Added new survey foot USCS group for deprecation support.

[mjmdavis]

• Closes survey foot phase out #1021
• Executed black -t py36 . && isort -rc . && flake8 with no errors
• The change is fully covered by automated unit tests
• Documented in docs/ as appropriate
• Added an entry to the CHANGES file

[Jellby]

From what I remember, units like fathom, chain and acre are survey units and should be defined in terms of the "survey foot", while their "international" equivalents are only defined by analogy, with no formal or official support. That was the reason the definitions look the way they do now. Not living in the US, I could of course be wrong.

There are also some errors in your changes: survey_link is duplicated, survey_foot_chain should be defined in terms of survey_foot_rod, not rod, etc.

[mjmdavis]

See the referenced issue for context regarding the Survey Foot deprecation.

Good catch on the dependencies. I’ll clear that up.

[Jellby]

Ah, I missed that, sorry. I guess it makes sense. I wonder however about the survey mile. The RFC says it will also be deprecated, so maybe only survey_mile = 5280 * survey_foot should be maintained, while survey_mile = 5280 * foot seems to be a hybrid with no support. On the other hand, chain, link, rod, furlong, fathom, acre are explicitly "re-defined" in terms of the international foot. I don't know what would be the best action with "cables_length" (rename to "cable_length"?) and "league".

[Jellby]

This would be my suggestion:

@group USCSLengthSurveySurveyFoot
    survey_foot = 1200 / 3937 * meter = sft
    survey_foot_rod = 16.5 * survey_foot = = survey_foot_pole = survey_foot_perch
    survey_foot_chain = 4 * survey_foot_rod
    survey_foot_link = 1e-2 * survey_foot_chain = = survey_link
    survey_foot_furlong = 40 * survey_foot_rod
    survey_foot_fathom = 6 * survey_foot
    survey_mile = 5280 * survey_foot = smi = us_statute_mile
    survey_foot_league = 3 * survey_mile
    survey_foot_cables_length = 120 * survey_foot_fathom

    survey_foot_acre = 10 * survey_foot_chain ** 2
    survey_foot_square_rod = survey_foot_rod ** 2
    square_survey_mile = survey_mile ** 2 = = survey_foot_section
    survey_foot_square_league = survey_foot_league ** 2

    survey_foot_acre_foot = survey_foot_acre * survey_foot = = survey_foot_acre_feet
@end

@group USCSLengthSurvey
    rod = 16.5 * foot = rd = pole = perch
    chain = 4 * rod
    link = 1e-2 * chain = li
    furlong = 40 * rod = fur
    fathom = 6 * foot
    league = 3 * mile
    cables_length = 120 * fathom

    acre = 10 * chain ** 2
    square_rod = rod ** 2 = sq_rod = sq_pole = sq_perch
    section = square_mile 
    square_league = league ** 2

    acre_foot = acre * foot = = acre_feet
@end