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The description on its file description page there is shown below.
Summary
DescriptionFlag of Tonga (construction sheet).svg
English: Flag of Tonga — unofficial construction sheet.
In the absence of definitive government specifications, this construction sheet serves to document the geometry that has been used in Wikimedia's representation of the flag of the Tonga since September 13, 2006. See File:Flag of Tonga.svg
Date
Source
Own work
The Flag of Tonga (the flag of King George) shall never be altered but shall always be the flag of this Kingdom and the present Royal Ensign shall always be the ensign of the
Royal Family of Tonga.
— Constitution of Tonga — section 47
Unfortunately the constitution neglects to define the geometry of the flag. It is therefore not surprising that different sources show a range of slightly different sizes for both the canton and the cross.
On September 13, 2006, Wikimedia Commons file: "File:Flag of Tonga.svg" was updated to match one of two unofficial construction sheets shown at Flags of the World (FOTW). Commentary at FOTW shows the geometry came originally from Album des pavillons, but recent versions of Album des pavillons show a slightly different sized cross.
This construction sheet matches the unofficial FOTW construction sheet that was used to create File:Flag of Tonga.svg. It also matches the Flag of Tonga shown at Encyclopedia Britannica.
I, the copyright holder of this work, release this work into the public domain. This applies worldwide. In some countries this may not be legally possible; if so: I grant anyone the right to use this work for any purpose, without any conditions, unless such conditions are required by law.
This image shows a flag, a coat of arms, a seal or some other official insignia. The use of such symbols is restricted in many countries. These restrictions are independent of the copyright status.