Posted on August 2, 2007. Filed under: Google Earth, Image Overlays |
In this post you will learn how to use the image overlay functionality provided in Google Earth to import existing images into the 3D display and drape them over the existing terrain. Many image file formats can be displayed in Google Earth including jpg, bmp, gif, tiff, tga, png, and a number of others. What this means is that you can import USGS topographic quadrangles, enhanced aerial photography and/or remotely sensed images, historical maps, raster image files exported from ArcGIS and other GIS software, and many others. Google Earth provides the capability to easily accomplish this task. Before we get started please download the image that we will use for this exercise at http://www.geospatialtraining.com/GoogleEarth/AbajoPeak.tiff. Also, make sure you have installed Google Earth (version 4). Let’s get started.
Open Google Earth 4 and enter Monticello, UT in the Fly To tab on the Search panel. Zoom to this area.
Click the “Add Image Overlay” Button
The Add Image Overlay dialog box will appear along with hash marks on the GE display that can be used to move and stretch your image.
Give your image overlay a name (Abajo Peak) and click the Browse button. Navigate to the image file that you downloaded (AbajoPeak.tiff) from geospatialtraining.com.
Your display should now look similar to the image below. This .tiff image is a USGS quadrangle topographic map for Abajo Peak in Utah. You’ll notice immediately that this particular image has not been georeferenced. It is simply displayed in whatever area you happen to have navigated to in Google Earth. Next, we’ll georeference our image.
Click the Location tab and then use a combination of dragging the green markers around the image along with manually entering the coordinates until your North, South, East, and West coordinates appear similar to the figure below. Now, I should point out here that I’m providing the coordinates for this particular image. You will need to know the bounding coordinates for each image that you import into Google Earth so that you can correctly georeference the image.
We will leave the items on the View, Altitude, and Refresh tab on their default values, but let’s discuss these items first. The Refresh tab allows you to automatically set the image overlay up to refresh to the latest view of the image. This is used when the image that you’re placing updates frequently, but in our case the image is static so we don’t have a need to refresh our image. The refresh functionality is useful when dealing with data such as weather radar images that are continuously updating. Refreshes can occur based on a time interval or based on a view that has changed. The altitude tab can be used to display the image a set distance above the ground for situations where you want the image to “hover” above the terrain. Again, in this case we do not need our image to hover, but it can be useful in some instances. Finally, the View tab can be used to set the current camera properties for the 3D viewer.
Now that we’ve defined the position of our image your GE viewer should appear as follows:
There are a number of things that we can do to enhance this view. First, we’ll set a transparency value on our image so that we can “see through” the image to the underlying terrain and vector data. With Abajo Peak selected in the Places panel, use the transparency slider to apply transparency to the image.
Your image should now have a transparency set. You may need to experiment with the slider until you determine the best setting. Notice that as you move the slider, the underlying terrain becomes visible.
Now, let’s turn on the Terrain layer in the Layers panel to drape the image over the underlying terrain.
Notice how the image now conforms to the underlying terrain.
Finally, let’s tilt the display and experiment with the orientation to get a better view of Abajo Peak. Use the navigation tools provided by GE to get a better look at the image and underlying terrain.
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