#### Description:

This attribute specifies the formatting system to use when AST Objects are written out as YAML through a YamlChan. It affects the behaviour of the astWrite function when they are used to transfer any AST Object to or from an external XML representation.

Type:

  String.

#### Applicability

##### YamlChan

All YamlChans have this attribute.

#### Formats Available

The YamlEncoding attribute can take any of the following (case insensitive) string values to select the corresponding formatting system:

- `"` ASDF`"` : Currently, this is the only schema supported by the YamlChan class. It is defined by the Space Telescope Science Institute. (see http://asdf-standard.readthedocs.io). See below for details of the support this class provides for reading and writing ASDF objects. In future, the YamlChan class may allow AST objects to be represented in other ways (e.g. AST Native format).

#### Notes on Reading ASDF WCS Information

This class does not currently support the complete ASDF WCS schema. When reading an AST Object from an ASDF YAML file, the following restrictions on the ASDF file apply:

- The ASDF spectral_frame, temporal_frame and composite_frame classes are not supported.

- Only the following celestial coordinate frames are supported: icrs, fk4, fk4noeterms, fk5, galactic, supergalactic, altaz, barycentricmeanecliptic.

- Earth locations must be specified using the WGS84 ellipsoid.

- Times must be specified in one of the following formats: iso, byear, jyear, jd, mjd.

- Only the following transform classes are supported: identity, scale, multiplyscale, remap_axes, shift, compose, concatenate, constant, fix_inputs, affine, rotate2d, rotate_sequence_3d, rotate3d, linear1d, ortho_polynomial (chebyshev only), planar2d, polynomial. In addition, all sky projections are supported.

#### Notes on Writing ASDF WCS Information

This class does not currently support the complete ASDF WCS schema. When writing an AST Object to an ASDF YAML file, the following restrictions on the AST object apply:

- Only Frames, Mappings and FrameSets can be written.

- Frames must be basic Frames or SkyFrames.

- The following SkyFrame systems are not supported: GAPPT, HELIOECLIPTIC, J2000, UNKNOWN.

- Only the following Mapping classes are supported: CmpMap, TranMap, UnitMap, ZoomMap, ShiftMap, WinMap, MatrixMap, PermMap, WcsMap, PolyMap, ChebyMap.

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