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MAPPING AS AN ECOLOGICAL INSTRUMENT. 
symbols to a minimum in order to avoid confusion, but if this 
precaution be taken, such a blocked-in map shows at a glance 
the general state of affairs on the area. For each map the 
appropriate symbols to be used must be decided for the 
individual case. The aim of a blocked-in map is to bring out 
certain salient features, and the symbols must be so arranged 
as to emphasize the essential points by obtaining the greatest 
contrasts and by avoiding all appearance of confusion by the 
judicious arrangement of a limited number of symbols. As 
this is the case, no convention has been adopted for the use of 
the different symbols, and it is probable that no convention ever 
can be adopted, though the matter has already been carefully 
discussed. In fact, it is often deemed advisable to vary the 
use of the same symbols in the different maps of a single 
investigation, and even to represent the same feature by 
another symbol, if such a course serves to emphasize the point 
for which each individual map is constructed. 
Another method of mapping, which proves of great use in 
certain cases, is that in which the plane table is used, the 
position of the various points being obtained by means of 
intersection of lines drawn from either end of the base line to 
the points. The method is best adapted to fairly level areas, 
as it is often necessary to sight over long distances. A base 
line, the longer the better, is selected so as to be at right 
angles to most of the points required. The plane table, 
with its telescope and ruler, is set up at one end of the base 
line, A, and a point A is marked on the plane table which 
coincides as nearly as possible with the point A on the ground ; 
a base line is drawn to scale on the plane table. The 
telescope is then trained successively upon each of the points 
required, and direction lines are ruled on the plane table 
towards them. The table is then set up again at B at the 
other end of the base line, great care being taken to orientate it 
so that the base line on the paper follows that on the ground. 
The same procedure is followed as before, but there is no need 
to rule the lines all the way, as only the intersection points are 
needed. The accuracy of the work can be tested by chaining 
