This could probably be classed as technical post (gosh!), and is probably only of interest if involved in hosting content to be displayed in Google Earth or Maps etc.
I’ve been meaning to write this up for a while, basically a set of tips for setting up Apache 2.X for hosting content for Google Earth. The same tips can probably be used for other webservers but won’t begin to look at the various ways it could be configured… Also these could easily be placed in httpd.conf, but using .htaccess as the example as thats probably easier, and not all hosting providers allow access to the file. Much of the stuff here originates from YSlow, but adopted for what I know about Google Earth.
These settings assume you not going to change the content much, so is good for serving up large static content, or the ‘chrome’ for a large layer. Regally updated content is likely up be output from a script anyway so that can setup its own headers, I’ve also done quite a bit of work on getting PHP to output good headers, so will post that in a follow up post.
First and foremost is setting up the MIME type, to ensure the Google Earth is launched when a user clicks a link, so these lines are added to the .htaccess file:
AddType application/vnd.google-earth.kml+xml kml
AddType application/vnd.google-earth.kmz kmz
AddType application/xml dae
Next is the Expires header, which tells a UserAgent how long it can cache the content, we set nice future dates, which mean in the main the content will stay cached for a while, potentially saving quite a bit of bandwidth.
ExpiresByType application/vnd.google-earth.kml+xml "access plus 30 days"
ExpiresByType application/vnd.google-earth.kmz "access plus 30 days"
ExpiresByType application/xml "access plus 30 days"
ExpiresByType image/jpeg "access plus 180 days"
I’ve included JPG here, as it’s often used with GE, esp. how with the introduction of PhotoOverlay. The same idea can of course be extended if you use png files for example.
Now Apache out of the box supports sending Last-Modified headers, which means conditional GETs can be made later, which will only return 304 Not Modified. I’ve seen suggestion to turn off Last Modifed headers (if you have far future Expires as above) – but after testing, I would NOT recommend this.
Next is Etags, now YSlow recommends disabling these in most cases, and Apache useually comes with them on, and can easily be misconfigured, esp. on clustered hosting, so they need to be turned off. I’ve dithered on this one for a while, it seems silly to disable a feature that could potentially be useful, however the clincher here is that I don’t think GE will use them, and besides we can save a few bytes be not sending them in the first place.
I’ve no evidence if it helps, but if you have enabled on the fly GZip compression, then you might get a small boost by turning if off. As GE doesnt support HTTP-GZip compression, and KMZ and JPG are both well compressed. The only small benefit might be if you have a KML based loader, that might get downloaded by the browser in which case it can GZip’ed. So turn it off globally in this folder, but turn it back on for KML:
AddOutputFilterByType DEFLATE application/vnd.google-earth.kml+xml
As a final tip, the following allows you to upload a index.kml file and it will be served as the index for a directory, doesn’t help performance, but useful in a few cases,
DirectoryIndex index.kml index.html index.php
Get all of the above in one file ‘ere, use or don’t use at will