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Forestry / Vegetation
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Change Detection in Colour : Presentation and Interpretation of Multi-Dimensional Image Data Sets
Anthony J. Lewis1,3, Qiang Tao2 and DeWitt H. Braud1
1Department of geography and Anthropology
Louisiana State University, Baton Rouge, LA USA 70803
Tel:504-388-6199 Fax:504-388-2912
E-mail: ajewis@cypress.cadgis.Isu.edu
2Coastal Studies Institute
Louisiana State University, Baton Rouge, LA USA 70803
Tel:504-388-4429 Fax:504-388
E-mail: qtao@cypress.cadgis.Isu.edu
3Senior Fellow, Department of Geography,
The National University of Singapore,
SINGPORE 119260 (From 12/96 to 12/97),
E-mail: geoal@nus.sg
Abstract
The perception and interpretation of colour enhance our understanding of the environment in which we live. Colour is used to present multi-spectral reflectance patterns in a colour television or standard colour schemes when energy outside of the visual range is recorded. In either case, the images generated utilize three layers of colours to present multi-spectral information. By incorporating the same colour theory and technology multi-dimensional false colour composite images that show change (spectral, temporal, polarization, look angel etc.) in a spatial context can be generated. To interpret these multi-dimensional false colour images correctly the interpreter must be cognizant of 1) the sensor parameters 2) the assigned colours, 3) colour theory and 4) the signal-target interaction. A standard procedure for assigning colours is recommended.
Introduction
Colour is a primary visual key to the perception and interpretation of our environment. Colour and its use are undoubtedly important contributors to a better understanding of the environment in which we live. Colour is defined by both physical laws and physiological perceptions. Physical laws describe colour in terms of wavelength and frequency; physiologically colour is described in the way that each of us" perceives" a certain colour. Description by physical laws is precise. In even through it may be subtle difference. To help reduce some of the subjectivity in using colour to identify and classify features, standardized colour chips, e.g. Munsell Colour Chart, are used as a reference source. Soil scientists are very familiar with Munsell Colour Charts and commonly use them to describe soil colour characteristics.
Primary Colours
Colour can be divided into 1) Additive primaries and 2) Subraractive primaries. The three additive primaries are Blue, Green and Red. The combination of these three additive primary colours results in white light. Subtractive primary colours are the result of white light minus one of the additive primaries. Although subtractive primaries are more logically thought of as white light minus an additive primary, subtractive primaries are also the combination of two additive primaries. For example, white light minus blue light results in the subtractive primary yellow. Yellow can also be described as red plus green.
Both types of primaries have special characteristics and are used different applications for colour. Additive primary colours are used to generate colours in colour TV's and colour monitors for computers, etc. Colour TV's utilize three electron guns to excite the blue, green and red phosphors on the monitor. In he photographic process, for both the production of slides and prints, subtractive colours must be used. With both slides and prints, the light that reaches the eye and therefore carries the colour information is filtered by the colour print.
Colour and colour theory are complex topics and beyond the scope of this paper. For more detailed discussions of colour and colour theory the reader is referred to Kueppers (1982) and Nemcsics (1993).
Colour and Colour Infrared Film
All colour films, slides and prints, regards of the manufactures, are tri-emulsion films (three layers of colour dyes). Standard or "natural" colour film has three layers, each layer sensitive to one of the additive primary colours. The dye assigned to each of these layers is the complementary subtractive primary colour. For example, a yellow dye is assigned to the blue sensitive layer. Although the colours on "natural" colour film may be different on different films, they are close to that of the original scene. Blue light reflect from the feature and recorded on standard colour film appears blue in colour. Reflected green light appears green, and reflected red light appears red.
When the assignment of colour days is not conventional, colours exhibited on the film are "unnatural." Such film is generally referred to as "false colour film." Colour infrared film, perhaps the most common type of "false colour film", has green, red and near-infrared sensitive layers as the three standard layers. Since these layers also exhibit sensitivity to blue light, an external yellow filter is usually attached to the lens of the camera to eliminate incoming blue light. When exposed and developed the green sensitive layers will appear blue, the red will appear green and the near infrared will appear red. If a object only reflects blue light, it will be black on colour infrared film.
For both conventional natural colour film and false colour infrared film, colour is used to present spectral differences in a scene taken at single point in time. The colours on the photograph provide the viewer (photo interpreter) with information on the way the object (s) absorb and reflect energy within the spectral bands being recorded. Since most photographs are captured using solar energy infrared energy from the sun. Therefore all colour film is a record of multi-spectral reflectance values collected at an instantaneous point in time.
For a more detailed description of colour and colour infrared film the reader is referred to almost any introductory test on remote sensing, e.t. Avery and Berlin (1992) and Lillesand and Kiefer (1994).
Multi-Dimensional False Colour Images
The discussion so far has focused on a brief explanation of colour and the different types of colour films. It is important to have a basic understanding of these concepts before attempting to visualize the presentation of other multi-dimensional data sets using colour. Using colour to view multi-spectral data is based on the same principles used to present other types of data sets (temporal, polarization, look direction etc.). Three different but spatial registerable data sets are dimensional models are presented below :
Multi-spectral model - S1T1; S2T1; S3T1
Multi-temporal model - S1T1; S1T2; S1T3
Multi-polarization model - S1P1; S1P2; S1P3
Multi-look angle model - S1L1; S1L2; S1L3
Multi-look direction model - S1D1; S1D2; S1D3
where S represents a spectral band; T represents a given point in time; P represents polarization; L represents look angle and D represent look direction. In all of these models there are only two parameters presented: one remains constant and the other varies. The reader should be aware that for a better understanding of the signal-target interaction, only one parameter should be varied in each model although in reality each model has more than two parameters. For example, for interpretive purposes time (T) should only be varied in the multi-temporal model and is assumed to be constant in all of the other models.
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