[mkgmap-dev] [PATCH] route recalculation sensitivity

Tue Mar 31 15:58:01 BST 2009

Bernhard Heibler wrote:
> I think I found the required bits. Seems to be the flag at 0x43 in the
> TRE header. Patch is attached.

> +		writer.putInt(0x110301); // Was 0xd0401 before. 31.03.2009 Changed to 0x110301 to increase route recalculation speed
>   

Judging by the source code and the "documentation", the use of this word
starting at 0x43 isn't yet known, and it does look as if the two
low-order bytes here haven't changed much.

*If* the important part of this patch is to change the value of byte
0x45 from 0x0D to 0x11 then we might postulate that the "recalculate
distance" is actually "EarthPerimeter/(12 * pow(2, n))" where 'n' is
that byte. (The '12' is just a fiddle-factor here).

Assuming Earth's perimeter is 40Mm, that gives an original
recalculate-distance of about 406m (yeah - I can believe that) changing
to the new one of about 25m as claimed by Michel Marti. Internally of
course the recalculate "distance" will actually be in some fraction of
degrees or radians - probably in "Map Units" in practice.

Again - assuming the 0x0D -> 0x11 change is the important bit, it leaves
the interesting question of what the other bytes do. It will be easy
experimentation to try other values of byte 0x45 to see if my postulated
relationship holds. Byte 0x44 changed from 0x04 to 0x03. What did *that* do?

Steve

PS: I must admit that this isn't where I expected the
recalculate-distance to be.