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Calibrating Google Earth Images

Note:

If the Google Earth Plugin no longer works on your machine,  see the topic on using Google Earth Pro from AirPhotoSE and follow the instructions on calibration there.

AirPhotoSE offers five grid systems for georeferencing of Google Earth images, three which use the latitudes and longitudes normally provided by Google, one which implements an arbitrary X-Y coordinate system and one which implements the European Petroleum Survey Grid (EPSG) grid library of over 3000 national and international systems.  They are selected on the Options File and Grid Preferences page.



Google uses a simplified version of the WGS84 / ETRS89 datum.  This was determined by NASA with many accurate satellite measurements.  The datum information contains an approximation to the ellipsoidal shape of the Earth.  Since the 18th century, many different astronomical measurements have been made and different datums applied by cartographers and geodesists in different parts of the world.  The national and international mapping systems of which there are thousands in existence have survived for historical reasons, and because changing land ownership survey at the large scales needed for this purpose is very expensive when done on the ground.



The UTM-WGS84/ETRS89 grid based on the NASA satellite measurements is accurate to better than 1 meter over the entire surface of the Earth. See the Calibration topic for more details.  It requires entry of two numbers for the East, North coordinates, a Zone number between 1 and 60 which covers the planet in meridian strips at 6 degrees of longitude and sign values for locations north or south of the Equator and east or west of the Greenwich meridian.

  

The position of a point in Google Earth can also be entered directly in latitude and longitude in the WGS84 datum either in decimal form or in degrees, minutes and decimal seconds.

   

If one of the thousands of national grids is required, the EPSG database must first be enabled in Options. 
       


Then, from the main menu, click on EPSG and follow this with a click on Select Grid.



A selection window appears showing the names of the many grids in alphabetical order along with the unique reference code which the EPSG committee has assigned to it.  Details on how to use this selection window are given in the EPSG Grids topic.



EPSG grids may be used for localisation and georeferencing by searching in AirPhotoSE's Google Earth implementation with Find in Grid  which converts a local or national grid to the datum and form needed by Google Earth. 



When the OK button is clicked, Google Earth "flies" to the point of interest and displays it at a simulated altitude of 2000 meters.  Zooming in or out with the mouse or dragging the image permits locating a desired point with high accuracy, depending on the quality of the Google image database at that point.  When the image is saved, it is calibrated (georeferenced) in the user-chosen grid system. 

Note:

User-determined X-Y grids are incompatible with all others and are not interchangeable or convertible to any of the other grid options.  UTM, Decimal Lat/Lon and Degrees may be used interchangeably.  EPSG grids are unique and can not be modified, although a different grid may be selected for the same area where applicable.  A rough check is made when saving a Google image to make sure that the area shown is within the boundaries of the applicability of the grid chosen, and if outside it, a warning is given, calibration fails and the image is saved with pixel coordinates only.