given later, The flight crew consisted of 2 pilots, a navigator, a camera 
operator and an electronics engineer. 
The aircraft positioned at Kaduna on the 5th October, 1959. The 6th October 
was used to complete the unloading operations, the setting up and testing of the 
APR equipment, the survey camera, and the 35mm recording camera. By that 
evening all the equipment was fully operational and on the 7th October a test 
flight was carried out of four hours duration which proved invaluable both from 
the technical and navigation angle. The first productive sortie was flown the 
following day. 
Before and after any APR flying it is standard practice to carry out at 
operational height a 'zero' setting of the hypsometer over a known datum point. 
For this purpose the aircraft must be flown over a flat area (preferably a large 
water body) of sufficient size to include (with plenty of room to spare) the one 
degree spread of the APR beam. When the datum is a small area, the setting 
will normally require three separate runs flown on the same or reciprocal 
headings. There must therefore be sufficient ground detail in the vicinity of the 
datum point to provide the navigator with reference points on which to align his 
runs. 
A first requirement therefore was to select two such datum points, one in 
the north and one in the south of the area. After several trials, two suitable 
sites were located on wide stretches of the Niger River between well-defined 
banks where the flow was even and the surface uninterrupted by rock or sand- 
banks. At each place a continuous record of the water-level was maintained by 
the ground levelling parties during the whole period of the flying operations. 
By means of looped circuits each APR line was extended to include a run 
over one of the datum points, both at the start and end of each run. In the 
interval between completing the main run and the circuit required to bring the 
aircraft on track with the datum run, the APR record could clearly be no longer 
relied upon and was therefore switched off. This break in the record, though 
undesirable, is inevitable in areas where no coast-line or suitable flat body 
exists over which the runs can be extended without introducing a turn. It was 
found that the height indicated by the APR over datum points drifted at rates 
between 17 and 28ft. per hour. These figures agreed closely with the diurnal 
barometric changes reported by metereological stations at Kano and Kaduna 
airfield. The same drift rates were found to apply whether the aircraft was 
flying at 6, 000ft. or 20, 000ft. For future work it would appear that a record- 
ing barograph situated in or close to the survey area would be an advantage,