In Figure 3, various subject categories are compared to determine which 
band may be used to distinguish which subject. The weaknesses of monochrome 
imagery are revealed here. It is not possible to separate non-vegetated 
surfaces from vegetated surfaces in any single band. Nor is it possible to 
subdivide either category by grey-tone alone. 
Colour Imagery 
Two forms of colour combination are produced by the photographic labora 
tories of the National Air Photo Library in Ottawa. Bands 4, 5 and 7 are 
usually colour coded using blue for 4, green for 5, and red for 7. The 
colour rendition of these prints is similar to that of colour-infrared film. 
The combination of bands 5, 6 and 7 is usually colour coded 5 = red, 6 = green, 
7 = blue. 
The advantages of colour over monochrome imagery are most impressive. 
a) All main subject categories can be separated on a single print by unique 
colours. It is particularly important for the geologist to be able to 
separate non-vegetated from vegetated surfaces. 
b) Different classes of vegetation can often be recognized. This may be 
important where a particular type of vegetation grows predominantly or 
exclusively on one particular rock type. 
The limitation of colour prints is that non-vegetated subjects are rarely 
represented by an extensive range of colours. Very often sands, rocks, soil, 
and concrete may all be represented by minor variations of colour from white 
to blue-white or fawn. This same limitation applies to the study of rocks and 
soils on colour-infrared film, and is one reason why normal colour film is 
often preferred to false-colour film for geological purposes. 
Of the two colour combinations, type 457 is preferred for the study of 
land areas because the colour differences between vegetated and non-vegetated 
areas (reds against whites) appear to the eye to have more contrast than the 
colour differences (blues against whites) of type 567. For water subjects, 
type 567 is preferred because of the striking visual contrast between its 
gold and reds representing silt-laden water and the black of silt-free water. 
For the same subjects on type 457, this contrast is between blues and black. 
APPLICATIONS 
The potential application of ERTS imagery for geological work may be 
grouped under four headings: 
a) Educational: Uses here include illustrations for reports, teaching guides, 
and for providing visual evidence of geological phenomena. 
b) As basemap underlays for other data, particularly for airborne geophysical 
data (gamma-spectrometer, magnetic) and geochemical data. Unfortunately 
the use of ERTS enlargements for geological field mapping or for aircraft 
navigation on geophysical surveys is limited by the resolution of the 
imagery.