Editing Geography Markup Language (section)

===Coordinates===

''Coordinates'' in GML represent the coordinates of ''geometry objects''. Coordinates can be specified by any of the following GML elements:

<syntaxhighlight lang="xml">
 <gml:coordinates>
 <gml:pos>
 <gml:posList>
</syntaxhighlight>

GML has multiple ways to represent coordinates.  For example, the <code><gml:coordinates></code> element can be used, as follows:

<syntaxhighlight lang="xml">
 <gml:Point gml:id="p21" srsName="http://www.opengis.net/def/crs/EPSG/0/4326">
    <gml:coordinates>45.67, 88.56</gml:coordinates>
 </gml:Point>
</syntaxhighlight>

When expressed as above, the individual coordinates (e.g. ''88.56'') are not separately accessible through the [[XML]] [[Document Object Model]] since the content of the <code><gml:coordinates></code> element is just a single string.

To make GML coordinates accessible through the XML DOM, GML 3.0 introduced the <code><gml:pos></code> and <code><gml:posList></code> elements. (Although GML versions 1 and 2 had the <code><gml:coord></code> element, it is treated as a defect and is not used.) Using the <code><gml:pos></code> element instead of the <code><gml:coordinates></code> element, the same point can be represented as follows:

<syntaxhighlight lang="xml">
 <gml:Point gml:id="p21" srsName="http://www.opengis.net/def/crs/EPSG/0/4326">
    <gml:pos srsDimension="2">45.67 88.56</gml:pos>
 </gml:Point>
</syntaxhighlight>

The coordinates of a <code><gml:LineString></code> geometry object can be represented with the <code><gml:coordinates></code> element:

<syntaxhighlight lang="xml">
 <gml:LineString gml:id="p21" srsName="http://www.opengis.net/def/crs/EPSG/0/4326">
    <gml:coordinates>45.67, 88.56 55.56,89.44</gml:coordinates>
 </gml:LineString >
</syntaxhighlight>

The <code><gml:posList></code> element is used to represent a list of coordinate tuples, as required for linear geometries:

<syntaxhighlight lang="xml">
 <gml:LineString gml:id="p21" srsName="http://www.opengis.net/def/crs/EPSG/0/4326">
    <gml:posList srsDimension="2">45.67 88.56 55.56 89.44</gml:posList>
 </gml:LineString >
</syntaxhighlight>

For GML data servers ([[Web Feature Service|WFS]]) and conversion tools that only support GML 1 or GML 2 (i.e. only the <code><gml:coordinates></code> element), there is no alternative to <code><gml:coordinates></code>. For GML 3 documents and later, however, <code><gml:pos></code> and <code><gml:posList></code> are preferable to <code><gml:coordinates></code>.