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:
If you’re having crashing or graphics issues with Google Earth,
Google recommends deleting the disk
cache files as one possible fix.
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.
The last post covered the basics of
the Google Earth disk cache, and how
it lets you use Google Earth even when
you don’t have an Internet connection.
Today I’ll talk about the simplest
method to create a Google Earth disk
cache file for an area – doing it
manually
To create a localized Google Earth
cache file manually:
Start up Google Earth while connected to the Internet
Go to Tools => Options, and select the Cache tab:
Click the “Clear disk cache” button to remove all data from the cache and
cache index files. This maximizes the
available amount of space for your
cached data, so that you can cover
more area with the cached data.
Set the Memory Cache Size to the minimum allowable value of 16MB; this
will force Google Earth to put data in
the disk cache more quickly. Click the
“Clear memory cache” button, then exit
the Options screen
In Google Earth, navigate to your area of interest.
Set your viewing altitude (the “Eye alt”) for the desired imagery
resolution. Higher altitudes mean
lower resolution, but the data will
take up less space. Lower altitudes
give you higher image resolution, but
at the cost of more disk space. If
there are major changes in elevation
over the area, you may have to change
your altitude to match them to keep
your relative height; if the terrain
elevation drops, but your “eye alt”
stays the same, the image resolution
will decrease because you are further
away from the surface.
Wait for the “data loading” indicator at lower right to show that
all the data for that area and
resolution has been downloaded; it
will display as a solid light-blue
circle when downloading is complete
for that area. You’ll also see the
imagery change from blurry to sharp.
With the mouse or cursor keys, move the view to an adjacent area that
overlaps the first area at least
partially, and wait for the “data
loading” indicator to show once again
that all the data has been downloaded.
Repeat this process until you’ve covered your entire area of interest.
If want data at several different resolution levels, you could repeat
steps 5-9, but at different viewing
altitudes.
When you’re finished with steps 5-10, you might consider zooming all
the way in to the closest level at one
point, then all the way out to the
planetary view level. It seems to me
that this prompts Google Earth to
write all of its data cached in RAM to
the disk cache, but I could be wrong
about this. It also seems as though
there can be a time delay between
viewing a location, and when it’s
cached to disk, so waiting 10 minutes
after you’ve finished covering the
area of interest might be a good idea.
You now have a set of Google Earth
data for that area saved in the disk
cache. If you take your computer
offline and start up Google Earth, it
will tell you that it can’t log on to
the server and download data, but it
can still read in whatever data it has
in the cache. You can copy this
cached data (see this previous post
for where to find it) into a different
directory, and then copy it back into
the cache directory when you need it,
or even copy it to a different
computer.
For small areas, manual caching of
data won’t take too long, but for
larger areas, manually scrolling to an
area, waiting for the data to
download, then scrolling to another
area gets old very quickly.
Fortunately, there are several free
applications that can partially
automate the process, and I’ll cover
some of those in the next few posts.
Best Answer
Have a look at the Plex.Mark! plugin.