KML - Keyhole Markup Language
Keyhole Markup Language (KML) is an XML-based language for managing the display of 3D geospatial data. KML has been accepted as an OGC standard, and is supported in one way or another on the major GeoBrowsers. Note that KML by specification uses only a single projection, EPSG:4326. All OGR KML output will be presented in EPSG:4326. As such OGR will create layers in the correct coordinate system and transform any geometries.At this time, only vector layers are handled by the KML driver. (there are additional scripts supplied with the GDAL project that can build other kinds of output)
KML Reading
KML reading is only available if GDAL/OGR is built with the Expat XML Parser, otherwise only KML writing will be supported.Supported geometry types are
Point, Linestring, Polygon, MultiPoint, MultiLineString, MultiPolygon and MultiGeometry. There are limitations, for example: the nested nature of folders in a source KML file is lost; folder KML Writing
Since not all features of KML are able to be represented in the Simple Features geometry model, you will not be able to generate many KML-specific attributes from within GDAL/OGR. Please try a few test files to get a sense of what is possible.When outputting KML, the OGR KML driver will translate each OGR Layer into a KML Folder. (you may encounter unexpected behavior if you try to mix the geometry types of elements in a layer, e.g.
LINESTRING and POINT data. The KML Driver will rename some layers, or source KML folder names, into new names it considers valid, for example '
Layer #0', the default name of the first unnamed Layer, becomes 'Layer__0'.KML is mix of formatting and feature data. The
Limited support is available for fills, line color and other styling attributes. Please try a few sample files to get a better sense of actual behavior.
Encoding issues
Expat library supports reading the following built-in encodings :- US-ASCII
- UTF-8
- UTF-16
- ISO-8859-1
The content returned by OGR will be encoded in UTF-8, after the conversion from the encoding mentionned in the file header is.
If your KML file is not encoded in one of the previous encodings, it will not be parsed by the KML driver. You may convert it into one of the supported encoding with the iconv utility for example and change accordingly the encoding parameter value in the XML header.
When writing a KML file, the driver expects UTF-8 content to be passed in.
Creation Options
The following creation options are supported:- NameField: Allows you to specify the field to use for the KML
element. Default value : 'Name'
ogr2ogr -f KML output.kml input.shp -dsco NameField=RegionName
ogr2ogr -f KML output.kml input.shp -dsco AltitudeMode=absolute
Example
The ogr2ogr utility can be used to dump the results of a PostGIS query to KML:ogr2ogr -f KML output.kml PG:'host=myserver dbname=warmerda' -sql "SELECT pop_1994 from canada where province_name = 'Alberta'"How to dump contents of .kml file as OGR sees it:
ogrinfo -ro somedisplay.kml
Caveats
Google Earth seems to have some limits regarding the number of coordinates in complex geometries like polygons. If the problem appears, then problematic geometries are displayed completely or partially covered by vertical stripes. Unfortunately, there are no exact number given in the KML specification about this limitation, so the KML driver will not warn about potential problems. One of possible and tested solutions is to simplify a line or a polygon to remove some coordinates. Here is the whole discussion about this issue on the Google KML Developer Forum, in the polygon displays with vertical stripes thread.See Also
Source : http://www.gdal.org
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