def to_string(self):
        """
        Turn a CRS dict into a PROJ.4 string. Mapping keys are tested against
        ``all_proj_keys`` list. Values of ``True`` are omitted, leaving the key
        bare: {'no_defs': True} -> "+no_defs" and items where the value is
        otherwise not a str, int, or float are omitted.

        Args:
            crs: A CRS dict as used in Location.

        Returns:
            str. The string representation.
        """
        def filt(x):
            return '+'+x[0] in PROJ4_PARAMS.keys() and x[1] is not False

        items = []
        for k, v in sorted(filter(filt, self.items())):
            items.append(
                "+" + "=".join(
                    map(str, filter(
                        lambda y: (y or y == 0) and y is not True, (k, v)))))
        return " ".join(items)
Turn a CRS dict into a PROJ.4 string. Mapping keys are tested against
        ``all_proj_keys`` list. Values of ``True`` are omitted, leaving the key
        bare: {'no_defs': True} -> "+no_defs" and items where the value is
        otherwise not a str, int, or float are omitted.

        Args:
            crs: A CRS dict as used in Location.

        Returns:
            str. The string representation.