MAPPING AS AN ECOLOGICAL INSTRUMENT. 
725 
be the first map of the area that was made from an actual 
survey. During the lapse of this hundred years the shape of 
the Point changed considerably, the tip growing out in a S.W. 
direction, so that it compared more closely with the condition 
of things at the present time. The fourth map is that made 
from a fairly recent Ordnance Survey. The Ordnance maps 
are very useful as the starting point of an ecological survey, 
but they are not nearly accurate enough for the purposes of a 
detailed investigation, consequently the construction of a 
general field map of the whole of the selected area is essential. 
General Field Map. 
This may with advantage combine the main geographical and 
vegetation features, because it is usually found that a very close 
connection exists between the two factors. For convenience 
sake the scale used at Blakeney was 1/2500, and specially ruled 
squared paper was obtained so that the map might be charted 
directly to this scale. The selection of a convenient base line 
is all-important. If a permanent line can be obtained, so much 
the better — if not, an imaginary line must be fixed by means of 
permanent bench marks made in appropriate positions. At 
Blakeney a line of telephone posts, stretching from the Point 
right into Cley, gives a most convenient system upon which to 
base the work. With an extended area it may be necessary 
and advisable to work with a composite base line, the angles 
between the different sections being carefully measured by 
instruments. 
The measuring up of the base line needs great care, and 
should always be done with a chain, and on no account with a 
tape, as the latter is apt to stretch and is also blown about by 
the wind, thus introducing inaccuracy at the very outset of the 
work. A few perpendiculars from the base line must then be 
raised at convenient intervals with the aid of the theodolite, the 
perpendiculars being marked out with barber’s poles and five 
foot ranging sticks. Intermediate perpendiculars, for con- 
venience in working, can then be raised by means of the cross 
