Wednesday, March 03, 2010

Using Superoverlay for Your Google Earth Project

You know those times that you have a large scanned map of your field area or project site and you want to use Image Overlay to add the map to a Google Earth project? If you want the file size to remain small and portable, you need to re-sample the map and lose all your useful information, or you can try to keep the map as the original file size and then you have an enormous KML to share. What to do? Use Superoverlay!

Superoverlay, developed by Valery Haronusov, has been around as long as I've been dabbling in Google Earth, I just never gave it a try. Until now.

I've been working on a Google Earth project based in the Grenadines. One of the files that I needed to add to the project folder is a nautical chart (shown above) that in digital form is a 110 MB file. It would be too unwieldy - both geographically and in digital file size - to try and do an Image Overlay for this file so I needed to make it smaller and so used Superoverlay. Superoverlay cuts up the raster file into rectangular chunks and creates smaller raster tiles. This makes it so that when you zoom in, the tiles that draw are more clear and have the nice resolution that you want maintained. You can do this by hand. But why?

I followed the Superoverlay instructions, which are great. Because the chart was in UTM, I needed to re-project to the WGS84 coordinate system. Superoverlay uses the world file (i.e., *.jpx, or *.tfw) for spatial reference. The spatial reference is read directly from the world file. The output I got for the KMZ file of the nautical map was 10.5 MB.

There was one slight problem. The nautical chart looked great but I have other files in my project that need to draw over the chart, and the chart was not drawing on the bottom. This had something to do with the DrawOrder tag. I contacted Valery to see how this could be fixed and he found this ordering issue to be a bug and he fixed it!

If you want your Superoverlay map to draw under your other files, use a negative number in the Start draw order box (circled in green above). I found that -20 gave me the order that I was looking for. By the way, it may seem like you can just reorder your KML files in the folder and place the Superoverlay map at the bottom and then this problem is fixed (like in ArcGIS). It doesn't work that way in Google Earth, unfortunately.

Shown above is the mapping of the shallow marine habitat near Mayreau and Union islands with the nautical chart layered underneath. Just the way I wanted it.

If you're interested in GTOPO30 maps for the world (shown below), one has been created by Valery using Superoverlay. Click here to find it.

This is how to make the file shown above. So, if you have super huge raster datasets, what do you do? Use online storage. Here is some useful information from Valery on the process of these larger Superoverlay projects:

"I use Dropbox and Amazon S3 storage for tiles. For huge rasters I use Global Mapper + Superoverlay or KML2KML. Global Mapper makes large tiles (2048 x 2048) from huge and Superoverlay makes small tiles (256 x 256) from large. And with the last one, we can copy to cloud web storage (S3) an absolute "authoring" independent low cost solution. I use Cloudberry as an Amazon S3 explorer."
Thanks, Valery! This is all great stuff and so helpful.

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