Google Earth Accuracy

Google Earth is an amazing source of data.  The intuitive ease of use combined with an extraordinary collection of data (including aerial photos, road networks and even user created information) has made it popular for all.

You may have used Google Earth and wondered how accurate it is.  As you look at Google Earth you can see some areas where the information doesn’t quite ‘fit’.  Examples I’ve seen include kinks in straight roads areas where the heights are patently wrong (ramped airport runways and sloping harbours).

So where do these errors come from?

First of all there’s the easily apparent errors in the imagery.  The kinked straight road I mentioned above was at the join between 2 swathes of images.  When aerial imagery is captured the scale is different from the edges to the middle.  There are ways to correct these errors (called ortho-rectification), but they are costly and not perfect.  I don’t know if the GE errors are due to these imperfections or possibly GE doesn’t use orth-rectified images at all.

ELEVATIONS

The other error is in the elevation information (called digital elevation model (DEM) data).  After a bit of research I’ve found that the original DEM data was captured by a space shuttle mission in 2000.  The data has errors in the actual heights captured (16m was the target accuracy), the coarseness of the data (30-90m spacing) and the fact that missing portions of data were interpolated.  There have been updates to the data to increase the accuracy, but there’s scant information about the areas where the increased accuracy is available, and where future updates will be provided.

The software that we use (AutoCAD Civil 3D) has the ability to import imagery and height data from Google Earth, and we have compared GE height data on some of our development sites and our conclusion is that it’s good for an indication of height/slope and aspect of land, but not sufficient for detailed design of any sort.

LICENCING

Google Earth is free for non-commercial use, but anything beyond that requires the purchase of upgrade licences.  We are licenced to use Google Earth, but the licence we’re under states we can’t pass the data on to you.

So in light of all this, if you require topographical information for your development, give us a call.  We have access to a variety of other data sources, and if necessary, we can take the topo measurements ourselves.

We would be happy to talk to you about your specific requirements.

SOURCES

http://blog.121pcs.com/2008/11/srtm-data-and-google-earth/

http://www.gearthblog.com/blog/archives/2007/06/google_earth_more_re.html