RGeo, calc zone

gistfile1.rb

    def self.calculate_offset(point, normal, distance)
      utm = utm_factory.point(point.x, point.y).projection
      lon, lat = RGeo::CoordSys::Proj4.transform_coords(proj4_utm, proj4_geo,
                                                        utm.x + distance * normal.x,
                                                        utm.y + distance * normal.y)
      new_pos = factory.point(lon, lat)

      (point - Vertex.new(x: new_pos.x, y: new_pos.y)).length
    end

    def self.utm_factory
      @utm_factory ||= RGeo::Geographic.projected_factory(projection_proj4: WGS84_UTM_PROJ4)
    end

    WGS84_UTM_PROJ4 = '+proj=utm +zone=23 +south +ellps=WGS84 +datum=WGS84 +units=m +no_defs'

# ---- snip ----

      long_temp = (longitude + 180) - ((longitude + 180) / 360).to_i * 360 - 180
      zone_number = ((long_temp + 180) / 6).to_i + 1

      zone_number = 32 if latitude >= 56.0 && latitude < 64.0 &&
        long_temp > 3.0 && long_temp < 12

      # Special zones for Svalbard
      if latitude >= 72.0 && latitude < 84.0
        if long_temp >= 0.0 && long_temp < 9.0
          zone_number = 31
        elsif long_temp >= 9.0 && long_temp < 21.0
          zone_number = 33
        elsif long_temp >= 21.0 && long_temp < 33.0
          zone_number = 35
        elsif long_temp >= 33.0 && long_temp < 42.0
          zone_number = 37
        end
      end

# --------- sniiiiiiiiiiiip ----------
    UTM_ZONE_LETTERS = ('C'..'X').to_a - ['I', 'O']

    def utm_letter_designator(latitude)
      base, limit = -80, 84
      current = base

      zone_letter = UTM_ZONE_LETTERS.find do |letter|
        rstart, rend = current, current + 8
        current = rend
        range = if letter == UTM_ZONE_LETTERS.last
                  (rstart..84)
                else
                  (rstart...rend)
                end
        range === latitude
      end

      unless zone_letter
        raise OutsideUTMBoundsException, 'Outside UTM zone bounds'
      else
        zone_letter
      end
    end