Google Earth is a geospatial browser produced by Google that allows interactive exploration of the world from data servers on the Internet. An integral part of Google Earth is a built-in “cache” that is stored locally on the user’s computer, and allows the user to later revisit previously viewed regions (even if the network link is disrupted or unavailable). The Google Earth cache is filled as the user interacts with Google Earth (i.e., flying and zooming the interface). The Google Earth Offline Cache Pre-loader (GEOCP) is a tool that allows users to specify a region to cache, and controls Google Earth to automatically and systematically fill the Google Earth cache. While connected to the Internet, the user specifies a latitude/longitude box, and a desired height above the ground. GEOCP then commands Google Earth to view the requested area in a lawnmower pattern, which has the side effect of bringing those areas into the Google Earth cache. When finished, the user is free to disconnect the network and continue to view the area.
The ability to visit cached places without Internet access is of particular value to disaster responders. While responders may have good Internet access at their point of origin, they generally will not have access once they deploy to the field. Consequently, responders who wish to use Google Earth in the field have to be sure that the Google Earth cache contains data that they will need. The conventional way to do this is to manually “fly” over the disaster area with Google Earth. This approach, however, is tedious and prone to errors. GEOCP seeks to aid disaster responders by automating this process. This both decreases the amount of time a responder must devote to “preparing” Google Earth for deployment, and increases the accuracy (and completeness) of the coverage.
One limitation of Google Earth is that it has a single cache store. Sometimes users need to back up and restore the cache, particularly when dealing with an intermittent network connection that might accidentally cause a carefully generated cache to be partly overwritten. Backing up and restoring the cache is as simple as copying two files, but in practice, this is error-prone for non-experts. GEOCP lets the user simply back up and restore cache files without the need to navigate folders or worry about copying two files at once.
GEOCP is a Visual Studio application that consists of a graphical user interface for user interaction and area specification, and utilizes the recently published Google Earth COM API (see http://earth.google.com/comapi/index.html ) to programmatically operate Google Earth to visit the specified area in the same way as an interactive user would.
The primary Google Earth COM APIs utilized by GEOCP are: 1) IApplicationGE.ViewExtents, which allows the user to select the desired area by importing the current area Google Earth is viewing; 2) IApplicationGE.SetCamera, which allows GEOCP to instruct Google Earth to travel to a desired point within the area, just as if the user typed the latitude/longitude specification in the Fly To box; and 3) IApplicationGE.Stream ingProgressPercentage, which allows GEOCP to wait until Google Earth has received all the relevant data at that location, just as a user does by watching the “Streaming ...” area at the bottom of the Google Earth screen.
This work was done by Anne Wright of Perot Systems and Randy Sargent of Carnegie Mellon West for Ames Research Center. This software is available for use. To request a copy, please visit https://software.nasa.gov/software/ARC-16089-1