Theme 1: The method of two orthoimages
	Errors in a digital terrain model (DTM) can be estimated from parallaxes of corresponding points measured in two overlapping orthoimages. Orthoimages are derived from two overlapping aerial images using the tested DTM. Corresponding points in orthoimages are found by image matching. Shifts in positions of corresponding points can occur due to: Errors in orientation parameters Errors in DTM Provided correct orientation of aerial images, errors in DTM can be calculated according to the formula: dh    height error h    flying height b    length of base dX = XL - XR    measured parallax