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Contour Elevations

I have received contours as outputs from my projects and I use a Phantom 4 to collect my images and they are not corrected with GCPs.

I was wondering how the elevation of the contours are calculated?  I compared the elevation value of contours to what the elevation is stated in Google Earth and they do not coincide.

How can I know if the contours from my projects are accurate to real world elevations?

Without ground control points, the accuracy of your project is limited to the inherent accuracy of the GPS satellites and the receiver in the Phantom.  This is really not a very good number.  

A bigger problem might be  the elevations provided  Google Earth.  Those are from a Digital Elevation Model (DEM) of the entire United States and should not be used as a reference source.  They are informational only.