The Placemark Family
There are a number of KML objects that define the location of one or points, and use connecting lines and areas to create geometry. They are all structured under the <Placemark> element.
KML: Geometry
Placemark Children
<Point>
Defined by longitude, latitude and (sometimes) altitude. A Point contained within a Placemark defines the location of the Placemark's icon. The default (Point) Placemark uses a yellow pushpin icon.
<LineString>
Commonly refered to as Paths, these are defined by a series of coordinates connected by straight lines. Combined these segments form one LineString. If the coordinates are located at altitude the LineString can be extruded to the ground to create an object that looks like a wall (extruded panels filled) or fence (unfilled).
<Polygon>
The outer edge of a Polygon is defined by an Outer Boundary which contains a LinearRing, that is similar to a LineString with the two ends connected by a line segment. They can also be created with an inner boundary to create a hole in the polygon.
<Model>
The KML for 3D models, created by SketchUp or other modeling programs are contained within the <Placemark> element. Models are supported in the Collaborative Design Activity (COLLADA) file format. Good examples of the use of <Model> can be found in 3D Buildings and Gallery layers that appear by default in Google Earth.
<MultiGeometry>
A container for multiple geometry elements associated with the same feature. Can be used to list multiples or combinations of Points, LineStrings, Polygons and/or Models, as "one" object.
Links
Getting Started With Placemarks
Google Tutorials
Brief guides explaining Point-Placemarks, LineStrings (Paths) and Polygons.
KML Tutorials
Tutorials created for Google Earth workshops taught by UAF's Geography Department introduce Points, Paths, Polygons and Models.
3D Warehouse
An online repository of models where users can search for and download SketchUp/Google Earth models created and upload by Google or other users.
KML Examples
These KML files use a mixture of elements, but the fundamental information is plotted
USGS Earthquake Locations plotted in real-time as Point-Placemarks and Tectonic Plate Boundaries defined using LineStrings.
Point-Placemarks that map out the spread of Avian Flu across the world. Created by Declan Butler
Track the Volvo Ocean Race which is traced in Google Earth using LineStrings and Point-Placemarks.
U.S. Geothermal Resources visualized as translucent Polygons and Balloons showing the inventory for each state
Hawaii Volcano Observatory's use of Polygons to define Lava Hazard Zones on the Big Island, Hawaii.