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KML and supports the overlay of KML files and their compressed KMZ versions, as provided by the underlying Google Maps API. While this is a very powerful feature, there are some severe limitations and there is virtually nothing can do to "fix" problem KML files -- I just pass the URL to Google and they figure it out or fail to display anything.

Not all KML files are valid. Google provides a table of KML elements supported in Google maps at KML Elements Supported in Google Maps. I get no notification if a provided URL is an invalid KML file, so I can't provide any feedback or error messages back to you.

Note that large and/or complex KML files may not display properly.

I've noticed that Google seems to cache the KML files, such that if you change the contents of your KML file, it may take awhile before your updated version is used as the overlay. Again, nothing I can do about that. However, if you change the name of your KML file, that would force Google Maps to retrieve the new data.

Finally, note that the URL file you specify must be accessible by Google, without any passwords, so you'll need to upload your personal KML file to a web server somewhere.

Even so, this feature can be fun. Try

and see the GPS track of my first Cross-Country Solo flight (originating in the San Francisco Bay Area). Or:

for some key USA Civil War battlefields.

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