Ashton elevation line data

Digital Elevation Model data record the terrain height variations from the processed point-located data recorded on an airborne geophysical survey.  The aircraft altimeter data records the height of the aircraft above the ground and the aircraft GPS records the height of the aircraft above the ellipsoid.  Subtracting the two values enables the height of the terrain beneath the aircraft relative to the ellipsoid to be calculated.  This ellipsoidal terrain height is corrected for the variation between the ellipsoid and the geoid (the n-value correction) to produce terrain heights relative to sea level.

数字高程模型（Digital Elevation Model，DEM）数据源自经处理的机载地球物理勘测点位定位数据，用于记录地形高程的起伏变化。机载高度计数据记录飞行器相对于地面的高度，机载GPS则记录飞行器相对于参考椭球面（ellipsoid）的高度。将两组高度数值相减，即可计算出飞行器下方地形相对于参考椭球面的高程。通过参考椭球面与大地水准面（geoid）之间的差值（即n值改正）对该椭球面地形高程进行校正，最终得到相对于海平面的地形高程。