July and August during the rainy season following the line of existing roads. During
August and September ground control was provided for these strips to a pattern
of four levelled points every seventh overlap and two points midway between. A
large part of this control was in practice a by-product of essential levelling which
was required to connect the bench-mark at Bussa to a datum near Samanage at
the extreme northern limit of the reservoir.
The low-level control strips (flown between 3, 000ft. and 5, 000ft, a.m.s.1.
Nos.10-31 in the Diagram) were then set up in the Multiplex which was used to
give out photogrammetric spot heights on the identified positions of the selected
35mm principal points at intervals along each APR strip where it crossed a
control run. The zones of control so provided are indicated by open circles on
Diagram B and include additional heights given out by Wild A8 using the
existing large-scale cover of the Kurwasa Dam site (runs prefixed "A" on the
Diagram) and those of the Yelwa Town site, which was controlled for close-
interval contouring.
As can be seen from Diagram B all but one of the N-S/APR lines were
held by at least three and,in some cases four,such control zones, so that it was
possible to reduce the line initially, using the terminal controlonly, and then
to compare the APR values with the intermediate control. Barometric condit-
ions during the survey were remarkably stable and Henry's formula when
applied to the four or five APR strips having the greatest variation in the
barometric pressure gradient gave a maximum irregularity of only * 1 ft..
The fact that the residuals at the groups of intermediate control points seemed
to be entirely random was considered sufficient further evidence that no
sudden changes occurred in pressure gradient along any strip.
The final adjustments made to the profiles were therefore carried out as
linear corrections applied between the terminal control points with later
datum corrections to minimise the small discrepancies at check points in the
zones of intermediate control. The table below gives the distances bridged
by the APR profiles and the mean residual errors at intermediate zones
before datum correction :-
