(33) 
Typical exposures are from 1/100 to 1/200 of a second at f 11 or f 8. Results 
of a statistical study of scene brightnesses were reported recently 20 . Photo 
graphic altitudes range from 7,920 ft. above the terrain for forestry to 16,000, 
20,000 or even 35,000 ft. above sea level for topography. The temperature 
at the camera is normally controlled to 68°F (20°C). The Photographic Survey 
Corporation controls to 65°F and uses a humidifier attached to the magazine 
which maintains the humidity inside the magazine at about 50 to 60%. 
England 
All camera mounts are supported on either rubber or sponge rubber. The 
Directorate of Colonial Surveys states that their A-11 and Type 46 mounts for 
the K17 and F49 cameras have natural frequencies of about 700 cycles per 
minute and mean angular velocities in service of about 0-01 radians per second. 
They have a stabilized mount under test and are constructing a tilt indicator. 
The stabilized mount is very bulky and heavy, and the tilt indicator is likely to 
be preferred and expected to give greater accuracy. Camera port glasses have 
been used for most of the photography but a significant amount of work is done 
without them. Tolerances for them vary but may be as low as a maximum 
wedge angle of 4 seconds. 
Photography is generally done only in clear weather, but some work at 
relatively low altitudes has been done under high cloud. Worst conditions 
under which photography is carried out is under a complete overcast or in clear 
weather with haze. Exposures are generally determined by means of tables or 
a calculator based on solar angle. Typical exposures range from 1/50 of a 
second at f 5 - 6 to 1/200 of a second at f 11 for 6" wide angle lenses and to 1/300 
of a second at f 11 for longer focal lengths. The Directorate of Colonial Surveys 
have an auxiliary camera used to photograph a thermometer, an air speed indi 
cator, a Ivollsman altimeter, an artificial horizon, a watch, and a Veeder counter. 
Where data is available temperature at the camera has varied from 25°h to 
82°F and humidity from 15% to 78%. Photographic altitudes range from 1,000 
ft. to 27,500 ft. 
Finland 
The Zeiss Aerotopograph's special mount for the horizon camera is used. 
Supporting materials are springs and rubber. 1 he two horizon pictures and a 
level are used for tilt indication. No camera port glass is used. Photography 
is done only in clear weather with typical exposures of 1/100 to 1/200 of a 
second at f 4• 5 to f 6• 3. In special cases an auxiliary camera is used to photo 
graph a statoscope. Photographic altitudes are 2,000 and 4,000 metres. 
France 
The National Geographical Institute uses mounts supported on Apex 
anti-vibration rubber buffers. Universal mounts, as shown in Fig. 11 are adjust 
able for drift and for aircraft trim but not for roll. A panoramic mount, shown 
in Fig. 12, allows tilts of 30 to 60°. All the IGN aircraft use camera ports except 
the NC-701. These are made of plate glass with one surface re-worked to ensure