Google Earth Tutorial: Image Overlays

29Jun09

Google has been pushing to get more 3D models into the 3D Buildings layer, particularly representations of bridges.

Their implementation of the buildings can be problematic however, as the satellite imagery on the terrain below remains visible, including the image of the bridge. This is sometimes a problem for tall buildings, but it destroys the illusion of realism for bridges.

I created a bridge for Google Earth (St. Johns bridge in Portland, OR).  It is a part of the 3D Buildings layer, and if you visit the site in Google Earth, this is what you see:

Continue reading this post to see a tutorial on how to solve this.

To me, it’s kind of confusing. When I first showed this bridge to my wife, she’s like “whats with the part underneath?”

What can be done about this?  My solution is to use the Image Overlay feature to cover up the redundant image.  I do this with two parts, a ground overlay, and a shadow. The parts could be combined into one texture, though I like the flexibility of having them separate.

Follow the steps below to create your own image overlays for Google Earth.

Step 1.) In Google Earth, locate the area you want to overlay. For most bridges, this will be a small stretch of visible bridge over the water. In the example of the St. Johns Bridge I had a large area to cover and chose to break my overlay into 3 parts.

Turn off ‘terrain’, and fit the area completely within your visible screen area. Make sure you are looking straight down, and rotate your view so that the bridge is horizontal.

Step 2.) Press F11 or choose View -> Full Screen. This will maximize the google earth viewport, and provide the highest resolution. You need the area you want to cover fully visible, with a little buffer on each side.

On windows, press the ‘Print Screen’ button to copy your screen to the clipboard, or use a screengrab application.

On Mac, press Command-Shift-3 to make a screengrab. It will save the screenshot to your desktop

Step 3.) Open the image into photoshop or other image editor. On windows, you will need to create a new blank document and ‘Paste’ the previously copied image into the new document.

Crop your image to encompass just your bridge, with a slight border around it. For video card textures, multiples of 2 are used for the dimensions of texture. Google Earth seems to work fine with odd-shaped images, though it’s probably good practice to follow traditional texturing conventions. I recommend 1024×256 as the final dimensions of your overlay. It’s clear enough to look good relatively close up, but small enough to not overtax the computer.

Step 4.) Add a new layer, name it ‘Overlay’ and Paint! In photoshop, the most common method is to use the ‘Clone Stamp’ . Grab your source from the original screen grab, then select and paint onto the overlay layer above.

Use a broad feathered brush to keep a nice blend with the background.

Step 5.) When complete, your bridge should be invisible, and your overlay layer should look like a fuzzy blob. Delete or hide the original screengrab, to leave only the overlay layer on a transparent background. Save this as a 24-bit png file.

Step 6.) Insert into Google Earth. Locate your bridge area again, and click the ‘Overlay button’

Name Your Overlay and click ‘Browse’ to locate your .png file.

Step 7.) Position your overlay. While the overlay properties window is still open, position your overlay to cover the bridge. When your overlay is positioned correctly, choose ‘OK’ from the overlay window to save your overlay placement.

Step 8.) Add a shadow overlay, if desired. Because this is a soft generic shadow, the resolution can be very small.

Step 9.) Save your kml/kmz and distribute!

Look at the image below to see the progression:

To see this all integrated within Google Earth, click the image below. You will need the Google Earth Plugin:

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