376 
SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962 
A more detailed study of ice, the evaluation of its thickness and the total 
amount in the sea could be undertaken only after we had obtained a true 
picture of the distribution of hummocks on the sea-ice surface. The interpreted 
morphological indications, which related only to the form, shadow and the 
location of the photographic image of natural objects, can now be used to 
classify the hummocky aggregations and give a quantitative appraisal [5] of 
this natural phenomenon, which for a long time remained rather obscure and 
unrepresented. 
Besides, an analysis of numerous vertical aerial photographs covering exten 
sive water areas of the Central Polar Basin allows a correct judgement of the 
existing relationship between distribution of open-water areas in the midst of 
ice and the relief of the sea bottom, i.e. the bathymetry of the North Polar 
Ocean [3]. 
Such are, in general outline, the main conclusions obtained from the inter 
pretation of aerial photographs of sea-ice, which greatly added to our know 
ledge of this natural phenomenon. But these conclusions by no means cover 
all possibilities of the aerial photographic survey. Many new possibilities have 
yet to be discovered. At the same time, the possibilities which have gained 
general recognition should be added to those which we did not regard as suffi 
ciently representative owing to the local character of their initial data. By 
force of their novelty, they are fully in agreement with the above conclusions 
and therefore deserve special attention. 
In this connection we must first of all point out the ability of the ice sheet to 
be, under certain conditions, an indicator of tidal wave propagation. 
A large number of interpreted aerial photographs of the Ob inlet have 
revealed a special variety of ice cracks which we called outflow cracks. By 
combining camera and field interpretation, we succeeded in establishing an 
identical character of the photographic image of the cracks and the outflows 
from the fishermen’s holes (fig. 1). Fig. 2 shows an outflow character of these 
cracks, and their general appearance, as recorded on a vertical aerial photo 
graph, is illustrated in fig. 3. Transferred onto an ice-patrol report chart these 
cracks interpreted from aerial photographs create, in their combination, a 
pattern strikingly similar to the location of co-tidal lines. Along the shores, 
they run parallel in two or three rows, sometimes merging to form one line. As 
they move farther away from the shore they bend along an arc and intersect the 
Ob inlet along the parallels. As our observations have shown, the form of these 
cracks is extremely stable and the cracks retain their bright colouring even 
after the ice has disintegrated. To all appearances, the interpretation and 
mapping of outflow cracks in the sea bays which indent deep into the Arctic 
shores can be used to judge, as a first approximation, the nature of the tidal 
wave propagation without resorting to the data provided by tide gauges. 
The role played by the ice sheet in shaping the shoreline and the bathy 
metry of the shore is another very interesting feature.