Fig. 1. Crop marks of “Celtic” fields and road system, with probable settlement, in the Lin 
colnshire Fens west of Spalding (looking east, about British National Grid reference TF188205) 
(Photograph by J. K. S. St Joseph; British Crown Copyright reserved) 
of ancient fields because of the basic need to make plans of large areas. There 
are other reasons almost as important. We would like to examine these and 
at the same time introduce views of field systems of quite different date and 
form from the “Celtic” but sometimes lying over these early remains. 
There are in fact four separate but interlocking operations to which air 
photography must or can make a really important contribution. These are 
1) the search; 2) analysis; 3) planning and 4) presentation. 
The search may obviously be made in the air by an experienced observer, 
who will then photograph the remain he recognises; or it may be made by 
examining air photographs already taken for general purposes. In this latter 
case overlapping vertical or near vertical air photographs are invaluable (cf. 
fig. 2) and without doubt one of the best ways of searching them is through an 
Oude-Delft stereoscope, which we in R.C.H.M. have occasionally been able 
to use. One of the objects of such a search is ultimately to build up a picture of 
the total recoverable remains of ancient fields. It should be mentioned that 
this will never be complete in an absolute sense, for destruction of all periods 
has obliterated most remains of “Celtic” fields in the heavier good arable 
lands, but equally we consider it quite wrong to look on the surviving down-