third, fourth, or more gene-
After photography every second,
rally, atl photo is supplied with a systematic scheme of
dots. The scheme may cover the photo totally or partly. In the
latter case a central part is to be preferred. According to
terminology of statistics the photos supplied with scheme are
regarded as clusters while the units defined by the scheme re-
present the elements. It is easy to see that the design is
unbiassed if a cluster has only one element and it falls on
the photo nadir. Later it is assumed, however, that the
clusters have more elements. It is further assumed, that photo-
interpretation of elements into area classes is made correctly.
Sampling on truly vertical photograph -" An illustration
It is assumed that the inventory area consists of four field
classes as shown in the Figure 1. The elements of a cluster
can be expected to fall on the classes according to the dis-
tribution of the respective cluster (photo) area. The sampling
can be regarded unbiassed if the distribution of photo areas
into classes is equal to the true field distribution.
The number of the photos in the illustration equals that of
the classes. The proportions of the area classes are estimated
on the phote scale by lengths of a,.b, c, and d as:
4 4 y 4
Pa z2.a,/s, Bg: Z b/s, Pc =2Zc./s, and By.” 2. d,/s, where
i ea subscript for a photo and
4
S =Z(a,+b,+c,+d.)
i i "1 i
The class lengths on the photo scale are presented in measures
of datum level (see Fig.) in the following table:
Terrain class
Photo a | b € d
Length