Photographic interpretation can be defined as: “the act of examining photographic images
for the purpose of identifying objects and judging their significance”.
Principles of image interpretation have been developed empirically for more than 150 years.
The most basic of these principles are the elements of image interpretation. They are: location, size,
shape, shadow, tone/color, texture, pattern, height/depth and site/situation/association. These
are routinely used when interpreting an aerial photo or analyzing a photo-like image. A well-trained
image interpreter uses many of these elements during his or her analysis without really thinking
about them. However, a beginner may not only have to force himself or herself to consciously
evaluate an unknown object with respect to these elements, but also analyze its significance in
relation to the other objects or phenomena in the photo or image.
Elements of Interpretation
1. Location
There are two primary methods to obtain precise location in the form of coordinates. 1) survey
in the field using traditional surveying techniques or global positioning system instruments, or 2)
collect remotely sensed data of the object, rectify the image and then extract the desired
coordinate information. Most scientists who choose option 1 now use relatively inexpensive
GPS instruments in the field to obtain the desired location of an object. If option 2 is chosen,
most aircraft used to collect the remotely sensed data have a GPS receiver. This allows the
aircraft to obtain exact latitude/longitude coordinates each time a photo is taken.
2. Size
The size of an object is one of the most distinguishing characteristics and one of the more
important elements of interpretation. Most commonly, length, width and perimeter are measured.
To be able to do this successfully, it is necessary to know the scale of the photo. Measuring the
size of an unknown object allows the interpreter to rule out possible alternatives. It has proved
to be helpful to measure the size of a few well-known objects to give a comparison to the
unknown-object. For example, field dimensions of major sports like soccer, football, and
baseball are standard throughout the world. If objects like this are visible in the image, it is
possible to determine the size of the unknown object by simply comparing the two.
3. Shape
There is an infinite number of uniquely shaped natural and man-made objects in the world. A
few examples of shape are the triangular shape of modern jet aircraft and the shape of a
common single family dwelling. Humans has modified the landscape in very interesting ways
that has given shape to many objects, but nature also shapes the landscape in its own ways. In
general, straight, recti-linear features in the environment are of human origin. Nature produces
more subtle shapes.
4. Shadow
Virtually all remotely sensed data is collected within 2 hours of solar noon to avoid extended
shadows in the image or photo. This is because shadows can obscure other objects that could
otherwise be identified. On the other hand, the shadow cast by an object may be key to the
identity of another object. Take for example the Washington Monument in Washington D.C.
While viewing this from above it can be difficult to discern the shape of the monument, but with
a shadow cast, this process becomes much easier. It is good practice to orient the photos so that
the shadows are falling towards the interpreter. A pseudoscopic illusion can be produced if the
shadow is oriented away from the observer. This happens when low points appear high and high
points appear low.
5. Tone/Color
Real world materials like vegetation, water and bare soil reflect different proportions of energy
in the blue, green, red, and infrared portions of the electro-magnetic spectrum. An interpreter
can document the amount of energy reflected from each at specific wavelengths to create a
spectral signature. These signatures can help understand why certain objects appear as they do
on black and white color imagery. These shades of gray are referred to as tone. The darker an
object appears the less amount of light it reflects.
6. Texture
This is defined as the “characteristic placement and arrangement of repetitions of tone or color
in an image.” Adjectives often used to describe texture are smooth (uniform, homogeneous),
intermediate, and rough (coarse, heterogeneous). It is important to remember that texture is a
product of scale. On a large scale depiction, objects could appear to have an intermediate texture.
But, as the scale becomes smaller, the texture could appear to be more uniform, or smooth. A
few examples of texture could be the “smoothness” of a paved road, or the “coarseness” a pine
forest.
7. Pattern
Pattern is the spatial arrangement of objects in the landscape. The objects may be arranged
randomly or systematically. They can be natural, as with a drainage pattern of a river, or man-
made, as with the squares formed from the United States Public Land Survey System. Typical
adjectives used in describing pattern are: random, systematic, circular, oval, linear, rectangular,
and curvilinear to name a few.
8. Height and Depth
Height and depth, also known as “elevation” and “bathymetry”, is one of the most diagnostic
elements of image interpretation. This is because any object, such as a building or electric pole
that rises above the local landscape will exhibit some sort of radial relief. Also, objects that
exhibit this relief will cast a shadow that can also provide information as to its height or
elevation. A good example of this would be buildings of any major city.
9. Site/Situation/Association
Site has unique physical characteristics which might include elevation, slope, and type of
surface cover (e.g., grass, forest, water, bare soil). Site can also have socioeconomic
characteristics such as the value of land or the closeness to water. Situation refers to how ትሄ objects in the photo or image are organized and “situated” in respect to each other. Most power plants have materials and building associated in a fairly predictable manner. Association refers to the fact that when you find a certain activity within a photo or image, you usually encounter related or “associated” features or activities. Site, situation, and association are rarely used independent of each other when analyzing an image. An example of this would be a large shopping mall. Usually there are multiple large buildings, massive parking lots, and it is usually located near a major road or intersection.
Lithostructural Geological Mapping
The interpretation methodology involves a process of identifying small-scale features to provide the evidence for large structures to build up the interpretation of the data. The surface traces of features such as bedding/banding/foliation fabrics, bedrock surficial cover contacts, faults/fractures/lineaments, and possible dykes/veins are annotated directly on-screen to the appropriate GIS layer. The fully attributed layers of the lithostructural interpretation map normally include the following:
- Geological boundaries
- Foliation/bedding
- Faults fractures/lineaments dykes & veins
- Alteration anomalies possibly related to mineralisation
- Potential targets for exploration
During annotation, reference would be made to the available geological mapping to help guide interpretation and identify lithology and stratigraphy. In some cases it is also important to map geomphological features which may influence the dispersion of trace elements or host placer deposits, and these will be included when appropriate.
Alteration Mapping
Alteration Mapping
Spectral mapping using data from satellite systems such as Landsat or ASTER to differentiate lithologies and alteration mineral suites, this combined with structural analysis is used to map hydrothermal systems and other indications of mineral deposits. This involves processing the data to generate ratio images to map any alteration minerals that may represent the surface expression of mineralized zones, which are highlighted on the final map, and in combination with the lithostructural interpretation provide a basis for target selection.
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