The theme of orthorectified and satellite images’ precision of Google Earth is a record question in search engines; in these days where confusing precision with tolerance is as easy as losing the GPS in the taxi, it would be good to make a couple of analysis about whether or not to use these data for serious work.
Now there´s in Google different images provider, some of they announced that it will soon have more material after the launch of a new satellite with higher resolution and greater coverage per day.
From the satellite is called Worldview I, which operates in conjunction with a Quickbird satellite; the images became a pixel resolution of 50 centimeters (currently 1 meter) and with captures of 600.000 square kilometers per day, which before was currently obtained in a week.
This tends to confuse those who only focus on the final product (maps) and not in the intermediate survey products and its photogrammetric origin which is what let us not only having products but their accuracy and relevance. When using Google Maps material for local purposes it can be confirmed the inaccuracy, sometimes up to 30 meters, as a satellite image requires a dense first order network of local checks for rectification. It’s not that is a bad issue, the fact is that it serves for Geographic Web purposes which is the way Google Earth call this layer.
We understand than Google got very accuracy content in some countries, but for those who still want to make cadastre with ortos from Google Earth, here is an example of what you will find:
This is La Jaguita, Comayagua, in Honduras; note how is seen at the images’ junction the repetition of the same buildings complex, 36.51 meters to the Northeast.
We used two GPS Magellan Mobile Mapper, one as a base, other for lifting points and then making the differential correction, then compared to an orthophoto created with aerial photography with 5,000 flight three years ago; GPS data agree well with the ortho but not with any of Google Earth images, one of them is 23 meters northeast, the other at 19 meters, but the displacement vector has a different angle. In this respect it is not possible put into question a system that is the best that can have many of our villages where map data will not arrive in the next 10 years an to make matters worse is free, the important thing is that we are aware of their relevance and limitations. In order to be meditated, I leave this point in terms of Google Earth, that paragraph nobody reads and just press the “OK” button:
e) NO ADVICE OR INFORMATION, WHETHER ORAL OR WRITTEN, OBTAINED FROM GOOGLE OR ANY THIRD PARTY OR BY USING THE SOFTWARE SHALL CREATE ANY WARRANTY NOT EXPRESSLY STATED IN THESE TERMS AND CONDITIONS.