude and longitude too) into a grid reference and display it on a Google map. Of course the Google map includes zoom, map view and satellite view. It is also handy for just displaying latitude and longitude on a map and you can also download the grid square as a KML for Google Earth.
There are other programs out there but most require you to download and install them on your computer and they are still a work in progress & only cover some of the grid squares. This one covers them all though a very few (11 out of total 536 large grid squares) of the irregular ones haven’t been proven yet and is web based so you don’t have to install anything on your computer. It also provides more information than any of the others I have seen. For example it also gives you the latitude and longitude of the corners of the square as well as the center of the square and its size (height & width etc.) in nautical miles (not all squares are the same size or shape).
Lastly there is considerable documentation on exactly how the WWII German naval grid system works that you won’t find anywhere else that I know of.
I contacted the author, Jan Kockrow, and he is an engineer and a former post war German Navy radio operator and also an avid sailor. Future enhancements might be:
(1) Release of the complete code under an open source license to allow people to build their own grid converter service. E.g. a U-boat community website might want to provide their own grid converter with their own interface so they can use this code and build upon it.
(2) Some additional calculations to be determined.
All in all this is the most comprehensive and accurate grid converter I have found & it is easy to use as well.