476 
Marks that result from plant density differences are best recorded on 
vertical photographs, whereas oblique views may be superior for detecting 
faint marks based on plant color or height differences. In the latter in 
stance, the oblique photographs are best obtained in mid-morning or mid-after 
noon during the drier summer months. This timing appears most suitable for 
detecting faint tonal differences and minor plant-height variations. 
Crop or plant marks have been useful for revealing evidence of past Roman 
landscapes in Great Britain. Numerous Roman military remains such as camps, 
forts, battlefields, and roads have been discovered by plant marks as defined 
on aerial photographs. 
OBTAINING AERIAL IMAGERY 
There have been numerous trials of various photographic and non-photogra 
phic sensors conducted to determine their suitability for archeological explo 
ration. However, since the major objective of each investigator tends to differ, 
a listing of rigid specifications would be inappropriate, if not presumptuous. 
On the other hand, there are general recommendations that can be made regarding 
the conduct of archeological flightsj those described here will refer to con 
ventional optical imagery unless otherwise stated. 
Angle of View 
As a general rule, vertical photographs are preferred for reconnaissance 
flights and for detailed photogrammetric mapping of known sites or excavations. 
However, as noted in the previous section of this report, oblique photographs 
are commonly specified for detecting certain types of crop or plant marks. 
Season of Year 
For the detection and evaluation of most archeological sites, the drier 
seasons of the year are preferred over wetter periods, because the loss or 
retention of moisture by various soils provides more striking tonal contrasts 
during dry seasons. Of course, the density and condition of covering vege 
tation is an additional seasonal factor for consideration. It is obvious that 
growing-season photography is required for the detection and evaluation of crop 
or plant marks, for example. 
Reconnaissance flights over humid regions are likely to be more successful 
when masking deciduous plants are leafless. And soil marks are most readily 
discernible after plowing but prior to the establishment of an agricultural crop. 
In summary, the photographic season must be selected on the basis of specific 
project objectives; there is no single period of the year that is "best" for 
all forms of archeological exploration. 
Time of Day 
This specification is largely governed by the desired sun angle on the date 
of photography. For any given latitude and day of the year, the sun’s declination 
can be determined in advance from a solar ephemeris or from special charts avai 
lable through aerial film manufacturers. 
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