coarser than would have resulted from using 70 line screens. 
In the example on page 20 the screened negatives are printed. On the 
left are the negatives of photo no. l and at the right hand side the 
negatives of photo no. 2. Studying these negatives we are inclined 
to choose negatives B and D because of the cohtrast in these negatives 
and the details shown. 
ee that fig. G and fig. I give the best positive result, and not H and 
In the example on page 21 with the printed positive imagery however we 
S 
J. Printing of the images H, J. K and L in a darker tone will not: give 
a better result. Careful study of the images shows that the darkest 
parts have closed screen dots and the whitest parts have no screen dots 
at all or very small ones. 
It is clear that the choice of screens mentioned here above under Ca) 
and (b) is very important to guarantee that photomap producers offer 
the best possible product to the map user. 
Application of Orthophotomap Products 
The application of orthophotomap products to the solution of mapping 
problems has been well documented in many papers on the subject. 
(Ref.: Baker, Ball, Fleming). rthophoto products appear to serve 
a dual purpose, that of the map product itself or as a tool in: the 
preparation of line maps. As reported (Ref.: Fleming 1971) about two- 
thirds of Canada remains te be mapped at 1:50,000 scale and much of 
this remaining area is devoid of man-made features and has terrain 
that is difficult to portray cartographically. This would appear to 
be an ideal area for the production of orthophotomaps as map products. 
Orthophoto production has been used in map revision both for evaluation 
of the map base for accuracy and content and aiter evaluation for the | 
actual planimetric updating. One method used in revision compilation 
is to prepare film positives of the orthonegatives and by direct trans- 
fer to a dye-scribe copy of the map, scribe new changes and make re- 
quired deletions to the old map base. » 
Another use of the orthophoto product is a tool in the preparation of 
the standard line map. Several factors, such as the availability of 
high altitude aerial photographs and better sensitized dye-scribe 
materials are contributing to the success of such a map compilation 
system. The U. S. Geological Survey is using a system patterned along 
the flow diagram (shown on the following page) to produce both ortho- 
photomap products such as orthophotoquads and standard line symbol maps. 
Note on the flow diagram that advance field completion may be done on 
the orthophoto at nominal map scale so that this information is 
readily available to the cartographic scriber at the later stage of 
map compilation. 
The orthophotoquad is produced in map format with no attempt at carto- 
graphic enhancement (color separation), and generally reproduced as 
a continuous-tone or ozalid product. 
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