Map of Kashubians

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The same map but in worse quality (black-and-white) can be found here on page 122:

http://docplayer.pl/57273906-Instytut-kaszubski-acta-cassubiana-tom-xvii.html

Kartuzy is called "the capital of Kashubia" and Wejherowo-Reda-Rumia is the "Kashubian Tri-City":

https://en.wikipedia.org/wiki/Kashubian_Tricity

The same data as in the map from the OP is in the table below but only by county, not by gmina:

 

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For comparison Kashubians in the German Empire in year 1892:

 

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And for comparison my map based on the year 2005 data by Jan Mordawski:

 

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If we go back to year 1800 then at that time Kashubians were still around 55% (or about 14,200 people) in Kreis Lauenburg-Bütow (at that time it was one Kreis not two) and over 25% (or about 10,450 people) in Kreis Stołp.