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selected and were examined with the zoom transfer scope. All were 
located in areas of research interest, where imagery (either "Quick- 
Look" prints or positive transparencies) was available for the year 
1973. The results of this examination were below expections for 
the following reasons: 
- A continued inability to discriminate between river banks 
and shore leads, or between islands and surrounding open 
water; 
- an inability to identify many islands shown on the maps; 
- some marked discrepancies, so far unexplained, in the 
positions of islands relative to the shore, when one ERTS 
image was compared with another; 
- distortions, so far unexplained, of shorelines used in 
image to map scaling with the Zoom Transfer Scope; and 
- an inability in later break-up stages to detect differences 
between fast ice and moving ice as well as between "dirty" 
ice and "clean" ice. 
In an attempt to heighten the discrimination between land 
and water areas several images were examined using a colour 
densitometer. There was no gain in the amount of interpretable 
data, compared with the results gained by using the Zoom Transfer 
Scope (Figures 5 and 6). The problem lies in the range of density 
levels provided in the "Quick-Look" prints. 
The change in position of islands, shoreline distortions 
and the inconsistent shapes of islands seem to be due to a 
combination of two factors. Firstly, the use of "Quick-Look" imagery 
may imply acceptance of uncontrolled local distortions in the 
printed image which are beyond the capability of the Zoom Transfer 
Scope to eliminate. Secondly, differences in publication date, 
aerial photography, interpretation and compilation exist in the 
1:250,000 maps. It is possible that some changes in the shape and 
position of islands represent genuine channel changes caused by 
erosion and deposition over 20-25 years. This possibility is given 
more weight because maximum agreement between ERTS imagery and the 
maps occurred where bedrock control of channel morphology is 
dominant (for example, near Wrigley and Arctic Red River), while 
maximum distortions appear in areas of more easily worked materials 
(for example, Birch Island and Old Fort Point). The changes in 
shape between frames, however, indicate that significant 
distortions are inherent in the imagery. This can exceed 0.5 KM 
in positional error. This exceeds the ability of the system to 
accurately detect the important shore leads. 
Perhaps the best illustration of the problems encountered 
during interpretation can be given by examining the test plots of