Examples in other pages of documentation here are still using version 3 so are incompatible with the updated demo map and app. The algorithm is the same but uses the alphabet "6789B5NPQC4MXRD3LWTF2KJHG".For example KFPX-WT7D/5 has the optional checksum character 5 which validates the characters in the code.
An optional checksum is available at any level of code precision.
It uses the same system as the PPS checksum (but modulus 31 rather than 23).
It makes a postcode verifiable and protects against transcription and OCR mistakes.
It is a character added to the code based on a mathematical calculation on the characters to check that it has been entered correctly. It is designed to trap codes with at least one incorrect character or codes with at least two transposed characters or codes with a missing character, while also catching many more - even if the incorrect character is the checksum itself.
As variations of the OpenPostcode can exist with different parameters, these are also included in the checksum calculation - codes generated for one region will mostly fail in another.
Many codes randomly created or mistyped with characters outside the allowed character set will also stand out as invalid codes.