Tom's Blog

Sunday, March 9, 2014

Simple georeferencing of Google Earth air photos

As I discussed in an earlier post, Google Earth has some up-to-date imagery that is not available through other sources. I found the image of the Black Earth Rettenmund Prairie area especially useful once I figured out how to tweak the image.

Here is the procedure I used:

  • With Google Earth select just the area you want. I turned off all the labels and icons so that I had a pure image. Also, F11 will give you a full screen.
  • Click on the icon for Save Image and put the image in a folder.
  • Open the image in Photoshop (or other software) and fix the brightness and contrast. This was important for me because my image in Earth was dark. However, all the detail seemed to be there so I did not lose anything by making these adjustments.
  • Open ArcGIS, using a reference air photo for the same area. I used the 2010 ortho air photo for Dane County, available from
  • Open the Georeferencing toolbar. I find it preferable to turn off the Autoadjust button.
  • Bring  the Google Earth image into Arc. If necessary, click Fit to Display to bring the image on the screen with the reference image.
  • Choose 3 or 4 good points on the photo to be referenced. Using Drawing, put a red dot (marker) at each location. (See my photo, which has 4 red markers.)
  • Use the Arc Effects tool to adjust the transparency of the Earth image. Move it around so that the points to be used for georeferencing are near those of the reference image. You want to be able to see the reference points on both images at the same time.
  • Using the Georeferencing tool, put in control points. Click on each red dot first, then click on the same location in the reference.
  • For a small image, such as the one I have here, only 3 or 4 control points are necessary.
  • Click Update Georeferencing in the Georeferencing tool. The Earth image should line up with the reference image.
  • Voila! 
  • The center of the image below is from Google Earth and the border contains part of the 2010 ortho-photo.

The details of the image, including when it was taken, are given when you click View>Status Bar in Earth.

My photo here is fairly small because of limitations of the web, but my Google Earth image was so sharp that I could see every fire break, every footpath, and every fence line. I was able to draw the best map of Rettenmund Prairie I have ever had.

I don't know the source of this October 2013 image, and I could get no information from the Wisconsin State Cartography Office about it. I don't guarantee that all parts of Dane County have such a good image.


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