Two points A and B situated 10 m and 40 m respectively above datum, are imaged on a vertical photograph, taken from an altitude of 2000 m with a camera of focal length 150 mm. The photo co-ordinates of the points about fiducial axis are measured as follows:

	x (mm)	y (mm)
a	+ 50.00	+ 100.00
b	- 100.00	+ 50.00

Calculate the horizontal ground distance AB (in kilometers).