This article is the first of two KML articles previously published on GeoChalkboard, and has been reproduced here with permission. It is based on course material published by Geospatial Training Services. We will be publishing the second article on Friday.
This article is the first of two articles that cover the use of KML Regions in Google Earth for displaying large GIS datasets. In this first post we’ll cover some basic introductory information that will give you some background information on KML Regions and the various ways in which they can be used in Google Earth.
What are Regions?
KML Regions allow you to add very large datasets to Google Earth without sacrificing performance. This functionality allows for the loading and display of data only when it falls within the display and occupies a certain portion of the screen. Typically, Regions are used to supply distinct levels of detail for your data where fine details are presented only when zoomed in far enough on the display. In the KML object model, Regions can be contained within any Feature which means Placemarks, Network Links, Overlays, and Containers. Most commonly, a Region is used to affect the visibility of Placemarks or Ground Overlays. The <Region> element defines this concept, and is composed of two important concepts including the bounding box and level of detail or LOD.
A bounding box, defined by the <LatLongAltBox> element, describes an area of interest defined by geographic coordinates and altitudes. This element, similar to <LatLongBox> contains the child elements <north>, <south>, <east>, and <west> that define the geographic boundaries of the Region. In addition, the bounding box also has elements that define the minimum and maximum altitude (<minAltitude>, <maxAltitude>). A Region is considered “active” or visible when the bounding box is within the display and the level of detail requirements are met.
Level of Detail
The level of detail or LOD is defined with the <Lod> child element of <Region>. It defines a range, specified by <minLodPixels> and <maxLodPixels> that determines the visibility of data within a Region. This ensures that large amounts of data are only loaded when enough pixels are available to display the data adequately. When the Region takes up a relatively small percentage of the screen, the LOD allows you to specify a dataset with a lower resolution. The <Lod> value units are defined by square pixels. Data must occupy an area greater than <minLodPixels> and less than <maxLodPixels> to be visible.
Primary elements specific to Region include:
- <LatLonAltBox> (required) – A bounding box that describes an area of interest defined by geographic coordinates and altitude.
- <minAltitude> – Defaults to 0; specified in meters above sea level
- <maxAltitude> – Defaults to 0; specified in meters above sea level
- <north>, <south>, <east>, <west> – Used to specify the latitude and longitude coordinates of the bounding box
- <Lod> – Abbreviated for Level of Detail. Describes the size of the projected region on the screen that is required in order for the region to be considered “active”. Composed of the following child elements:
- <minLodPixels> – Measurement in screen pixels that represents the minimum limit of the visibility range for a given Region.
- <maxLodPixels> – Measurement in screen pixels that represents the maximum limit of the visibility range for a given Region.
- <minFadeExtent> – Distance over which the geometry fades, from fully opaque to fully transparent. Value is expressed in screen pixels and is applied at the minimum end of the LOD limits.
- <maxFadeExtent> – Distance over which the geometry fades, from fully transparent to fully opaque. Value is expressed in screen pixels and is applied at the maximum end of the LOD limits.
Now that we’ve covered some basic information on KML Regions you can click here to see an example (courtesy of Google). You’ll want to make sure you already have Google Earth installed. Experiment with different viewpoints and watch when the Region comes into view and out of view, depending on how much of the screen area it requires.
In the second article we’ll cover some of the more advanced Region features, show you an example of how to create Regions, and discover how to use Arc2Earth to create Regions from your ArcGIS Data.
This article was written by Eric Pimpler at GeoSpatial Training Services. Geospatial Training Services provide a
range of geoweb courses.