On GoogleEarth accuracy

Google Earth is becoming commonly used for many applications but what about it’s accuracy? Is a point in Google Earth (or Google map, since this is the same images) really is where it seems to be?

The first inaccuracy source is that Google seems to round decimal latitude and longitude to 10^-6. That’s about 10 cm on the ground.

The second source of error depends on the processing of each satellite or aerial image. This is a process called orthorectification and its accuracy can vary widely. This error will depend on the area you’re looking at in Google Earth. You probably won’t have the same accuracy in Europe and Africa as it can depend on the digital elevation model (DEM) of the area.

If you know precisely the coordinates of a particular point that you can see on Google images, you can evaluate the accuracy on this area. Let’s see an example with a reference geodesic point.

Reference geodesic point

Zoom to the maximum: with a perfect orthorectification the blue mark should be exactly on the center of the white square. This is not the case, so we know that for this area we have an accuracy of about 1-2 meters.

This accuracy can be considered as quite good already (not enough though to be used as a reference) but probably worsen in mountainous areas.

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