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DISCUSSION ON THE GENERAL REPORT OF SUB-COMM IV-3 163 
fragt, der durch die Aufnahme bei bedecktem 
Himmel erzielt werden kann, statistisch erzielt 
werden kann, dann kann ich zwei Zahlen nen- 
nen, die für Deutschland gelten. Wir haben, wie 
Dr Härry soeben erwähnte, die Zahl von etwa 
25 Tagen im Jahr, bei denen die Bewölkung ein 
Achtel oder kleiner ist. Wir haben in Deutsch- 
land im Durchschnitt einiger Jahre, für die wir 
das genau verfolgt haben, 22 Tage festgestellt. 
22 Tage im ganzen Jahr, an denen der Himmel 
mit ein Achtel oder weniger Wolken bedeckt ist. 
Wenn wir jetzt die Forderung stellen, dass der 
Himmel zumindestens sieben Achtel mit Wolken 
bedeckt sein soll, dass aber die Wolkenhöhe 
natürlich nicht niedriger sein darf als die Flug- 
höhe, weil wir ja durch die Wolken hindurch 
nicht photographieren können, dann ergab eine 
statistische Auswertung, die wir mit dem Deut- 
schen Zentralamt für Wetterdienst ausgeführt 
haben, dass wir für zwei, und zwar für die 
beiden in Deutschland hauptsächlich vorkom- 
menden Flughöhen, 2.500 m und 1.900 m, dass 
wir für diese beiden Flughöhen pro Monat be- 
reits zu 19 oder 15 Bildflugtagen kommen. Das 
heisst, dass im Durchschnitt jeder zweite Tag 
für den Bildflug geeignet ist. Das scheint ein 
ganz bedeutender Fortschritt zu sein, den man 
in der praktischen Luftbildaufnahme beachten 
sollte. Danke. 
Herr Dr Hànnv: Ich danke Herrn Prof 
Schwidefsky für seine ausserordentlich interes- 
santen Ausführungen, die für uns vom Stand- 
punkt der praktischen photogrammetrischen 
Produktion aus sehr ermutigend sind. Und ich 
móchte gleich übergehen und Herrn Fagerholm 
bitten, uns seine Ausführungen zu geben über 
eine konkrete Durchführung einer Aufnahme 
und Auswertung für Grossmasstab-Photogram- 
metrie aber eben auf Grund von Luftbildauf- 
nahmen, die bei extrem ungünstiger Beleuchtung 
ausgeführt wurden 
Dr P. O. FAGERHOLM: When Dr Hürry a 
couple of days ago asked me to present a report 
on this investigation I had a feeling that our little 
investigation might be of too little value for 
such an eminent group as this. However, espe- 
cially since having heard what Professor 
Schwidefsky said about these investigations in 
aerial photography when the sky is cloudy, I 
think it might be of interest to give some infor- 
mation about the work. 
First, I may say that this investigation was 
started late last Fall, and it was only possible to 
carry it through by the co-operation given to us 
by various Swedish Governmental Agencies, We 
made the investigation within the Swedish 
Hydrographic Department, and the project 
engineer was one of my assistants. He was Mr 
Thunberg and I should like to pay tribute to him 
for his good work. 
The background for the investigation was 
that we were interested in having a possibility 
for getting photogrammetric surveys of rather 
limited areas without waiting for aerial photog- 
raphic weather to arrive in the country. Also, 
the nice weather has to be used for the larger 
photographic missions and cannot be used for 
rather small jobs. 
In the first place, Sweden is pretty far north. 
The southern part is about 59° latitude, and 
the northern part extends up past the Arctic 
Circle. That means that the sun is very often 
low; the elevation angle to the sun is small. 
The aerial photography I startel with was 
made on 16th September. It was heavily cloud- 
ed, a very dark, grey day, where you certainly 
get out of the mood and do not like to do any- 
thing in the way of aerial photography. It was 
raining, the cloud height was about 270 metres. 
The Geographical Survey Office made their 
aerial photography from 250 metres, that was 
with an aeroplane flying at 230 kilometres an 
hour. The film was a fast one, 400 ASA. It was 
an Ilford film. The shutter speed was only 1/425 
of a second. 
Four flights were made over the little terri- 
tory, and three of these models were well enough 
positioned for our purpose and were used. In 
the test area we had placed about 200 signals, 
most of them cardboard pieces — white card- 
board — 6 X 6 cms. Some signals were also 
painted on rocks or pavements. I should add 
perhaps that we were very well aware of the fact 
that the flying height was much lower than 
needed for accuracy purposes, but we thought 
we would try under very unfavourable condi- 
tions. In the test area we made a small geodetic 
net of high precision, composed of 13 points. 
The scale was determined with a geodimeter 4. 
The 13 points after adjustment gave a standard 
error of = 2.2 mm in x and + 1 mm in y. 
After the flight we surveyed as accurately as 
we could. It was frosty and partly snow, and so 
there were very unfavourably conditions for 
accurate geodetic surveys. We surveyed about 
130 signals which could be regarded as remain- 
ing in the position from air photography. After 
some sorting out of a few which certainly had 
been moved by dogs or children — it was just in 
Stockholm, this area — we had about 100 left 
with an error margin of 10 mm. The diapositives 
were made without the use of logetronics. I