Field Methods in Remote Sensing

Preface

Practitioners of remote sensing will at some point need to learn how to obtain field data suitable for the various needs of their projects. Effective field data are best obtained through thoughtful planning, thorough knowledge of valid sampling techniques, accurate location-finding procedures, and reliable field measurements. Unfortunately for the beginner, few remote sensing research reports provide thorough accounts of the methods that were followed in the field. Instead, they concentrate on laboratory procedures such as data correction and processing. 

However, the methods of measuring field data have as much influence on the reliability of the final product as do laboratory procedures.Field procedures are important and should always be included in final reports as a service to readers who would like to validate, replicate, or educate. The purpose of this book on field procedures is to ease the way for the person who has a background in the fundamentals of remote sensing and laboratory methods but little practical knowledge of the field methods that may be needed for remote sensing projects. 

The readers I am envisioning include the following: students with some background in the fundamentals of remote sensing and image data processing who want to begin a project requiring field work; professionals with government agencies who may have field skills but need guidance applying them appropriately to remote sensing; and teachers who want to supplement a remote sensing course with a project requiring field work but whose field experience is limited or rusty. The field methods discussed encompass project planning, sampling plans suitable for selecting spectral training sites or accuracy assessment sites, finding locations in the field using a global positioning system, obtaining reflectance spectra from objects in the field, and basic measurement methods for studies of vegetation, soils, and water. 

The goals of remote sensing projects cover a wide range. The most fundamental is to produce a map of some selected surface features. Others may be to calibrate sensors with the response of surface features, to validate or evaluate the final product, to model the spectral response of a material and its biophysical characteristics, or to develop or test image data processing techniques. Because the approaches to field work in these various types of studies may be similar to each other, this guidebook will not differentiate field methods on that basis. 

Examples in the book usually assume that a map is the intended result. Measurement methods, particularly for vegetation, vary widely. Where the choice of measurement methods is large, the selection of an appropriate method depends on the amount of precision and detail needed for the final product. As objectives become increasingly specialized, the method of measurement may be less widely known, and often its effectiveness may be considered controversial among professionals. The plan for this book is to provide several basic measurement methods for vegetation, soils, and water/snow that might be applied to many types of projects. 

For situations in which more specialized measurements are needed, I have provided a bibliography, in Appendix 1, on advanced field methods. For example, many studies require field data quantifying vegetation cover density. Chapter 6 provides some methods for obtaining these data, in this case by pacing, in which a measurement is taken at each step, or by the analysis of small areas. Methods for deriving volume or weight of vegetation are also presented. For more advanced or specialized methods, the reader would refer to the bibliography in Appendix 1. 

The outcome of field work in remote sensing varies widely depending on project objectives. The simplest result may be an aerial photograph annotated with current ground cover types. Another outcome of field work may include a notebook of field data sheets containing measurements and observations for each sample site. In any case the strategy is to relate field data with image data as accurately and thoroughly as possible. The goal of field work should be that data collected from a variety of field sites are representative of the surface “seen” by the airborne or satellite sensor. The immense difference in scales between the ground and the image makes this an especially challenging task. 

The discussions of field procedures in this book are always focused on the needs of remote sensing projects that use sensors in the 400- to 2,500-nm wavelength range, that is, reflective radiation. When thermal infrared or microwave wavelengths are used, many of the field methods described here would still apply, but a number of others would need to be added.

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Field Methods in Remote Sensing