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I'm very new to GIS.

I'm trying to understand 3D city maps in the format of CityGML, as well as extract and identify individual buildings. Currently my coordinate system (SRID) is set as 4326, which is EPSG::4326. This is supposed to be just latitude / longitude, and the unit is supposed to be degree, right?

But the data turn out like this:

    <bldg:Building gml:id="gml_V1OLK6F8Q9IXH0YK8T2J4L2PF9BROTDEYFHK">
  <gml:boundedBy>
    <gml:Envelope srsName="urn:ogc:def:crs:EPSG::4326" srsDimension="3">
      <gml:lowerCorner>317106.513996383 79684.7738185953 15.3638404876802</gml:lowerCorner>
      <gml:upperCorner>317119.656396384 79713.6622185921 27.0662941325891</gml:upperCorner>
    </gml:Envelope>
  </gml:boundedBy>

Also this is a polygon:

  <gml:posList srsDimension="3">317106.581796386 79690.3543185918 15.3638404876802 317106.581796386 79690.3543185918 27.0662941325891 317106.699996386 79690.958318593 27.0662941325891 317106.699996386 79690.958318593 15.3638404876802 317106.581796386 79690.3543185918 15.3638404876802</gml:posList>

So you can see an individual point has coordinate like

"317106.581796386 79690.3543185918 15.3638404876802".

How can I understand these numbers?

  • 2
    Your assumption is right. You better ask the data provider about it. Maybe the original data has no CRS, and your GIS software sets EPSG:4326 by default and writes it into the file.. – AndreJ May 10 at 6:19

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