The curvature of the Earth begins about 5000 miles down (oops, that is near the center of the Earth). To "SEE" the curvature of the Earth you must start at about +13,000 feet above Mean Sea Level (although the curve is very slight) and to see a definitive curve to the Horizon you should be somewhere near 45,000 feet AMSL. These elevations can be checked using a good understanding of Geometry, or perhaps a clear, unrestricted view from any altitude. JMHO,,