TEACHING THE PRINCIPLES OF OPTICAL REMOTE SENSING USING GRAPHICAL TOOLS DEVELOPED IN TCL/TK

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1 TEACHING THE PRINCIPLES OF OPTICAL REMOTE SENSING USING GRAPHICAL TOOLS DEVELOPED IN TCL/TK M.J. Barnsley and P. Hobson, Department of Geography, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, U.K. Abstract: This paper describes a number of tools that have been developed to help teach undergraduate students the basic principles of optical remote sensing. It addresses the gap that exists in available teaching materials, prior to introducing students to digital image processing. The tools are developed in a scripting language known as Tcl/Tk, which is available for a wide variety of computing platforms. The current set of tools cover: (i) the linear and non-linear mixing of spectra, (ii) the impact of sensor spectral-sensitivity curves on detected reflectance, (iii) the relationship between leaf and vegetation-canopy optical properties, and (iv) the bidirectional reflectance properties of vegetation canopies. Each tool is controlled by a simple Graphical User-Interface (GUI). They are freely available for educational purposes via the World Wide Web. INTRODUCTION Most undergraduate textbooks and courses on environmental remote sensing adopt a similar structure. This can be characterized broadly as follows: (i) an introduction to the physical principles of remote sensing, (ii) an overview of sensor and platform technology, (iii) a review of the principal techniques for digital image processing and (iv) a discussion of selected applications of remote sensing. While this approach is entirely valid, it has a number of inherent weaknesses and limitations. First, in separating physical principles, sensor design, data processing and applications, the linkages between these components are often underplayed. Consequently, students frequently do not appreciate the impact on the detected signal of apparently subtle differences in, for example, the spectral bandwidth of two or more sensors operating in homologous spectral wavebands. Second, the focus on the technical details of specific sensors and platforms results in emphasis being placed on the technology as opposed to the science of remote sensing. These problems are frequently compounded by the paucity of teaching resources that deal with remote sensing sensu stricto (c.f., digital image processing). Consequently, practical classes typically focus on how to process one form of remotely-sensed data (i.e., multispectral images) after they have been acquired. The possibilities for independent investigation of the principles of energy interaction at the Earth surface or of the image formation process are typically much more limited. 1

2 DEVELOPING TEACHING AND LEARNING TOOLS IN A SIMPLE SCRIPTING LAN- GUAGE (TCL/TK) To remedy this situation, we have begun to develop a number of Computer-Aided Learning tools in a scripting language, known as Tcl/Tk (Tool Command Language/Tool Kit; pronounced tickle-tee-kay ), originally developed by John Ousterhout (Ousterhout 1994, Welch 1995, Johnson 1996). Tcl, in common with many other scripting languages (e.g., AWK and Perl), is a weakly-typed (i.e., string-based), interpreted language; while Tk provides a powerful set of widgets (e.g., frames, canvases, labels, buttons, scale bars, scroll bars and entry boxes) that can be used to construct sophisticated graphical user-interfaces (GUIs) (Sunsoft 1997a). In the context of this paper, the primary advantages of Tcl/Tk are: a) its simplicity and flexibility; b) the potential for rapid application development (RAD), including the creation of sophisticated graphical user-interfaces (GUIs); and, c) its availability and uniformity across a range of computing platforms including Microsoft Windows (3.x, 95 and NT), Macintosh, and various flavours of UNIX (including Solaris and Linux) ensuring direct portability of code between platforms. The last of these three points distinguishes it from other graphical scripting languages, such as Microsoft s Visual Basic (Ousterhout 1997). Although we do not exploit the capability here, Tcl/Tk can also be used to generate embedded programs within HTML web pages (known as Tclets ), in addition to standalone applications. In terms of the former, plugins are currently available for several of the most common Internet browsers, including the Netscape Navigator and Microsoft Internet Explorer (SunSoft 1997b). A SIMPLE EXAMPLE OF A TCL/TK SCRIPT To illustrate the flexibility, power and simplicity of Tcl/Tk, a simple example script is presented below: button.b -text "Press this button to exit" -command.b configure -fg black -bg white pack.b exit This script illustrates some of the most basic features of Tcl/Tk. The first line of the script creates a button widget (.b), places some text on the button and associates a command with that button (this command is enacted when the button is pressed). The second line of the script configures the previously created widget, altering the foreground and background colours associated with the button. The third line packs the button widget into the main frame (window). Although this is a trivially simple script, it illustrates how easy it is to use Tcl/Tk to create a simple graphical interface (Figure 1). 2

3 Figure 1 Simple GUI created using the Tcl/Tk script listed above. SIMULATION SOFTWARE We have developed four main teaching packages in Tcl/Tk to date. Each of these is discussed briefly below. Linear and Non-Linear Mixing of Soil and Vegetation Spectra The first package explores the interaction of electromagnetic radiation in the visible and nearinfrared with a surface that consists of some mixture of vegetation and soil (Figure 2). The user selects the proportion of ground covered by vegetation and the software displays both the pure spectra for the vegetation and the soil, as well as the spectrum for the mixed surface. The user can also select the type of spectral mixing, namely linear (based on single-scattering only) or nonlinear (taking into account multiple-scattering between the vegetation and soil). This provides a simple introduction to the issues underlying spectral mixture modelling and spectral un-mixing in later image processing exercises. The package is currently being enhanced to simulate mixing of two or more spectra read from a user-specified file. Figure 2 Spectral mixing simulation Impact of Sensor Spectral Bandwidth on the Detected Spectral Response The second package explores the impact of spectral bandwidth on the signal detected by the 3

4 sensor (Figure 3). The user can control the upper and lower limits of the sensor s bandpass: the software displays these, superimposed on examples of soil and vegetation spectra, and calculates the minimum, maximum and mean reflectance for these two surfaces in that spectral waveband, assuming a square-wave sensor spectral response. This software is currently being extended to operate on spectra provided by the user and to convolve these with the actual spectral sensitivity curves of a number of satellite sensors (e.g., Landsat-MSS and TM, SPOT-HRV and NOAA- AVHRR). Figure 3 Simulating the effect of sensor spectral bandwidth on the detected spectral response. Effect of Leaf and Soil Optical Properties and LAI on Vegetation Canopy Reflectance The third package follows on from these by investigating the effects of variations in leaf and soil optical properties and leaf-area index (LAI) on vegetation canopy reflectance (Figure 4). The program is based on the simple Adding model (Cooper et al. 1982), assuming a vegetation canopy composed of three homogeneous, plane-parallel leaf layers over a uniform soil substrate. The user can vary the leaf reflectance, leaf transmittance and soil reflectance to simulate the canopy reflectance in different spectral wavebands. The results can be plotted as a function of the contributions either from the different leaf and soil layers within the canopy, or from singlescattering and various orders of multiple-scattering events. One purpose of this software is to explain the reasons for the asymptotic relationship between reflectance and leaf-area index (LAI) 4

5 in the red and near-infrared spectral wavebands. It also helps students to understand the design of spectral vegetation indices. In common with the other three tools, separate pop-up windows explaining the use of the software and the background to the underlying model are available to support the student during the exercise (Figure 5). Figure 4 Simple canopy reflectance model Figure 5 Pop up help text window The SAIL Canopy Bidirectional Reflectance Model The final package is based on the SAIL model of vegetation canopy reflectance (Verhoef 1984) and is used to examine the influence of leaf-angle distribution (LAD), sky radiance and the Sun-target-sensor geometry on the bidirectional reflectance of a vegetation canopy (Figure 6). In common with the other packages described above, this implementation of the SAIL model allows the student to save or print the results either the raw data (Figure 7) or a 2-D plot (absolute reflectance or normalized to nadir; Figure 8) from individual model runs for subsequent analysis. We are now working on the implementation of other canopy reflectance models. Figure 6 Graphical User-Interface to the SAIL model of vegetation canopy bidirectional reflectance. 5

6 Figure 7 Data window for the SAIL model Figure 8 Plot of data from the SAIL model SUMMARY This paper has outlined the need for a set of interactive tools to teach the physical principles of remote sensing and the fundamentals of the image formation process. A scripting language, Tcl/Tk, has been used to develop four such tools, each of which is controlled by a simple graphical user-interface. The tools are freely for educational purposes from our WWW site (Barnsley, 1997). Anyone intending to make use of this software will also need to download a copy of the Tcl/Tk interpreter, which is available free-of-charge or for a nominal fee from SunSoft (Sun- Soft 1997a). We welcome any feedback in respect of potential enhancements to this software or exchange of additional teaching and learning tools for remote sensing developed in Tcl/Tk. REFERENCES Barnsley, M.J., 1997, Cooper, K., Smith, J.A., and Pitts, D., 1982, Reflectance of a vegetation canopy using the Adding method, Applied Optics, 21, Ousterhout, J., 1994, Tcl and the Tk Toolkit, (New York, Addison-Wesley). Ousterhout, J., 1997, html Johnson, E.F., 1996, Graphical applications with Tcl and Tk, (New York, MIS Press). SunSoft, 1997a, 80/research/tcl SunSoft, 1997b, sunlabs.com:80/research/tcl/plugin/index.html Verhoef, W., Light scattering by leaf layers with application to canopy reflectance modelling the SAIL model, Remote Sensing of Environment, 16, Welch, B.B., 1995, Practical programming in Tcl and Tk, (New Jersey, Prentice Hall). 6