the average scale of photography was 1/40, 000.
All other flying was carried out at altitudes of 10, 000ft. and below. Three
tie strips of combined photo/APR were flown at.approximately 9, 000ft. across
the main N-S photographic lines and included several trig.points which had been
premarked for identification. These strips as well as the high-level runs 12,
13, 14 subsequently proved to be unnecessary and were never used. The
remaining flying involved the APR record only, together with its associated 35mm
locating camera: these runs were flown at heights varying between 6, 400ft. and
10, 000ft. a.m.s.l. along the lateral overlaps of the survey cover.
The pattern of the flying carried out is shown in Diagram A, the APR lines
actually used for the subsequent photogrammetric work being shown in Diagram
B. More detailed information on flying heights, drift, wind force etc. is given
in Appendix I at the end of the paper.
The precision of the navigation could scarcely have been improved on, and
no difficulty seems to have been experienced by the navigator in flying the low-
level APR lines where they were required in the centre of the lateral overlap.
The very small camera tilts (maximum forward 1i? and lateral 1$9) undoubted-
ly contributed to the accuracy and consistency of the APR readings.
Very few troubles were experienced with the equipment. Initially the auto-
matic gain control of the receiver failed to operate under conditions of maximum
reflection: this was traced to a hair-line crack in a printed circuit. A
magnetron had also to be replaced. Near the end of the survey the hypsometer
record showed a periodic variation varying from + 1.5ft. to + 5ft. in amplitude;
both period and amplitude varied with the temperature of the outside of the
hypsometer. The exact cause of the defect was never established but it proved
possible to minimise its effect by drawing a mean line through the small
periodic variations where these were present in the record. For all practical
purposes the error introduced on this account was negligible in the final analysis.
Otherwise the equipment gave no trouble whatever and the 'cleanness' of the
final record is evidence of the stable conditions, and the small amount of
extraneous noise.
The best operating period for the APR runs was found to be before midday;
the afternoons almost invariably brought the formation of cumulus and a marked
increase in turbulence. The full programme of APR and survey photography
(including the extra flying carried out as a precaution) was completed in nine-
teen days, of which five days, or twenty-six percent, was required for the APR
lines flown along the lateral overlap of the vertical photo cover. The total area
