Extracting Geolocation Image Data with Carrierwave and RMagick on Heroku

item_image.rb

class ItemImage < ActiveRecord::Base
  belongs_to :item
  
  mount_uploader :image, ImageUploader
  
  before_save :extract_geolocation
  
  def extract_geolocation
    img = Magick::Image.read(image)[0] rescue nil
    
    return unless img
    img_lat = img.get_exif_by_entry('GPSLatitude')[0][1].split(', ') rescue nil
    img_lng = img.get_exif_by_entry('GPSLongitude')[0][1].split(', ') rescue nil
    
    lat_ref = img.get_exif_by_entry('GPSLatitudeRef')[0][1] rescue nil
    lng_ref = img.get_exif_by_entry('GPSLongitudeRef')[0][1] rescue nil
    
    return unless img_lat && img_lng && lat_ref && lng_ref
    
    latitude = to_frac(img_lat[0]) + (to_frac(img_lat[1])/60) + (to_frac(img_lat[2])/3600)
    longitude = to_frac(img_lng[0]) + (to_frac(img_lng[1])/60) + (to_frac(img_lng[2])/3600)
    
    latitude = latitude * -1 if lat_ref == 'S'  # (N is +, S is -)
    longitude = longitude * -1 if lng_ref == 'W'   # (W is -, E is +)
    
    self.lat = latitude
    self.lng = longitude
    
    if geo = Geocoder.search("#{latitude},#{longitude}").first
      self.city = geo.city
      self.state = geo.state
      self.zipcode = geo.postal_code
    end
  end
  
  def to_frac(strng)
    numerator, denominator = strng.split('/').map(&:to_f)
    denominator ||= 1
    numerator/denominator
  end
end