def parse_file_name_starting_position()
            groups = @local_file_name.scan(/([NS])(\d+)([EW])(\d+)\.hgt/)[0]

            if groups.length != 4
                raise "Invalid file name #{@file_name}"
            end

            @latitude = groups[1].to_i * (groups[0] == 'N' ? 1 : -1)
            @longitude = groups[3].to_i * (groups[2] == 'E' ? 1 : -1)
        end
def parse_file_name_starting_position()
            groups = @local_file_name.scan(/([NS])(\d+)([EW])(\d+)\.hgt/)[0]

            if groups.length != 4
                raise "Invalid file name #{@file_name}"
            end

            @latitude = groups[1].to_i * (groups[0] == 'N' ? 1 : -1)
            @longitude = groups[3].to_i * (groups[2] == 'E' ? 1 : -1)
        end
Returns (latitude, longitude) of lower left point of the file