234 
Oliver and Tansley. 
enable the surveyor to map with greater relative accuracy than in 
the method of squares, where there are no bars, and this is highly 
desirable in cases where such detailed work is worth undertaking 
at all. A gridiron can easily be mapped by one person. 
Fig. 79. Mode of laying out the tapes in the Gridiron. 
The specimen gridiron shewn in Fig. 80 forms the upper left- 
hand corner of the square shewn in Fig. 78. The cliffs forming the 
edges of channels are put in as continuous lines, the boundaries 
of associations as dotted lines, while the contour lines, one-tenth of 
a foot apart, are represented by lines composed of alternate dots 
and dashes. The surfaces of the “ pans ” and channels, where 
they are of bare mud or are covered only with algae ( Rhizoclonium , 
Lyngbya, Chroococcus), are in black. The association symbols 
used are the same as in the square (see above). The “ spot ” levels 
were taken with the theodolite and levelling staff, reading to of a 
foot, and were corrected to a datum (the normal water level at low 
tide at a certain spot in the channel of the main stream). They 
represent hundredths of a foot above this datum, e.g. 375 means 3.75 
feet above datum. The contour lines are necessarily approximately 
correct only; a very much greater number of readings would be 
necessary to obtain complete accuracy. They have been drawn 
partly with the assistance of the association boundaries. This 
course is justifiable in view of the fact that in other gridirons, where 
the readings are more numerous in the immediate neighbourhood 
