GML Linearring should not have srsName attribute

Describe the bug The OpenLayers GML formats have an issue serializing a geometry to GML: <gml:LinearRing> element should not have srsName attribute.

Reference: http://www.datypic.com/sc/niem21/e-gml32_LinearRing.html

To Reproduce Steps to reproduce the behavior:

1. Create an ol geometry
2. Write features from GML format of this geometry
3. Output GML (both version 2 and 3) will have an srsName

<gml:exterior>
  <gml:LinearRing srsName="urn:x-ogc:def:crs:EPSG:4326">
    <gml:posList srsDimension="2">38.5571 -75.7071 38.4631 -75.6991 38.4551 -75.3501

Expected behavior <gml:LinearRing> does not have srsName attribute.

<gml:exterior>
  <gml:LinearRing>
    <gml:posList srsDimension="2">38.5571 -75.7071 38.4631 -75.6991 38.4551 -75.3501

[ahocevar]

I guess the most generic way to achieve this is a change in our GML serializers so the srsName is only set on the “outermost” geometric entity. So in the case of a polygon, only the gml:Polygon would get the attribute. Or e.g. for a MultiPoint, only the gml:MultiPoint and not its point members.

[ahocevar]

Correct. In GML2, a LinearRing is a geometry primitive, which can have a srsName, but the spec states in 4.3.3

Since a LinearRing is used in the construction of Polygons (which specify their own SRS), the srsName attribute is not needed.

Also in GML3, the LinearRingPropertyType, which is used for the rings of a polygon, is made of a sequence of LinearRings, which have a hierarchy of AbstractRing -> AbstractCurve -> AbstractGeometricPrimitive -> AbstractGeometry. For that, the spec states

Any geometry element that inherits the semantics of AbstractGeometryType may be viewed as a set of direct positions. All of the classes derived from AbstractGeometryType inherit an optional association to a coordinate reference system. All direct positions shall directly or indirectly be associated with a coordinate reference system. When geometry elements are aggregated in another geometry element (such as a MultiGeometry or GeometricComplex), which already has a coordinate reference system specified, then these elements are assumed to be in that same coordinate reference system unless otherwise specified.

OpenLayers also has a LinearRing geometry. If such a LinearRing is serialized in GML, its srsName attribute makes sense and is needed to indicate the SRS. For the rings of a polygon, there is nothing wrong about proposing the removal of the srsName, at least for GML2. But for both GML2 and GML3, the XML is still valid with the srsName on a LinearRing, also for the rings of a Polygon.