#GML geometry

The deck reads and writes a subset of GML.

GML (ISO 19136) is a framework from which an application schema builds a format: hundreds of element types and no fixed document shape. The deck implements the geometry vocabulary, which stays the same across application schemas and is what WFS responses, INSPIRE data and GML embedded in other documents carry.

def doc as geo.Document init geo.readGml($wfsResponse);
def out as string init geo.writeGml($doc);

#What is read

Unlike KML and GeoJSON, a GML polygon's inner rings are not dropped: each comes back as a track of its own after the outer ring.

Geometries are found at any depth, so an unknown application schema still reads:

<wfs:FeatureCollection>
  <wfs:member>
    <app:Summit>
      <app:name>Peak</app:name>
      <app:where><gml:Point><gml:pos>47.0 11.0</gml:pos></gml:Point></app:where>
    </app:Summit>
  </wfs:member>
</wfs:FeatureCollection>

That reads as one waypoint called Peak. A feature's name (any prefix, gml:name included) names the geometries inside it, inherited inwards. A bare <gml:Point> as the whole document, as a WFS returns for a single geometry, also reads.

Wrappers between a geometry and its coordinates are descended through rather than listed: a polygon's <gml:exterior>/<gml:LinearRing>, a curve's <gml:segments>/<gml:LineStringSegment>, and others of the same shape. Descent stops at MAX_NESTING (100) levels.

#Axis order

GML does not fix whether a position is lat lon or lon lat; the coordinate reference system does, and the common spellings of WGS 84 disagree:

The same two numbers name different places depending on that attribute. Both of these are Innsbruck:

<gml:Point srsName="urn:ogc:def:crs:EPSG::4326"><gml:pos>47.3122 11.3889</gml:pos></gml:Point>
<gml:Point srsName="EPSG:4326"><gml:pos>11.3889 47.3122</gml:pos></gml:Point>

The srsName nearest the geometry applies; a geometry without one inherits it from the closest enclosing element that has one. Names are compared case-insensitively.

No reprojection is performed. Coordinates are read as WGS 84 degrees whatever the CRS. A document in a projected system (a national grid, a UTM zone) reads eastings and northings as degrees, and the range check usually rejects them with an error.

#posList and srsDimension

<gml:posList> is a flat run of numbers; srsDimension says how many make one position:

<gml:posList srsDimension="3">47.0 11.0 500 47.1 11.1 600</gml:posList>

Without srsDimension, 2 is assumed. Any value other than 2 or 3 raises, and so does a count that does not divide into whole positions, rather than shifting every coordinate after a stray number. A <gml:pos> is one position whatever its length; a third number is the elevation.

#What is written

writeGml writes a GML 3.2 gml:FeatureCollection (GML_NAMESPACE) with the document name as gml:name and one gml:featureMember per waypoint, route and track segment. Each holds a gml:Feature with a gml:name and a gml:Point (gml:pos) or a gml:LineString (gml:posList), in GML_CRS (urn:ogc:def:crs:EPSG::4326), latitude first.

Positions are written two-dimensional (srsDimension="2"), so elevations are dropped, as are descriptions, symbols and times. A path of fewer than two points is not written.

The output is not schema-valid against any application schema, since GML has no generic one. It carries geometry between readers of the same subset; to conform to a specific schema, wrap the geometry yourself.