1. Introduction to Earth Observation by Remote Sensing

Chapter 1

Introduction to earth observation byremote sensing

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1.1. GEOSPATIAL DATA ACQUISITION 38

1.1 Geospatial data acquisition

Are we making sufficient progress on social well-being, sustainable economicdevelopment, and environmental protection? Do we have the right informationavailable at the right time at the right place to take adequate decisions? Instru-mental to achieving such progress is the availability of geospatial data, the moreso the more drastically we exploit the planet Earth. Fortunately, we can noticethat the awareness increases worldwide of the importance of having access toreliable, detailed, timely and affordable geospatial data. Equally important, thetechnology of acquiring and providing geospatial data quickly develops stimu-lated by the rapid advances of information and communication technology.

This book is about basic principles of geospatial data acquisition. Geospatialdata acquisition is central to earth observation. Earth observation is gathering ofinformation about physical, chemical, biological, geometrical properties of ourplanet; it helps us to assess the status and monitor changes of the natural andcultural environment. Thus, mapping, monitoring, and also forecasting are the Earth observationuses of earth observation. Earth observation gives us geospatial data. Geospa-tial data acquisition can be taken as a starting point in our development cycle ofobserving - analyzing - designing or planning - constructing or developing - ob-serving, etc. Geospatial data are very valuable, so we rather not consider theiracquisition as the first stage of a linear system - as we know it from materialseconomy - which starts with acquisition and ends via production and consump-tion at disposal.

The following examples illustrate the diverse need for geospatial data:

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• A land administrator should have an up-to-date record of property bound-aries. Property boundaries usually coincide with observable terrain fea-tures, which s/he needs to have surveyed.

• A civil engineer has to design a highway. One of the criteria for findingan optimum alignment of the new road is to balance cut and fill volumesin road construction. Thus, s/he needs information about the shape of theground surface. Calculation of actual transportation costs of material canbe based on re-surveying terrain relief after construction.

• An urban planner may want to identify areas of informal settlement. Thedifferent types of houses and their configuration need to be determined.The municipality may furnish infrastructural improvements based on adevelopment plan for the identified areas. The urban planner will haveto monitor the impact of the provisions before proceeding to further plan-ning.

• An agronomist is interested in forecasting the overall agricultural produc-tion of a large area. S/he needs the size of fields per crop and data onbiomass production to estimate the yield. Observing soil properties andmonitoring degradation will improve forecasts.

• An environment analyst is worried about pollutants of waste disposal sites.S/he has to detect the dump composition and determine volumes.

• A climatologist would like to understand the El Niño phenomenon. Tothis end s/he needs data on spatial patterns of sea surface temperatureat different times, data on sea levels and sea currents, the direction and



velocity of surface winds, information about the processes of interactionbetween ocean and land surfaces, etc.

These few examples indicate already that our earth observation interest is onobjects, scenes, and phenomena, which are different in spatial extent. More-over, we are interested in different aspects of objects, different levels of detail,and all that at various times and different repeatability rates. Accordingly weuse a wide range of different methods and techniques of earth observation. Alsoobvious from the above examples may be the fact that solving higher order prob-lems, such as sustainable economic development, requires more data than thosewe can obtain by earth observation. To satisfy the information requirements ofgeo-scientists, engineers, land and water managers we conduct interviews (eg,in adjudication for land administration), take field samples and analyze them inlaboratories, observe using in situ sensors (eg, a tide gauge), do land surveying,make aerial surveys using cameras and scanners, or we take a more synopticview by an instrument on a space shuttle, an orbiting space station, a reconnais-sance satellite, or an earth resources satellite. We use meteorological satellitesfor forecasting weather; Meteosat looks at our planet from really remote outerspace at some 36,000 km above mean sea level.

The following should help to delineate the subject area of this book and placesome terms, which you will come across in literature. You may find them beingused by some authors indifferently or by others with a specific but geo-disciplinedependent flavour. In this book we try to give them a specific but not disciplinedependent meaning and use them consistently.



Remote sensing (RS) has been defined in many different ways. A sufficient def-inition for this book is: remote sensing is the art, science, and technology of ob-serving an object, scene, or phenomenon by instrument-based techniques. ‘Re-mote’ because observation is done at a distance without physical contact withthe object of interest. We can either use detection and real-time display devicesor recording devices of energy, which is emitted or reflected from an object ora scene. The energy can be light or another form of electromagnetic radiation, Remote sensingforce fields, or acoustic energy. An example of a remote sensor is a conventionalcamera. Light reflected from an object passes through the lens and the light-sensitive film detects it. At the moment of exposure a latent image is recorded.Developing and fixing the film in the photo lab generates a definite record, thephotograph. This image is then subject to interpretation. Today most remotesensors are electronic devices. The data recorded by such sensors, eg, a scannerdetecting thermal emission (heat), used to be converted to images - for visual in-terpretation. Accordingly we still refer to the record produced by an electronicsensor as (remote sensing) image.

RS defined as above is applied in many fields, including architecture, archeol-ogy, medicine, industrial quality control, robotics, extraterrestrial mapping, etc.The interest domain for this book is, however, earth observation and even morespecifically earth observation from airborne or spaceborne platforms. Earth ob-servation does not only rely on RS but also on sensors that allow us to makein situ observations. The principles of sensing in physical contact with an ob-ject are beyond the scope of this book. Limiting the spatial interest of RS toobjects that can (and are relevant to) be located on the surface of the Earth - us-ing a geodetically defined coordinate system - we use the term geospatial dataacquisition (GDA). The outcome of GDA is not simply an image as obtained by Geospatial data acquisition



converting sensor recordings, but adequately processed or interpreted data fromthe sensor. Processing/interpretation and validating require knowledge aboutthe sensing process and yields data readily suited for analysis, eg, in a GIS. Typ-ical products derived from geospatial data are orthophoto maps, “satellite im-age maps”, topographic maps, thematic maps such as land use maps, land usechange statistics, etc.

Placing geospatial data acquisition in the historical perspective we can take Sur-veying and Mapping as starting point. About a century ago Photogrammetryevolved as a sub-discipline of surveying and mapping, offering an extension tothe traditional ground-based methods. Photogrammetry and other aerial sur-veying techniques could replace to a large extent observing directly in the fieldby measuring terrain features indirectly, on images in the office. The next tech- Historical perspectivenological extension was Remote Sensing, which has enabled us to see phenom-ena our eyes cannot see. We can detect, eg, thermal emission and display itsuch that we can analyze it with our eye-brain system. The professional orga-nization International Society of Photogrammetry was renamed to InternationalSociety of Photogrammetry and Remote Sensing (ISPRS) in 1980. Technologi-cal development continued; now we have a journal of Earth Observation andRemote Sensing. The ITC Journal, which reported on research in photogramme-try, photo interpretation, and cartography became the International Journal ofApplied Earth Observation and Geoinformation.


1.2. REMOTE SENSING FOR EARTH OBSERVATION 43

1.2 Remote sensing for earth observation

Why remote sensing, what is different from other ways of earth observation?This section will generically answer the question using the example of El Niño.

Figure 1.1: Sea sur-face temperature asdetermined from NOAA-AVHRR data. Courtesy ofNOAA.

Global warming and the rapid change of climate have an enormous impact onour well-being. In early 1998 we observed particularly abnormal weather inmany parts of the world. There was very heavy rain in otherwise dry areas,causing landslides in the Andes. There was drought and huge forest fires inIndonesia. The devastating weather coincided with a strong rise of sea watertemperature in the Eastern Pacific Ocean. Fishermen of Peru have named thephenomenon of water temperature change El Niño, because it happens aroundChristmas, not every year but in intervals of 4 to 9 years. The last El Niño eventwas in 2006. The 1982 El Niño event caused an estimated economic loss of 8.2billion $. The most direct impact of the observed cyclic temperature rise is the



interruption of the fishing season in the Eastern Pacific. If we better understoodthe causes of El Niño and its effects on the climate, we would have a better RS provides dense data for

large areasstarting position for taking preventive and mitigation actions. To develop andsupport a theory, we have to make observations, among others on sea watertemperature. How does temperature change in space and time? To find outwe could place buoys in the ocean and continuously measure the temperaturethere. The west to east extent of the Pacific Ocean is roughly 16,000 km. Is thespatial variation of water temperature so small that we get sufficient informationby placing buoys with a spacing of 2000 km? Or would we need a network ofsmaller meshes and possibly a different spacing in the west-east direction andthe north-south direction? The thermal scanners on board of the meteorologi-cal/environmental NOAA satellites can provide us with data at 1 km spacing.We process the recordings of the scanner - which includes correcting the atmo-spheric distortions of the thermal emission of the water - to calculate surfacetemperature and derive sea surface temperature maps (Figure 1.1).

By comparing the temperature measurements at the buoys with the recordingsof the scanner we can calibrate the processing and thus obtain accurate temper- RS benefits from in situ

observationsature values for a network of much higher density than the one of the buoys.This principle of “ground control” applies to most RS methods.

A thermal scanner gives us the temperature of the sea surface not the tempera-ture of subsurface currents, while the latter is possible with buoys. In general,the observations by RS relate to a thin layer of the Earth’s surface, which may be RS observes surface

featuresconsidered a limitation of RS. Being interested in subsurface features we have touse additional information on how they manifest themselves in surface features.



The EuropeanMeteosat provides a new image of the same area every 30minutes RS offers high surveyrepeatability(since 1977). NASA’s NOAA satellites have a revisit time of 24 hours.

The NOAA satellites do not only have a thermal scanner on board (labelledAVHRR) but also other instruments, which provide us with data on atmospherictemperature, the concentration and distribution of ozone in the stratosphere, etc.To acquire data for studying a complex phenomenon like El Niñowe have to rely RS is “multi-channel”on sensors on various platforms. NASA’s scatterometer aboard the QuikSCATsatellite provides us with information on speed and direction of ocean winds(Figure 1.2). NASA uses OrbView-2 data to study global warming; it can alsoprovide us with ocean biomass estimates (Figure 1.3). We use spaceborne radarand laser altimeters to determine sea level changes (Figure 1.4).

The Pacific Ocean is known for its unfavourable weather and current conditions.It is quite difficult to install and maintain a network of measuring buoys in thisregion. A RS approach is specifically suited for areas that are difficult to access.A related topic is that of acquiring global or continental data sets. RS allows datato be acquired globally using the same or a similar sensor. This enables meth- RS is the only wayods for monitoring and change detection. RS has become an important methodof earth observation and for many applications it is the only suitable one - sincean increasing number of issues are of global concern, such as climate change,environmental degradation, natural disasters, and population growth.

Data on sea surface temperature acquired for studying El Niño could be used



Figure 1.2: Ocean sur-face wind as determinedfrom scatterometer mea-surements by QuickSCAT.Courtesy of NOAA.

years later for studying the influences on algae bloom around the Pacific islands. RS can serve severalpurposesSea surface temperature maps are not only of interest to researchers; they are

used by fishing companies to guide their vessels to promising fishing grounds.Moreover, RS images can also prove useful for studies on phenomena we are notyet (fully) aware of.

The validity of the statement that RS is cost-effective is sometimes hard to assess,especially when dealing with spaceborne remote sensing. Consider an interna- RS is cost-effectivetional scientific project that studies the El Niño phenomenon. Installation andmaintenance of buoys cost a lot of money. Meteorological satellites have already



Figure 1.3: Ocean (andland) biomass as deter-mined from OrbView-2data. Courtesy of Orbim-age.

been paid for and the data can be considered “free”. Further use of RS thenbecomes cheap. However, in order to obtain accurate and “in depth” sea watertemperature values, we still have to place buoys for calibration purposes and fordetermining subsurface characteristics.

The above statements on RS characteristics are supported by an example of ascientific problem related to a global phenomenon. However, they are applica-ble also to other applications. You may analyze yourself whether all the abovestatements would also apply, eg, to the monitoring of (illegal) urban growth



Figure 1.4: Sea surfaceelevation above meansea-level as determinedfrom spaceborne radarand laser altimeters.Courtesy of the Universityof Texas.

around cities using aerial photographs. You will probably conclude that youdo not have all the knowledge required to comment on these statements in theurban context. You may even find it difficult to identify which remote sensingtechnique meets your own particular information requirements. After studyingthe textbook Principles of Remote Sensing you may expect to be better equippedto consider these issues.



2. Electromagnetic energy3. Spatial referencing4. Platforms & sensors 9. Aerial photography 10. Radar & laser scanning 14. Subsurface remote sensing 6. Geometric operations 11. Atmospheric corrections 5. Visualization & radiometric operations 7. Visual interpretation 8. Digital classification 12. Thermal remote sensing 13. Spectrometry

Figure 1.5: Relationshipbetween the chapters ofthis textbook and the re-mote sensing process.


1.3. STRUCTURE OF THE TEXTBOOK 50

1.3 Structure of the textbook

Figure 1.5 relates the chapters of this book to the overall remote sensing process.Chapter 2 reviews the relevant properties of electromagnetic energy, which is re-flected or emitted by the Earth’s surface and travels through the atmosphere andmay be even outer space before reaching the remote sensor. Chapter 2 also in-troduces the basics of sensing electromagnetic energy. Chapter 3 is very short; itpoints at determining position on the Earth’s surface. Spatial reference systemsand spatial positioning are prerequisites for dealing with geometric aspects ofearth observation, which is done in Chapter 6. Chapter 4 is on technology, dis-cussing airborne and spaceborne platforms and sensors, which detect and recordelectromagnetic energy radiated by the Sun or the Earth. Active sensors are sen-sors which do not depend on an external source of energy - they emit their ownand sense what is reflected by the Earth’s surface; they are treated in Chapter 10.Chapter 9 is also a special chapter; it reviews aerial photography, which is thebasis of the oldest remote sensing method. Sensing alone only provides us withraw data, which need to be processed. We have to apply radiometric processingto obtain data that more readily reveal physical object properties; Chapter 11 in-troduces methods of ‘image restoration’ and ‘atmospheric corrections’. Chapter5 deals with different techniques of converting data to actual images and en-hancing remote sensing images to facilitate their interpretation. Visual imageinterpretation and information extraction by algorithm driven classification areintroduced in the Chapters 7 and 8. Special Chapters are again the last three (12,13, 14) on thermal remote sensing, spectrometry, and subsurface remote sensing.



Summary

Many human activities and interests are spatial in nature. We need geospatialdata for navigating and planning, for mapping, monitoring, modeling, and de-cision making. This introductory chapter has exposed the concepts of earthobservation, remote sensing (RS), and geospatial data acquisition (GDA). Var-ious examples were given to illustrate the utility of earth observation by remotesensing, indicate the variety of applications and the diversity of data acquisitiontechniques.



Questions

The following questions can help to study Chapter 1.

1. To what extent is geospatial data acquisition applied by your organiza-tion/company?

2. Which methods of in situ observation and which remote sensing methodsare applied by your organization/company to acquire geospatial data?

3. Remote sensing data and derived products are available on the internet.Locate three web-based catalogues or archives that comprise remote sens-ing data.

4. Following the El Niño example, outline how the statements about RS relateto a particular case of your organization (eg, land resources, geology, etc).

These are typical exam questions:

1. Explain, or give an example, how in situ and remote sensing methods maycomplement each other.

2. List three possible limitations of remote sensing data.


Documents

1. Introduction to Earth Observation by Remote Sensing