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 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 <description> tags will
not carry through to output. Since GDAL 1.6.1, folders containing multiple geometry types, like POINT and POLYGON, are supported.
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 <description> tag of a Placemark will be displayed in most geobrowsers as an HTML-filled balloon. When writing KML, Layer element attributes are added as simple schema fields. This best preserves feature type information.
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.
The content returned by OGR will be encoded in UTF-8, after the conversion from the encoding mentioned 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.
The following dataset creation options are supported:
ogr2ogr -f KML output.kml input.shp -dsco NameField=RegionName
ogr2ogr -f KML output.kml input.shp -dsco AltitudeMode=absolute
The driver supports reading and writing to files managed by VSI Virtual File System API, which include "regular" files, as well as files in the /vsizip/ (read-write) , /vsigzip/ (read-write) , /vsicurl/ (read-only) domains.
Writing to /dev/stdout or /vsistdout/ is also supported.
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
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.