Textures 
Textures and patterns on aerial photographs are usually the means by 
which features of the landscape are distinguished and named in the image, 
for example, an exposed rock,a field or a wood. This is achieved in large 
part because most photographs have a ground resolution of 1 to 3 metres, 
which is appreciably smaller than most of the features the geologist is 
trying to recognise. The resolution of ERTS imagery is between 200 and 300 
metres for most subjects, although there are individual cases where a high 
contrast subject, a 30 metre wide road on a band 5 image for example, may 
be recognised.The use of texture in ERTS imagery is thus to differentiate 
landscape units, rather than to put names to them. One of the more interesting 
discoveries about ERTS images of Canada is that almost every geological 
province has a texture that is individual and largely specific to that 
province. Figure 1. gives eight examples of such textures from Canadian 
imagery. 
Reflectance 
Most photogeologists would regard differences in grey tone as the least 
useful source of information on an aerial photograph. The ERTS system on the 
other hand, is designed to provide an abundance of information on scene 
reflectances in four separate parts of the energy spectrum. Undoubtedly the 
most efficient way to make use of this information is by computer. But few 
geologists have either the time for or the access to a suitable computer. The 
concern of most geologists is rather, 1 which band is most useful for what, 
and what about colour. 1 Figures 2 and 3 attempt to provide a general answer 
to these questions. 
For purposes of analysis, most images can be divided into a small number 
of subject categories, Non-vegetated surfaces (outcrop, soils, roads, urban 
development), Vegetated surfaces, Water, Water with suspended sediment, Snow 
and ice. In Figure 2, the range of grey tones is described for each of these 
categories in each monochrome band. Band 4 (green energy) images often have a 
low range of grey tones because of scattering in the atmosphere. Ground 
resolution suffers because of this. There does not appear to be any unique 
application for this band. In band 5 (red energy), vegetation is recorded in 
dark grey tones, while non-vegetated surfaces reflect strongly and appear 
as pale greys. Despite the differences between the two categories, it is 
not possible to separate the two at a single density level. Thus it is not 
possible to describe any one grey level as representing a 1 vegetated'or a 
'non-vegetated' surface. Water is relatively transparent in this band, so that 
suspended sediment plumes and reflections of bottom sediments in shallow water 
are all recorded in paler grey tones. Band 6 (first solar infrared band) often 
has a pleasing appearance in spite of the modest range of grey tones of the 
land areas. Water is usually black in sharp contrast to the land. Both 
vegetated and non-vegetated surfaces have a similar range of grey tones, and 
can rarely be separated. Band 7 (second solar infrared band) is similar to 
Band 6, but with a lower range of grey tones. There does not appear to be any 
unique application for band 7.