RE: Various examples of GML documents for Point, Polygin, and Circle

["Carl Reed OGC" <creed@opengeospatial.org>]

As promised, some examples of light weight GML encodings for point, polygon, and circle. I am sure there are others!  FYI, the IETF is using GML 3.1 to define a point geometry for use in PIDF-LO. Their use cases are being driven by emergency services requirements for expressing locations of IP address location in buildings (using a DHCP location payload).

 

Anyway -

 

Here is one for Circle:

 

 <gml:CircleByCenterPoint
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:gml="http://www.opengis.net/gml"
xmlns:xlink="http://www.w3.org/1999/xlink" xsi:schemaLocation="
http://www.opengis.net/gml
http://schemas.opengis.net/gml/3.1.1/base/geometryPrimitives.xsd"
numArc="1">

<gml:pos srsName="urn:ogc:def:crs:OGC:1.3:CRS84">115.832
-31.939</gml:pos>
<gml:radius uom="km">0.5</gml:radius>

</gml:CircleByCenterPoint>

One could remove the srsName if we know the default is WGS 84. Also note the flexibility of expressing the "uom" - units of measurement. the "uom" allows the application to express the units of measure in just about any units.

 

Here is a "richer" GML document for Polygon. This is what is used by the OGC OpenLS Core Specification as well as by OMA in the MLP API Spec.

 

<INWATERA_1M.GEOMETRY>

    <gml:Polygon srsName="EPSG:4326">

        <gml:exterior>

            <gml:LinearRing srsName="EPSG:4326">

                    <gml:coordinates decimal="." cs="," ts="">119.593002319336,-31.6695003509522 119.595306396484,

                    31.6650276184082 119.600944519043,-31.6658897399902 119.603385925293,-31.669527053833            

                    119.60050201416,-31.6739158630371 119.595664978027,-31.6728610992432 119.593002319336,

                    31.6695003509522</gml:coordinates>

            </gml:LinearRing>

        </gml:exterior>

    </gml:Polygon>

</INWATERA_1M.GEOMETRY>

 

This is a bit "rich" in terms of semantic content. However, this GML example would support  the expression of islands (holes) in the polygon geometry. If islands are not an issue, the GML could be reduced to:

 

 

    <gml:Polygon srsName="EPSG:4326">

            <gml:LinearRing>

                    <gml:coordinates decimal="." cs="," ts="">119.593002319336,-31.6695003509522 119.595306396484,

                    31.6650276184082 119.600944519043,-31.6658897399902 119.603385925293,-31.669527053833            

                    119.60050201416,-31.6739158630371 119.595664978027,-31.6728610992432 119.593002319336,

                    31.6695003509522</gml:coordinates>

            </gml:LinearRing>

    </gml:Polygon>

 

Again, the srsName could be ignored. Also, for expressing the srsName, we could use an OGC URN liternal along the lines of srsName=”urn:ogc:def:crs:EPSG:6.6:4326”>

 

These example obviously do not have the external references, such as xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance".

 

Now for a point. Can't get much simpler than this! This example is from the new OGC GML Point Profile document - well on its way to becoming an OGC standard.

 

<gml:Point srsName=”urn:ogc:def:crs:EPSG:6.6:4326”>

   <gml:pos>45.256 -110.45</gml:pos>

</gml:Point>

 

Carl Reed, PhD
CTO and Executive Director Specification Program
OGC

 

The OGC: Helping the World to Communicate Geographically

 

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