Field control - Photointerpretation
Keys
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5.2 Types of Photo-Interpretation Keys
5.2.1 Photo-Interpretation Keys of objects/features and conditions/patterns of a region
5.2.1.1 Photo-Interpretation Keys of objects/features of a region
5.2.1.2 Photo-Interpretation Keys of conditions/patterns of a region5.2.2 Analogue Photo-Interpretation Keys
5.2.3 Technical Photo-Interpretation Keys
5.2.4 Non-technical Photo-Interpretation Keys
5.2.5 Direct and Indirect Photo-Interpretation Keys
5.2.6 Selective/Elimination Photo-Interpretation Keys
| 5.1 Introduction |
In order to conduct a rigorous interpretation, analysis and processing of analogue and digital remotely sensed images, an appropriate guiding or classification scheme must first be adopted. This presupposes: |
 the selection of the appropriate and well documented ground truth data and
their correlation with their spatial and spectral characteristics as image features and |
| the development and use of the so called photointerpretation keys. |
Photointerpretation keys could be characterized according to: |
| the conditions and patterns or the types/objects of the natural and socioeconomic
reality recorded within the scene |
| the way they are used by the specialised scientist interpreter or the
appropriate interdisciplinary study group |
| the way and their principal descriptive characteristics are presented. |
The appropriate use of combined photointerpretation keys could become a valuable complementary tool in applying remote sensing methodology, supporting significantly the whole process by minimising additional field control. In photointerpretation and remote sensing science, there is, sometimes, a need to compare characteristic parts of different remotely sensed images, with data, facts and phenomena of the natural and socio-economic reality, in the form of empirical data, schedules, reports etc. So, we are going to look through the rationale and the technique used to compare the particular data of an aerial photograph with the relevant ground characteristics, in order to find out how to exploit this comparison’s results, which would be considered as a “model”. In this way, every time we see in an image a specific shape/form having the same external and internal characteristics as the model, we can identify, with satisfactory reliability, an object or series of possible object categories. So, we create the “Photo Interpretation Keys”, which are very useful “tools” in the photointerpretation process. By this term we mean pictorial, arithmetic (digital) or even descriptive documentation and reference data of a particular state, which become the fundamental knowledge of a scientific – technical field, in order to interpret some images for applications related to this field. Photointerpretation keys can be useful: |
| a. because they provide the quick processing of a considerable volume of aerial
photographs/images, b. because they can help the simultaneous acquisition of many data categories, given that personnel’s education on the use of the specific photointerpretation keys, could be easily done in a short period of time, in “series” , and c. when there is no possibility of access to data for a large area, other than some series of old aerial photographs, while at the same time we have to draw some kind of conclusions. |
Photointerpretation keys can be categorised according to: |
| a. the conditions and patterns or the objects/features of a region, b. their use and the person who created them, c. the means and method of the presentation of their characteristics.
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| 5.2 Types of photointerpretation keys | |
5.2.1 Photo-Interpretation Keys of objects/features and conditions/patterns of a region Considering a region as a model of point, surface and/or spatial data/features, we can examine it: i. either, as a totality of objects/features or units of features, ii. or, as a field where there are units of patterns or conditions. This means that a region could be examined horizontally or vertically. Depending on the approach, we may have photointerpretation keys of objects/features or of the conditions/patterns for a region. Each of the two above categories is divided into the following sub-categories: 5.2.1.1
Photo-Interpretation Keys of objects/features of a region |
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According to the above, we could have: |
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| a. Keys of specific objects/features (The simplest
photointerpretation keys) This kind of photointerpretation key is the photographic or descriptive presentation of a vineyard, an orchard, a pasture, an olive plantation etc. b. Keys of specific units of the same objects/features These are combinations of specific objects/feature keys. For example: big or small pine-trees, healthy or not healthy, thin or thick, that form a series composing a unit. The keys of this category refer to general regional characteristics of objects/features. |
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5.2.1.2 Photo-Interpretation Keys of conditions/patterns of a region As was mentioned above, a region can be considered as a totality of objects/features and of units of objects/features. From another point of view, we may characterise a region by its structure and function conditions and patterns, as an aggregation either of general conditions or of general regional functions. So, for example, a region’s irrigated cultivation can be identified, both as a unit of specific objects/features and as areas with possible homogeneity in hydrological, hydrogeological, productive etc. conditions. According to these criteria, it can be differentiated from neighbouring cultivation. This category can be further divided into two other basic sub-categories: |
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a. Keys of specific conditions and b. Keys of specific zones with homogeneous conditions (It’s a combination of keys of specific conditions) |
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It is obvious that the combination of photointerpretation keys (objects/features and conditions/patterns in a region) is the most effective method of exploiting their possibilities, as additional “tools” in photointerpretation methodology. 5.2.2 Analogue Photo-Interpretation Keys These keys result from the integration and the combined use, in particular accessible areas, of all the above-mentioned photointerpretation keys, in order to identify impenetrable areas with probable similar characteristic features and units of features and/or of conditions/patterns, and zones of homogeneous conditions/patterns. 5.2.3 Technical Photo-Interpretation Keys These are keys that could be defined according to specific principles of the relevant scientific field. Example: an experienced forester with a good knowledge of photointerpretation, creates a file containing photographs, under certain circumstances and for a specific period of time, of pine-tree disease cases. All this material is a technical key that could be created only by the scientist–expert on the relevant scientific field and not by a rural and surveying engineer–photointerpreter. A technical key could be a combination of objects/features and conditions/patterns keys. 5.2.4 Non-technical Photo-Interpretation Keys This key is to be used by photointerpreters, both no-experts and not experienced in the relevant scientific field. 5.2.5 Direct and Indirect Photo-Interpretation Keys These can be keys of objects/features and/or conditions/patterns of a region. Direct are the keys used for objects/features and conditions/patterns. They can be identified by a simple process on aerial photographs or other remotely sensed images (for example, a stadium, a cemetery, a refinery, a port etc). Indirect are the keys used for an indirect data acquisition (for example, estimation of a region’s population from the number of houses etc.). 5.2.6 Selective/Elimination Photo-Interpretation Keys Depending on the way and the method of the photointerpretation characteristic presentation, the photointerpretation keys can be divided into Selective/Elimination Keys. 5.2.6.1 Selective Keys |
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| a. Specific Description Keys These define the items/features or the conditions/patterns of a region descriptively in a technical report. For this key type, the photographic documentation is only indicative. b. Specific Reference Keys (Descriptive/photographic, based on the photointerpreter’s previous experience) With these keys it is possible to exploit the acquired photointerpreter’s experience and the photographic/descriptive material in his archive. c. Keys of Systematic Reference to descriptive/photographic documentation These result from systematic archiving of descriptions and photographs of all the photointerpretation studies and research relevant to the interpreted feature. |
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5.2.6.2 Elimination Keys |
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| a. Photointerpretation Mechanical Keys These are keys recorded on disks or punch cards, that estimate, according to a code, all the non-interesting objects, conditions, patterns. b. Dichotomous Photo-Interpretation Keys These identify the feature or the conditions we are interested in, by successive, logical elimination. |
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It is obvious that the creation of useful and effective photointerpretation keys requires systematic and appropriate, for each case, ground controls. Generally, photointerpretation keys, very often, replace the necessary ground controls, whose role is then limited to assuring or simply confirming the photointerpretation methodology’s reliability. Reference: Rokos, D. “Photointerpretation and Remote Sensing”, NTUA, 1979
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National Technical University of Athens |