Anamalous Altitude Readings

I live in Charlottesville, VA in Westminster Canterbury of the Blue Ridge (WCBR) and use the general address of 250 Pantops Mountain Road.

I believe I am getting erroneous altitude readings that are being used by other applications like Contour Map Creator and GMaps Pedometer which perpetuate them.

I will attach 2 images. One shows the altitude readings for an on-campus area. The other shows an image of a 10 ft block retaining wall that holds back a slope with an approximately 5 ft drop from a sidewalk and the end of a building to the top of the wall. Google earth shows a 1 foot altitude difference.



Could this be caused by a tree canopy?

thanks,
baumgrenze

Unless the area has been professionally surveyed and the data has been shared with GM, topographical accuracy can be assumed to be within ~20 feet.
[ LINK ]

 

Thank you very much for the link.
It was so easy to fall into the false assumption that satellite data provide the altitude via parallactic shift.
Will it be 'one day soon' that the precise but high viewpoint of satellites will allow such calculations?
thanks,
baumgrenze

I don't see how to use parallax for measuring altitude .  Too many features would  not measure accurately via that method, and how would the computer program decide which points can be trusted ?
 

What may occur is that the existing topographic map of your location area is calibrated against sea level datum..   So you pick X points on your map, and work out the error in their sea level.. 
then  adjust the topographic info  up or down...  (perhaps some tilt.)   

So you can only hope for the government to release accurate topographic maps.   but these are  drawings made from someone estimating slopes when viewing from one position a long way away. subject to parallax.

Its up  to them to judge that they have good accuracy when doing it.

But  google may use street view cars to record GPS height info and so calibrate based on direct measurement at known locations (the roadway.)  . This is alright for your purpose of building blocks.. not any good for wilderness with no roads.  Or  hikers have GPS and the assumption is that that their track can be trusted and outliers (such as a person  beginning a hang glider or parachute descent. ) can be detected and ignored. So calibrations can be done by hikers position reports ?