not for the purpose of correcting the APR results but rather to keep a check on 
sudden variations in pressure conditions that would necessitate reflying any APR 
line. 
Reduction of the APR profiles by means of these datum points was in fact 
never necessary since other more reliable data was available. It is however of 
interest to note that the subsequent analysis of line 6/7 using the datum points 
showed errors of +11ft. and +12ft. at the terminal points of the main run (i.e. 
where it crossed the ground control points). 
Adequate co-ordinate control for establishing the scale of the mapping was 
already available in the area, and no attempt was made therefore to calibrate 
the APR for the absolute height of the record, since this is only required if it 
is to be used for horizontal scaling of the overlap. In this connection it is 
worth noting that absolute calibration should be carried out twice a day, at the 
start and finish of a sortie. Consequently if the base airfield is unsuitable, and 
if there is no alternative within reasonable distance of the mapping area, a great 
deal of wasteful flying might be involved which could result in this application of 
APR being of doubtful value, and in certain conditions altogether impracticable. 
As already explained, absolute calibration was unnecessary on the Niger Survey. 
Doppler when combined with APR is only likely to be of value where measure- 
ment of the drift angle is critical in determining the slope of the isobaric surface 
by Henry's formula. A factor in determining the correction to be applied by 
Henry's formula is the sine of the latitude which in low altitudes approaches 
zero. In this case the latitude of the area was only 10°N. (i.e. sine 10° 0.2), 
and it was not considered that the additional accuracy to be gained in the reading 
of the drift angles by the introduction of Doppler would make any material differ- 
ence to the slope correction: Doppler was therefore omitted from the record. 
In practice the barometric conditions in the area proved to be so consistent that 
the variation in drift angle on any one run was insignificant and the maximum 
irregularity in slope of the isobaric surface experienced on any line was shown 
to be negligible. 
Navigation throughout was by visual methods, employing (for the combined 
survey-photography/ APR lines) the standard techniques of establishing master | 
runs and then building up on these by reference to the lateral overlap of strips 
already flown and processed in the field. These high-level photo/ APR runs which 
included one tie strip (No.1) were made at an altitude of approximately 20, 500ft. | 
above mean sea level, the camera used being the Wild RC8 fitted with Aviogon 6" 
lens on 9" x9" format. Since the mean ground level was in the region of 500ft.