Where am I? The 10:10 code...
Tuesday, July 6, 2010
A couple months ago, I came across a blog post discussing an encoding of geographic coordinates that might be “easier” to remember or exchange than latitude/longitude pairs.
The encoding, known as “10:10”, is able to identify to a 10 meter precision any location on the planet using a 10 character code. Without going into the merits of the scheme, I will just note that there is some good discussion in the comments section about ideas for its improvement.
- Calculate a single
pnumber representing a latitude/longitude pair.
- Calculate a
ttnumber representing a version of
pmodified by a checksum.
- Generate a 10:10 code by iteratively indexing into the alphabet.
We will separate each of these steps into words that can be tested individually and then combined to compute the 10:10 codes.
We will need some vocabularies and a namespace:
USING: kernel math math.functions ranges memoize sequences strings ; IN: ten-ten
The 10:10 code uses a 29 character alphabet, so we will need to define it:
CONSTANT: ALPHABET "ABCDEFGHJKMNPQRVWXY0123456789" MEMO: BASE ( -- base ) ALPHABET length ;
First, we compute the
: p ( lat lon -- p ) [ 90 + ] [ 180 + ] bi* [ 10000 * floor >fixnum ] bi@ [ 3600000 * ] dip + ;
Then, we calculate and add the checksum to create a
: tt ( p -- tt ) [ BASE * ] keep 10 [1..b) [ [ BASE /mod ] dip * ] map nip sum BASE mod + floor ;
Next, we convert the
tt value to a string (inserting spaces for
: tt>string ( tt -- str ) 10 [ BASE /mod ALPHABET nth ] replicate nip reverse >string [ CHAR: \s 3 ] dip insert-nth [ CHAR: \s 7 ] dip insert-nth ;
And, finally, putting it all together:
: ten-ten ( lat lon -- tt ) p tt tt>string ;
When I run
51.09559 1.12207 ten-ten (the included example for the
location of the Eurotunnel in the UK), it gives me a value of
MEQ N6G 7NY5. This is not the same as the original author, but matches
a version written in C# by another commenter as well as a version I
wrote in Python to validate my results.
You can find my implementation on my GitHub account.