[GRASS-dev] sq meters to acres

[Hamish]

Hi,

In my quest to fix, code comment, and abstract all the unit conversions in
GRASS, I notice that d.what.vect and v.what use 10.763649 for converting
sq meters to sq feet. I can't see where the precision in that number comes
from.

by my calculations-

to standard feet:
   1 / ( 0.0254 m/in * 12.0 inch )^2  =  10.7639104167097

to US survey feet:
   1 / ( 0.0254 m/in * 12.0 inch * 0.999998000 )^2 = 10.7639534724806


d.what.vect later goes on to use ( 5280 ft/mi ^2 / 640 acre/sq mi )
to get 43560. sq ft/acre.

( an acre = 1 chain * 1 furlong, or 66'x660' = 43560. sq ft )


I am always a bit confused by acres though, there doesn't seem to be a
US Survey feet version of them listed in my trusted Glover's pocket ref,
yet it would /seem/ to the layman that measuring acres would be US
Surveyors' bread and butter. But apparently the common use it to base
them on the standard 1'=12" definition and keep chains, rods, and
furlongs in standard feet too. shrug.

US survey acres are mentioned in the Wikipedia article, and ISTR the
unix `units` program does something completely insane basing the decision
of which version to (silently) give you based on if your locale is en_US
or something like that! anyway, I don't consider either of those in any
way definitive in this regard and was hoping there was a US surveyor in
the audience who might comment.


this may seem a bit of an anal retentive task to quibble over, but IIRC
grass made it all the way up to 6.2.something with r.report outputting
rather incorrect numbers for acres. (cue thread re. errors due to the
distortion of equal-area map projections over great distances/areas)


?,
Hamish