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delayed because of bad weather — the quality of the photographs was also inferior 
to that of the first series. A control series was therefore used — 1:15.000 Aviogon, 
15 cm 23x23 cm, which had been taken on 20th June, 1960, when photographing 
was done to decide control points. Most of the boundary points had been signalled 
by this time. 
Control point base. 
The determining of control point base for all models in the experimental area was 
done by terrestrial measuring (Wild Tl) of control points for a series in a scale 
of 1:25.000 Aviogon, 15 cm, 23x23 cm, photographed 20th June, 1960, after the 
control points had been signalled. For the other series — experimental and con 
trol — the control points were decided photogrammetrically. This precision was 
found to be satisfactory over the whole area. 
Signalling. 
The team who carried out the signalling in the experimental area had received 
detailed instructions, but even so had difficulty in placing the signals in such a 
way that they could be seen on the photographs, especially in woods. This was 
partly due to the men’s lack of experience in the use of air photographs, so that 
they had difficulty in deciding whether a signal would be visible or not. Because 
of poor weather during the period between signalling and photographing, some of 
the signals were destroyed, or became too grey in colour. 
The signals were either white painted patches on rock, or else white survey 
plastic — the size from 80 x 80 cm to 90 x 90 cm. 
The result was that out of about 1400 signalled boundary marks, only 950 were 
visible on one or both photos. 
Field work to locate points. 
Work in the field to locate points was done by two agriculturalists who had been 
on a forestry course (F and N) and one forestry man (A), each working in a 
different part of the experimental area. A and N had had some months experience 
in the locating of points in photographs, F had little experience in using air pho 
tographs. 
The series where signals were not used (the experimental series), were first marked 
by field work. Each of the three men marked all four series in succession in his 
own area. The order in which the series was marked was different for each. The 
same boundary points were to be marked on each of the series. 
Boundary points which could be identified were pricked on ordinary photo contacts 
with a sharp needle. Where this was not possible, some clear detail nearby was 
marked. The boundary point was then measured from this mark and a sketch 
made. The total of these measurings varied from 9.5% to 20.6% for the different 
senes.