Georeferencing Images from Google Earth
Using ArcGIS
Google Earth has great high resolution aerial imagery available free to the public. It's a great GIS viewer program and it's easy to use, however sometimes I need to do more with the imagery so I import it into ArcGIS by georeferencing it.
This tutorial will explain an easy and accurate way to georeference images from Google Earth in ArcMap. I will be using 2000 TIGER/Line data in Little Rock, AR for demonstration purposes.
I start off with a base map of the area of interest in ArcMap using the TIGER data. This is not a neccessary step, I just want to show how the image will line up properly to the data set when it is finalized.
Next open Google Earth and zoom in to the area of interest. I usually zoom out one magnitude more than my area of interest in ArcGIS so I will have enough aerial image to cover the area.
Now I will pick out 4 locations on the Google Earth map that are equally distributed and are good visual landmarks. This will allow us to easly find them in ArcMap when we get the image imported. Try to use features that are permanent (i.e. buildings, intersections, bridges) and are easy to spot in the image.
We need to get the coordinates for each of these points while in Google Earth. Right-click your point and choose "Edit" to pull up the properties of the point. Check the "Advanced" checkbox and click on the "Location" tab to view the latitude and longitude.
Open Notepad or any text editor and paste in the coordinates for each point in a tab-delimited format like the text below.
station xCoord yCoord 4 -92.263082 34.742178 2 -92.282934 34.741464 3 -92.283584 34.752602 1 -92.265551 34.754582
Next turn all the points off by unchecking them in the Places list and click "File > Save Image". Save the image in an accessable place for ArcGIS.
In ArcGIS make sure you have the Georeferencing toolbar open. If not turn it on by clicking "View > Toolbars > Georeferencing". Next add the exported image to your view via the Add Data button. Browse to your exported aerial image and add it to your layers. You will see a prompt regarding missing geospatial information, just ignore it. You will not be able to see the image immediately on the map, it will only appear in the layers list.
Click on the Georeferencing toolbar and select "Fit to Display" The image will now appear in your view. If you can't see the image, it may be because it is under other layers in the map. I usually turn off any polygon data that may interfere with my georeferencing. In this case I will turn of Urban Areas and Water polygons.
Now we will add the text file we created earlier as a data layer. Click the Add Data button and browse to your pointLocations.txt file. It will appear in the Source tab of the layers window.
Right-Click the text file in the source list and select "Display XY Data". Match up the X and Y fields respectively and click OK.
This creates a new data layer called pointLocations.txt Events and inlcudes the 4 points on the map now. Right-Click the layer and select Zoom to Layer to view the entire points. I usually change the color of the symbol to something bright so I can see it clearly.
Next turn off all the layers so all you see is the image and your points. We will now begin to georeference the image. Using the Move tool in the georeferencing toolbar move the image so it lines up with your points. It may be a bit off as far as zoom is concerned but thats ok for now. Just orient the image so it is square with the points.
Using the Add Control Point tool start to match your image to your points. First you click on the aerial image where your point originated in Google Earth (In this example I will be using a railroad overpass that I spotted earlier as a landmark). Drag your mouse from there to the point in ArcGIS that corresponds to it. Your image will shift and the point should be on top of your image in the correct place.
Repeat this process for each point on your aerial until all points are matched up. You may have a small amount of residual, just play around with it until you feel comfortable with the outcome.
Once you have finished georeferencing the image click on the Georeferencing toolbar and click "Update Georeferencing". Next click "Delete Control Points". At this point your crosshairs will disappear. Your image is now georeferenced. If you look in the folder where your image is stored you will see several new files that are used to display the image in ArcMap.
