Precision Aerial Photogrammetry from Very 
Low Altitude 
By P. O. Fagerholm and A. Thunberg, Swedish Hydrographic Dept. 
There is a certain need for very accurate photogrammetric surveys of 
limited areas e. g, of civil engineering building sites, harbours etc. The 
accuracy in the coordinates of signalled points should be so high that 
the construction people may use them as reference points. 
As l:o ordinary photography season is short in many countries 
2:0 the need for surveys very often turn up after the ordinary 
season, 
3:0 good weather during the season must be used for more ex- 
tensive photography missions 
the aerial photography for the purpose mentioned above must be 
possible to perform when weather conditions are unfavourable. 
This is the background for the study. 
The investigation was carried through by the Swedish Hydrographic 
Department with very valuable cooperation in various ways from the 
Royal Institute of Technology (Photogram. Dept.), Geographical Sur- 
vey Office, National Road Board and the State Power Board. Project 
engineer has been Mr A. Thunberg from the Hydrographic Dept. 
Test field: (Vig. 1). A rather flat area (/\ z about 10 metres), 400 
X 200 metres, situated in front of the National Maritime Museum 
(Sjohistoriska Museet) in Stockholm with some houses, leafy trees etc. 
Signals: About 200 points were signalled, some of them with white- 
painted cardboard squares 6 X 6 cm on wooden sticks some painted 
white (6 X 6 to 10 X 10 cm) on flat rocks, pavements, etc. Due to 
delay of photography about 14 days — for other reasons than clouds — 
many cardboard signals were destroyed by children and dogs, but they 
were partly restored before photography. The signals on flat rocks were 
later on marked with drillholes. 
Aerial photography. On the 16th of November at 12:30 a.m. four 
strips were flown by the Geographical Survey Office at about 250 m. 
altitude and about 230 km/hour (140 miles/hour). Camera Zeiss RMK 
15/23; no 21051 with Pleogon Lens. Shutter speed 1/425 sec, f- 
stop 5,6. Film Ilford Aerial N (400 ASA). As the clouds were extrem- 
ely heavy and dark — as a matter of fact it was also drizzling — and 
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