Free Geography Tools

Note: you should read this post first for general info about how to use automated Google Earth caching programs.

Next Google Earth caching program up for review is GE Cacher.  No install program, just download and unzip the executable into a folder, then run it from there; it will always run on top of other windows. You’re likely to get a security warning from Windows asking if you want to unblock network access for the program. The program has two tabs; first, for configuration:

Proxy data is read from the registry, and entered if needed (though it usually isn’t needed for most systems). If you’ve installed Google Earth somewhere other than the default directory, you’ll need to enter its location in the appropriate box.

In the “Use” tab:

Unlike the two other caching programs I’m reviewing, GE Cacher lets you set the latitude/longitude bounds, which is more convenient, and calculates the area covered. It also saves the entered data in a config file, so it shows up again if you re-start the program. On the downside, though, the time update interval is fixed, and you also have to set the altitude manually in Google Earth. Use this altitude to set the imagery resolution, and also to set the position spacing interval (here called the visible area width). On my system, the “Reload Google Earth to use GE Cacher” button either has no effect, or gives an error message.

You can also specify the location of the KML file to be used as the network link, unlike the other two which have a fixed KML file name and location. You’ll still need to click “Start caching”, then “Stop caching”, to create the KML file, then make a Network Link to it in Google Earth. Once you’re ready, with Google Earth running and the network link created, click on “Start caching” to begin the process. The program will show a bar graph indicator of progress next to “Area count”, and also an approximate estimated time remaining (“ETA”). The program is sometimes cranky in Windows Vista about exiting, which may require bringing up the Windows Task Manager “(Ctrl-Alt-Esc) to shut it down.

I found that the best results were obtained by setting the “Visible area width” lower than you think you’ll need, and also adjusting the Fly-To speed in Google Earth to the fastest setting. The update interval isn’t adjustable, and IMO the program’s fixed update interval is too short to ensure that all the data for an area is downloaded before moving to the next area. But apart from that, the program does do the job, and the ability to set the absolute bounds for the area is a big plus; there’s also decent documentation on the web page. The program doesn’t seem to be in active development any longer; a link is provided to the partial source code in Delphi, but only for educational purposes.

In a previous post, I described how to manually create a Google Earth disk cache file for an area. For small areas, this is probably the fastest approach, but for larger areas, doing it manually can get tedious. I’ve found three programs that can automate this process, but before reviewing them, it might be useful to go through common factors for all of them.

All three programs work by creating a KML file for a particular location, and then updating that file on user-specified schedule, typically every 5-10 seconds, with a new location. You need to create a Network Link in Google Earth to this KML file that updates on a regular basis as well; as the KML file is updated on a regular basis, the Network Link picks this up, and moves the Google Earth view to the new location, where imagery is downloaded and added to the cache.

To create a Network Link in Google Earth:

1. Open up Google Earth, and from the Add menu choose Network Link.

2. Click the Browse button, and select the KML file used by the cache automation programs (CashBild.kml in the example below):

3. Select the “Periodically” option for the Time-Based Refresh, and then set the refresh time to 5-10 seconds (more on this shortly).

4. Check the “Fly to View on Refresh” box; this tells Google Earth to move to the updated location in the Network link file.

When the location is updated in the network link, Google Earth will fly to that location, download the imagery and display it. But if the speed with which it flies to the location is too slow, then the location may shift to a new one before all the data is downloaded. To make sure this doesn’t happen, go to Tools => Options, Touring tab, and set the “Fly-To Speed” to a higher value than the default of 0.180:

Here I’ve set it midway, which should be fast enough for caching, but not too fast for regular fly-to operations. You can experiment with this setting to find the one that works best for you.

The caching programs I’ll be reviewing have the following options:

• Set the bounds of the area you want to cache. Two of them require you to specify the NW corner of the area in latitude/longitude, and the east and south bounds of the area from that point in distance (meters or kilometers); the third requires the E/W longitude and N/S latitude bounds. Because of how they work, they don’t function directly over the poles, or across the 180W/180E longitude transition (essentially the International Date Line)
• Set the spacing for successive points. This sets the distance that the view moves between one point and another. It would be convenient if the programs calculated the correct value for you, so that every bit of imagery in your designated area was covered, but they don’t. You have to set this manually, and the correct value will depend on the altitude the views are downloaded at (i.e. the image resolution).  Set it too small, and you won’t miss any imagery, but it will take longer; set it too large, and you might miss some areas. I’ve found that a reasonable rule-of-thumb is to set the spacing about 10-15% smaller than the altitude you set (see the next item), and then watch the first few Google Earth views to see if you’re getting enough overlap between successive views. If you’re getting too much overlap, increase the spacing; no overlap, decrease the spacing. You can also set the eye altitude you plan to use manually in Google Earth, and then use the Ruler tool to manually measure the field of view in both E/W and N/S. Take the smaller of the two values, decrease it by about 10-15%, and that should be a satisfactory spacing
• Set the altitude. Two of the programs let you set the altitude/elevation over terrain (Eye alt – elev in Google Earth), while the third requires you to set that manually. The lower the altitude, the higher the resolution of the imagery downloaded to the cache, but the more time it will take and the larger the final cache size. For some areas with very high resolution imagery, the eye altitude will need to be no more than a few hundred meters to capture the highest resolution imagery. For other areas where only Landsat imagery is available, this could easily be set to a kilometer or two. If you want the cache to contain the imagery appropriate for varying heights, you’ll have to run these programs with different eye altitudes for the same area. For GE Cacher, you’ll need to
• Set the update time.Two of the programs update the KML file with a user-defined time interval. The longer the time interval, the longer it will take, but too short an interval may not give Google Earth enough time to download all the data. The appropriate interval will depend on your Internet connection speed, and how fast the Google Earth servers can provide the data. Start with about 10 seconds, and then adjust it up or down based on whether all the data for a view is downloaded before it moves to a new view. Important: You should set the Network Link Time-Based Refresh interval to be no longer than the update time, otherwise you might miss some of the position updates. You might even try the shortest refresh interval available, one second, and see if that works for you; too often shouldn’t cause a problem, too seldom will. For the third program with a fixed update interval, GE Cacher, you should definitely set the Refresh to one second.

Once everything is set up, all the programs will have the option to start the caching, and manually pause or stop it before it’s completed if you want. As with the manual caching process, you might want to empty the current disk cache before starting, and set the memory cache as small as possible. When you’re done with a caching session, you should either uncheck or delete the Network Link to the KML file; otherwise, next time you run Google Earth, it will go to that file’s location/view and keep returning there regardless of where you want to see. If you want to save the cache data, copy the files from their location to another one for safekeeping.

Next up: reviews of the Google Earth cache automation programs.

When you view a location in Google Earth, it caches the imagery data (but not the elevation data), storing it locally so that as you scroll back and forth across different areas, it doesn’t have to download the same imagery over and over again from the server on the Internet. There are actually two sets of locally cached data:

Memory Cache: Data stored in system RAM. This information can be retrieved very quickly, but vanishes when you exit the application.

Disk Cache: Data stored on the hard drive, and which persists after program exit. So if you have data cached for a particular area in Google Earth, exit the program, and then return to the same area the next time you use Google Earth, GE will load the imagery information directly from the hard drive cache – a lot faster than having to download it again over the Internet.

The size of these caches can be set in the Tools => Options section, under the Cache tab:

The maximum values are the ones above, 500MB for memory cache, and 2GB (2000MB) for disk cache; apparently, Google doesn’t want people to download the entire imagery dataset for the whole planet to their systems :). If you’ve got the disk space, and most modern systems do, you should definitely set the disk cache size at this maximum value. Optimum value for the memory cache will depend on how much RAM you have installed on your system, and how much your OS uses. If you have a system with a relatively small amount of RAM (512 MB to 1 GB), making this memory cache too large might impede performance. On most modern systems, with 2GB or more of RAM, the maximum memory cache size of 500MB shouldn’t cause any problems.

Disk cache data is stored by default in the C:\Users\your_user_name\AppData\Local\Google\GoogleEarth directory in Windows Vista.If you can’t find it there, you can determine the actual location using the registry entry HKEY-CURRENT-User/Software/Google /Google Earth Plus, under the CachePath entry. There are two files: dbCache.dat which contains the actual data, and dbCache.dat.index, which presumably is the index file for the data. You can clear out all the disk cache data using the “Clear disk cache” button, which removes all the data but keeps the files there. To delete the files completely, you need to either log out of the server (File => Server Log Out) and then click the “Delete cache file” button you see above, or exit Google Earth and delete those two files directly from the cache directory. You can also copy them to another directory to hang on to them.

Why would you want to delete or copy these files? Two reasons:

1. If you’re having crashing or graphics issues with Google Earth, Google recommends deleting the disk cache files as one possible fix.

2. You can “pre-cache” a set of data for a particular area of interest, then copy them to a different directory on your hard drive. Then, when you want to look at this area in detail in Google Earth, you can copy these files back to the original directory, and Google Earth will access this data from the hard drive, a lot faster than downloading it again from the Google Earth server. If you have a decent Internet connection all the time, this is likely to be more trouble than it’s worth. But if you have a slow connection, this can speed things up enormously. And if you’d like to use Google Earth on your laptop, in a location that has no Internet connection, using cached data is your only option; Google Earth can access and use cached data when no Internet connection is present or available. You can copy this cache data from one computer to another as well.

Note: This only caches image  terrain data from Google Earth, not data from KML/KMZ files. If the KML/KMZ files are self-contained, you can load them into Google Earth and view the data, but if they’re network links, and you’re not connected to the network, the data in the links won’t show up.

Coming up next: how to cache Google Earth data manually.