In interpreting aerial images the precise characteristics useful for
a specific task are dependent upon the field of application. However,
most applications will consider the following basic characteristics:
| Shape |
This is the general configuration or outline
of an object. The shape of some objects may be distinctive enough
to be identified on this criterion alone, e.g. the Pentagon building
near Washington DC |
| Size |
The size of objects on images must be
considered in the context of the image scale. As an example, a small
shed might be misinterpreted as a barn if size were taken into consideration.
Additionally relative sizes among objects on images of the same
scale should also be considered. |
| Pattern |
This relates to the spatial arrangement
of objects. Repetition of certain general forms can give objects
a pattern that aids the image interpretation. For example the spatial
arrangement of trees in an orchard is distinctly different to that
of a forest. |
| Tone |
The relative brightness or colour of
objects on an image is refereed to as tone (or sometime hue). Photo
tones can for example be used to distinguish between deciduous and
coniferous trees. In black and white photographs drainage conditions
in soil, with the lighter areas being higher and drier and the darker
lower and wetter. |
| Texture |
This is the frequency of tonal change
on an image. It is produced by an aggregation of unit features that
may be too small to be discernible individually on the image, e.g.
tree leaves and leaf shadows, and is a product of their individual
shape, size, pattern, shadow and tone. It determines the overall
visual smoothness or coarseness of image features. As the scale
of the image is reduced the texture of an area or object becomes
finer and ultimately disappears. For example the smooth texture
of green grass as contrasted with the rough texture of green tree
crowns. |
| Shadows |
These have two opposing respects important
to interpreters (1) the shape or outline of a shadow gives an impression
of the profile view of objects, thus aiding interpretation (2) objects
within shadows reflect little light and are therefore difficult
to discern, thereby hindering interpretation. As an example shadows
cast by some large tree species, bridges, towers etc. can aid in
their identification on air photographs. Additionally, the shadows
resulting from subtle variations in terrain elevations, particularly
at low sun angles, can help in assessing natural topographic variations
diagnostic of various geologic landforms. |
| Site |
This is geographic or topographic location
and can be particularly important in the identification of vegetation
types. Certain tree species for example would be expected to occur
on well-drained upland sites, whereas others would occur on poorly
drained lowland sites. Additionally some tree species only appear
in certain geographic areas. |
| Association |
Is the occurrence of certain features
relative to others. An example of this would be a Ferris wheel.
Whilst is might be difficult to identify if standing near a barn
in a field it would be easy to identify in an area recognised as
an amusement park. |
| Resolution |
This depends on many factors
and it always places a practical limit on interpretation as some
objects are too small or have little contrast with their surroundings
to be clearly seen on the photo. |
