How to create accurate, custom overlays!
There are thousands of wonderful overlays for many different parts of the world to use in lessons, but what do you do if there is a lesson you want to teach where no overlays are available? One option is to create your own custom overlays to share lesson specific information, quiz questions, etc.
In this tutorial we will look at an easy way to create custom overlays that will be pinpoint accurate and convey the information you want. You will need one, perhaps two specialized pieces of software besides Google Earth. The first piece of software is a graphic editor that supports layers. Some commercial examples include Adobe Photoshop, Macromedia Fireworks, Corel Paint or similar. While those software packages are very expensive, a free, equally as powerful software program available for Windows, Mac and Unix is Gimp and is the one we will use for this tutorial (the Windows version is pictured). You can download the sample overlays .kmz file to see how they look in practice.
The second piece of software is optional; a good screen capture utility such as TechSmith's Snag-it. There are free alternatives to Snag-it as well for Mac, PC or Unix. On the PC you can try 'Screen Capture' while for Mac you can try a little widget called 'Capture'. It is not required however as every operating system offers a way to capture what is on your screen.
In brief, the trick is to take a screen capture of Google Earth where you want your overlay to appear, open the screen capture with a graphics editor, then create a layer or layers to hold your overlay graphics. You then delete the screen capture layer, save the file as a transparent png file and presto, a custom overlay!
Custom overlays are relatively simple to create!
Now for the details...
1. Take a screen capture of the Google Earth centered on the area of interest. You can either do a 'Print Screen' or use a custom capture utility like the ones described above.
2. Open your layer enable graphic editor and open the file you created. Different applications will behave differently, but essentially they will open the file as the background layer. Add a new layer.
Make certain that the new layer is set to transparent so you can use the screenshot as reference!
3. On the new layer, use your paint tools to create whatever you wish using the available paint tools.
You may also paste graphic information from other pictures, etc. One thing to watch for is that text is usually added as a separate layer due to how graphic programs handle the different fonts. If that is the case, when you type using the text tool a new layer is automatically created.
When you are happy with the text you can merge the layers down if you wish but be careful not to merge them all the way down as you will be deleting the reference screenshot layer soon.
4. Once you have created your overlay with text, drawings, etc. and are happy with it, you may then delete the underlying background screen capture that you used for reference.
It may look a little disconcerting, but don't worry, this is exactly what you want.
5. You will then want to save your file someplace where you can find it again. You may choose to save the image either as a .gif or .png. Both .gif and .png formats support transparency which will allow your drawings to lay on top of the specific geographic features you wish to highlight. I recommend the .png format for overlays since .png supports not only transparency, but more colors, so your overlay will not look mottled or blotchy. Do Not save it as .jpg as jpg does not support transparency.
Be certain that when you save the image you specify that the .png retain its transparency.
6. Once you have saved your image, then switch to Google Earth and use the Add:Image Overlay function to add your graphic to the display.
Using the 10 control points, tweak the graphic so that it is properly aligned where you need it.
In the sample above, it could as easily have provided a quiz question such as 'Quiz Question #3: Identify the feature that is circled' or something similar.
You can name the overlay, add descriptive text, set the view etc. as detailed in other tutorials.
You will want to save the finished product as a .kmz (NOT .kml!!!) so that you can then distribute the overlay with the images remaining intact. Another option would be to place the graphic on a web server placing a link to the graphic should you anticipate needing to alter the graphic later.
A neat feature about image overlays is that they can hug the ground, providing a 3D element to your 2D image.
I hope that this Nifty Trick has given you some Nifty Ideas on possible ways to engage your students (or just have some fun:-)