730 MAPPING AS AN ECOLOGICAL INSTRUMENT. 
changes that occur in a specified area during a period of years. 
As the areas are relatively small it is possible to re-chart them 
periodically, so that accurate comparisons can be made 
between the maps. When this re-charting is intended, it is 
necessary to mark the site of each grid permanently in some 
way — by means of stakes driven into each corner, by cairns of 
stones, if in very isolated places, or, failing these, by location 
by means of landmarks or by measurements with ths surveying 
instruments. The two charts illustrated show the same grid at 
an interval of five years, showing very clearly the permanence 
of the clumps of Silene maritima, which have somewhat 
increased in size, the growth of the Suceda fruticosa, while 
they also show how the Rumex on the upper part of the grid 
has completely disappeared. 
Transects. 
The principle of the transect is the same as that of the grid, 
except that the area under consideration is very long and 
narrow, being of any length and frequently about one decimetre 
in width. The transect is particularly useful as a means of 
illustrating the change in the plant associations where zonation 
occurs, such as in the transition from shingle bank to marsh. 
If the transect is run across the zones at right angles, the 
changes and gradations can be delineated within a very small 
compass. The variation in the groundwork vegetation can be 
clearly brought out by the judicious use of colours on the chart. 
In the course of a vegetation survey it is commonly found 
that certain peculiarities of plant distribution are correlated 
with the physical features of the area. This correlation can 
best be demonstrated by means of contour maps, for which the 
level of the ground at different points is determined, the contours 
being drawn from the data so obtained. To get the levels a 
certain low level is taken as the zero point. Then by means of 
a telescope and levelling staff a determination is made of the 
heights above the zero of numerous points all over the area, 
the number and distribution of the points being determined by 
the complexity of the system. To get the contours on the 
