Google Earth General

Google Earth is a powerful tool for viewing, creating and sharing GIS data. The latest improvements in the KML format allow storing attributes as structured data, which makes possible exchange and even editing of GIS data using Google Earth. Google Earth comes in four different versions (from Free to Enterprise). Make sure to read the Google Earth license agreement before using it.

What is KML

Keyhole Markup Language (KML) is an XML - based language for managing the display of geo spatial data in Google Maps and Google Earth. Since a KML file is a text file, its size might become quite large. Google Earth also takes a lot of RAM when large KML files are loaded. If possible split your datasets to subsets before converting them to KML.

What is KMZ

The compressed version of the KML with the extension KMZ. Actually this is a zipped archive and the contents can be extracted with any zip program. A KMZ file can contain one or more KML files together with images etc. The export function of ET GeoWizards expect a full file name (with the extension). The extension of the output file defines whether the file will be compressed (KMZ) or not (KML)

Google Earth version

ET GeoWizards exports KML version 2.2 files (this is the KML version which introduced support for attributes called in KML "ExtendedData"). Since it is impossible to find out which exactly version of Google Earth starts supporting KML 2.2, we recommend using Google Earth 4.2 or above.

Google Earth projection

For its reference system, KML uses Geographic Coordinate System (GCS) with WGS84 datum. In ArcGIS this projection is called GCS_WGS_1984. The export to Google Earth functions of ET GeoWizards project the data on the fly to GCS_WGS_1984. If the source data is in a projection that have different datum, the functions of ET GeoWizards do on the fly geographic transformations on the data.

• If the input data does not have a projection associated with it or have so called "Unknown" coordinate system, the data cannot be exported to KML.
• If the export functions cannot find an appropriate geographic transformation to project the input data to GCS_WGS_1984, they will not export the data. This might happen if the input data is in a very specific or outdated projection.

Consideration when exporting to Google Earth

1. The export function of ET GeoWizards supports multipart features, which are exported as MultiGeometry in the KML file. These features will consist of several not connected geometries and will have a single label point. If the same features are imported back, each geometry will be created as a single part feature with the same attributes.
2. The export functions of ET GeoWizards allow creating Labels for each Polyline and Polygon features. The Labels are created as follows:
   • For polygons - the label points of the polygons.
   • For polylines - the middle point of the polyline

   Using Labels is convenient way to display the name of a polyline or polygon feature. Since Labels are part of the feature they are also linked to the feature attributes, which can be displayed by clicking on the Label as well as by clicking on the feature.

   Point features are always labelled and the user can not turn the label option off.

3. Point Symbols. ET GeoWizards uses a set of the standard Google Earth marker symbols to display point features and Labels for polyline and polygon features. The user can select the marker to be used for each feature class. The symbols that can be used are:

   

   The size and the color of the symbols are taken from:

   • Size - assigned by the user
   • Color - randomly assigned
4. Exporting elevations: The export functions of ET GeoWizards allow three ways of exporting Z values for the features.
   • Z values from geometry - Only available if the exported dataset to be exported has Z values (PointZ, PolylineZ, PolygonZ).
   • Z values from a field - A numeric field is required
   • Constant Z values for all features

   Note that Google Earth uses elevation values in Meters. If the Z values of the dataset are in Feet, the user needs to indicate this in the export procedure.

   Representation of the elevations in Google Earth:
   • Z Type - how the Z values will be interpreted by Google Earth
     • Absolute - Sets the altitude of the coordinate relative to sea level, regardless of the elevation of the Google Earth terrain beneath the feature.
     • Relative - Sets the altitude of the feature relative to the Google Earth terrain in a particular location.
     • NONE - the Z values are ignored - the feature will be displayed on the Google Earth surface
   • Extrusion - Specifies whether to connect the geometry to the ground.
5. Attributes. All attributes of the features are exported and can be displayed in Google Earth. To display the attributes select the feature or its Label point.