[anoved]: `arcencode` is my Tcl implementation of the coordinate list compression algorithm used by MapQuest and Google Maps. It is useful for encoding a long sequence of coordinate data as a single comparatively compact URL parameter. https://github.com/anoved/arcencode%|%The code is available at GitHub.%|% Here is an excerpt from the Readme:

----

***About***

The `arcencode` package provides a Tcl implementation of the coordinate list compression algorithm used by MapQuest Platform Services and the Google Maps API. The algorithm converts a list of coordinate pairs to a string of printable non-space ASCII characters. Compression limits coordinate precision to three digits left of decimal and a default precision of five digits right of decimal.

***References***


   1. http://open.mapquestapi.com/common/encodedecode.html%|%MapQuest Platform Services: Compressed Lat/Lng Encoding/Decoding%|%
   2. https://developers.google.com/maps/documentation/utilities/polylinealgorithm%|%Google Maps API: Encoded Polygon Algorithm Format%|%

The `arcencode` code is derived from the MapQuest http://open.mapquestapi.com/common/encodedecode.html#js%|%JavaScript Sample Source%|%. The default five digit right-of-decimal precision is compatible with Google Maps API convention.

***API***

The `arcencode` package exports two procedures: `arcencode` and `arcdecode`. No
input validation or error checking is currently performed.

======
arcencode::arcencode points ?precision?
======

The `points` argument is expected to be a list of coordinate values containing
an even number of elements (conventionally, an alternating sequence of
latitude and longitude values). The `precision` argument specifies how many
digits to preserve right-of-decimal; the default `precision` is 5. The procedure
returns an encoded string representation of the point list.

======
arcencode::arcdecode strval ?precision?
======

The `strval` argument is expected to be a string value generated by `arcencode`
or a compatible source. The `precision` argument specifies how many digits to
recover right-of-decimal; the default `precision` is 5. The procedure returns a
list containing an even number of coordinate values (conventionally presumed to
be an alternating sequence of latitude and longitude values).

***Example***

These example values are drawn from the default MapQuest http://open.mapquestapi.com/common/encodedecode.html#example%|%Interactive Example%|%.

======
package require arcencode

set coordinates {
        45.967 -83.928700032549
        55 -83.928420000
        35 -83.97948699748273
        25.000000 -83.000000
        15.00000000000 -83.9279400000
        0.9600 -83.9275623435
        35.90 -0.90
        35.900 -83.00
        35.000 -83.000
        35.90000 -83.0000
        35.00000 -83.00000
        35.000004190 -83.00000123490
}

set encoded [arcencode::arcencode $coordinates]
set decoded [arcencode::arcdecode $encoded]
======

The contents of `encoded`:

======
w|pwGjig_Oggcv@w@~fayBd~H~b`|@yh~D~b`|@rftD~dutAkA_fgtEgiwyN?~cbtN~wnD?_xnD?~wnD???
======

The contents of `decoded` (compare to the input `coordinates`):

======
45.96700 -83.92870 55.00000 -83.92842 35.00000 -83.97949 25.00000 -83.00000 15.00000 -83.92794 0.96000 -83.92756 35.90000 -0.90000 35.90000 -83.00000 35.00000 -83.00000 35.90000 -83.00000 35.00000 -83.00000 35.00000 -83.00000
======

Note how the decoded values have been rounded to a uniform fixed precision.


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[Category Geography]
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