726 
MAPPING AS AN ECOLOGICAL INSTRUMENT. 
staff. An optical square may be used, but the result is far less 
accurate over long distances. 
The actual charting of a section of the map needs two 
operators, and can be done with the aid of one chain and one 
100 foot tape, but a very great saving of time and labour is 
effected if at least two chains and two or three tapes are avail- 
able for each unit of two workers. It is advisable to divide the 
map into sections of convenient widths bounded by adjacent 
perpendiculars from the base line. 
The chain is laid down along the perpendicular and secured 
with arrows. With the aid of a second chain or tape, the 
perpendicular distances of the salient points of the different 
physical and vegetation features are measured from the first 
chain and charted on to squared paper on the selected scale. 
These points are joined up, freehand, as the work proceeds. 
At first it is necessary to obtain the right angle for each 
measurement with an optical square, but after some experience 
this can be dispensed with. If there is much wind, all the 
longer measurements should be made with a chain, as the sag 
of a tape introduces considerable inaccuracy. The significance 
of the various features charted must be carefully indicated. 
For the sake of clearness it is best to use definite symbols or 
initial letters to represent the various plants, such symbols 
being indexed on the chart itself. On the general map letters 
are perhaps preferable, whereas on small detailed maps symbols 
are the more convenient. For instance, on the Erquy and 
Blakeney general maps, Salicornia europcea is designated by 
SI, Saceda maritima by Sd, Aster Tripolium by As or A, 
Silene maritima by Si, Obione portulacoides by O. The 
different associations of plants are indicated by combinations 
of the representative letters. Only the principal constituents of 
each association are indicated, and the relative dominance 
of the different species is shown by the order in which the 
letters are arranged. SI As Sd shows that Salicornia europcea 
is the dominant, Aster coming second, Suceda maritima third. 
SI As Sd signifies that while Salicornia is domin ant, Aster 
and Suceda are about equal in quantity, while SI As Sd 
