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Remote sensing: a tool to monitor and assess desertification

De
48 pages
A whole range of satellites and sensors allows environmental monitoring, comparisons in time and space, and then a better understanding of ecosystems and planet functioning. This document explains how to pass from satellite data to useful information for combating desertification.
Begni Gérard, Escadafal Richard, Fontannaz Delphine and Hong-Nga Nguyen Anne-Thérèse, 2005. Remote sensing: a tool to monitor and assess desertification. Les dossiers thématiques du CSFD. Issue 2. 44 pp.
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CSSFFD
Les dossiers
thématiques
Issue 2
Remote sensing:Remote sensing:
a tool to monitor and assess
desertification
Comité Scientifique Français de la Désertification
French Scientific Committee on DeserLes dossiers thématiques du CSFD
Issue 2
Managing Editor CSFD
Marc Bied-Charreton
President of CSFD
Emeritus Professor of the University of Versailles
Saint-Quentin-en-Yvelines (UVSQ)
Researcher at C3ED-UMR IRD/UVSQ
(Centre of Economics and Ethics
for Environment and Development)
Authors
Gérard Begni
Director of Médias-France
The French Scientific Committee on Desertification gerard.begni@medias.cnes.fr
Richard Escadafal
Researcher at CESBIO The creation in 1997 of the French Scientific Committee on
(Centre for the Study of the Biosphere from Space)
Desertification (CSFD) has met two concerns of the Ministries in for IRD (Institut de recherche pour le développement)
charge of the United Nations Convention to Combat Desertification. richard.escadafal@cesbio.cnes.fr
Delphine Fontannaz First, CSFD materialises the will to involve the French scientific
Research Engineer at Médias-France community versed in desertification, land degradation, and
delphine.fontannaz@medias.cnes.fr
development of arid, semi-arid and sub-humid areas, in generating Anne-Thérèse Hong-Nga Nguyen
Researrance knowledge as well as guiding and advising the policy makers and
anne-therese.nguyen@medias.cnes.fr actors associated in this combat. Its other aim is to strengthen the
position of this French community within the international context.
Contributors
In order to meet such expectations, CSFD is meant to be a driving
force regarding analysis and assessment, prediction and monitoring, Taoufiq Bennouna, Scientific and Technical
Adviser at OSS (Sahara and Sahel Observatory) information and promotion. Within French delegations, CSFD also
Antoine Cornet, Research Manager at IRD takes part in the various statutory meetings of the organs of the
Éric Delaitre, Researcher at the ROSELT/OSS (Long-Term
United Nations Convention to Combat Desertification: Ecological Monitoring Observatories Network / Sahara and
Conference of the Parties (CoP), Committee on Science and Sahel Observatory) Regional Coordination Unit of IRD
Frédéric Dumay, Design Engineer at the Laboratory Technology (CST), Committee for the Review of the Implementation
of Zonal Geography for the Development (LGZD) of the Convention. It also participates in meetings of European and
at the University of Reims Champagne-Ardenne
international scope.Monique Mainguet, Member of the University Institute
of France (IUF) and Director of the Laboratory of Zonal
Geography for the Development (LGZD) CSFD includes a score of members and a President, who are appointed
Bernard Toutain, Researcher at Cirad Emvt (Department intuitu personae by the Minister for Research, and come from various
of Animal Production and Veterinary Medicine of the French
specialities of the main relevant institutions and universities. Agricultural Research Centre for International Development)
CSFD is managed and hosted by the Agropolis Association that
gathers, in the French town of Montpellier and Languedoc-Roussillon Editor
region, a large scientific community specialised in agriculture, food
Isabelle Amsallem (Agropolis Productions) and environment of tropical and Mediterranean countries.
The Committee acts as an independent advisory organ; Photography credits
it has neither decision-making powers nor legal status.
Its operating budget is financed by subsidies from the French
Ministries of Foreign Affairs and for Ecology and Sustainable
Danièle Cavanna (INDIGO picture library of IRD),
Development. CSFD members participate voluntarily to its activities,
June Cools (EOWorks), Jean-Marc D’Herbès and
as a contribution from the Ministry for Research Éric Delaitre (IRD/ROSELT/OSS Programme),
Frédéric Dumay (LGZD), Nadia Imbert-Vier (European
Space Agency), Josef Jansa and Klaus Scipal (Institute More about CSFD:
of Photogrammetry and Remote Sensing, Vienna University
www.csf-desertification.orgof Technology), Lionel Jarlan (Météo-France),
Sandrine Jauffret (OSS), Édouard Lefloc’h andet
Christian Floret (CEFE – Centre for Evolutionary and
Functional Ecology), Marc Leroy (Médias-France),
Monique Mainguet (LGZD), Éric Mougin (CESBIO),
James P. Verdin and James Rowland (U.S. Geological
Survey), Seydou Traoré (AGRHYMET Regional Center),
Ann Tubbeckx (VITO), Nancy Walker (W.H. Freeman
and Company), Professor E. Zakarin (National Center for
Radio Electronics and Communications),
the POSTEL team (Pôle d'Observation des Surfaces
continentales par TELEdétection) as well as the authors Redaction, production and distribution of Les dossiers thématiques du CSFD are
of the pictures shown in this report. fully supported by this Committee through the backing of relevant
French Ministries. Les dossiers thématiques du CSFD may be freely downloaded
Design and production from the Committee website.
With special contribution from Olivier Piau (Agropolis Productions)om
the French Space Agency (Centre National d’Études Spatiales, CNES) the French Space Centre N’Études Spatiales, CNES)
Printed by Les Petites Affiches (Montpellier, France)
Translated by Catherine Tiné
Registration of copyright: on publication ISSN: 1772-6964
1,500 copies also available in French
© CSFD/Agropolis, December 2005Foreword
ankind is facing a world-wide concern, i.e.,Marc Bied-Charreton
desertification, which is both a natural phe-President of CSFD
nomenon and a process induced by humanEmeritus Professor of the University of Versailles
Saint-Quentin-en-Yvelines (UVSQ)M activities. Our planet and natural ecosys-
Researcher at C3ED-UMR IRD/UVSQ tems have never been so much degraded by our presen-
(Centre of Economics and Ethics for Environment and ce. Long considered as a local problem, desertification
Development) now belongs to global issues that affect us all, whether a
scientist, a decision-maker, a citizen from the South or
from the North. Within such a context, it is urgent to mobi-
lise the civil society and induce it to get involved. To start
with, people must be given the elements necessary to
understand better the desertification phenomenon and
its stakes. Scientific knowledge must be brought within
everyone’s reach, in a language understood by the great
majority. Within this scope, the French Scientific
Committee on Desertification has decided to launch a
new series entitled "Les dossiers thématiques du CSFD",
whose purpose is to provide appropriate scientific
information on desertification, its implications and sta-
kes. This series is intended for policy makers and their
advisers, whether from the North or from the South, but
also for the general public and for the scientific journa-
lists involved in development and environment. It also
aims at providing teachers, trainers and trainees with addi-
tional information on various fields. Lastly, it endeavours
to help spreading knowledge to the actors part of the com-
bat against desertification, land degradation, and pover-
ty, such as representatives of professional, non-
governmental, and international solidarity organisations.
A dozen reports are devoted to different themes such as
biodiversity, climate change, pastoralism, remote
sensing, etc; in order to take stock of the current kno-
wledge on these various subjects. The goal is also to set
out ideological and new concept debates, including
controversial issues; to expound widely used methodo-
logies and results derived from a number of projects; and
lastly, to supply operational and intellectual references,
addresses and useful websites.
These reports are to be broadly circulated, especially
within the countries most affected by desertification, by
e-mail (upon request), through our website, and in print.
Your feedback and suggestions will be much appreciated!
Redaction, production and distribution of "Les dossiers
thématiques du CSFD" are fully supported by this
Committee thanks to the backing of relevant French
Ministries. The opinions expressed in these reports are
endorsed by the Committee.
1Preamble
ince the dawn of time, the first hunters, and laterHubert Curien
the first shepherds and farmers, observed theirMember of the French
environment with their eyes and brain. They thusAcademy of SciencesSconceived systems of interpretation that enabled
them to know where to sow and plant, where to graze their
animals, and where to build their villages. Then several
thbig revolutions occurred, in particular during the 19
century: opticians invented telescopes and binoculars,
Niepce and Daguerre invented photography, and the
brilliant Nadar was the first ever to set up a photographic
camera in the gondola of a balloon. Aerial photography
was born.
Initially much used during World War I to locate the ene-
my's position, this technique expanded out of the milita-
ry field to become the essential tool of every cartographer
and town and country planner around the world. At the
beginning of the 60', the first meteorological satellites
appeared, which have become essential to short-term
forecasts. Earth observation satellites came out in 1972
with the US Landsat series, and the generation of high
resolution satellites began with the French SPOT satellite
in 1986. Today, a wide range of high, medium and low
resolution satellites and sensors are available to monitor
our environment, to make comparisons in time and space,
and to model our ecosystems and planet in order to know
them better.
Thanks to these means, lots of data are received daily, but
they are too often the privilege of scientists and people
in memoriam highly skilled in their processing. Their use in developing
countries, and especially in arid, semi-arid and sub-humid
Hubert Curien redacted the areas, began about two decades ago. The huge services
preamble of this report in January that these new techniques could supply, in particular to
2005, little before departing this life assess degraded areas and try to forecast trends, were soon
thon February 6 . We wish to pay him realised.
a special tribute, considering his Considering that such techniques should not remain the
constant concern to disseminate prerogative of technicians of developed countries, many
scientific and technological results cooperative actions have been implemented and are still
to the greatest possible majority. ongoing within bilateral or international frameworks. To
allow development stakeholders and decision-makers to
We will miss a great man who had been General use the results obtained, it is necessary to popularise such
Director of the French National Centre for Scientific results and to provide information regarding their limits
Research (CNRS), President of the French Space and costs.
Agency (Centre National d’Études Spatiales, CNES), This is the aim of the current CSFD publication, and I
Minister of Research, Prrench Academy congratulate the Committee and the authors on their
of Sciences and President of the Cirad Ethics efforts in making accessible to a wide audience the com-
Committee. plex steps required to convert data recorded onboard satel-
lites into useful information.
2 Remote sensing, a tool to monitor and assess desertification
??Table of Contents
4 30
Remote sensing, a support Remote sensing: successes,
to the study and monitoring of limits, open questions
the Earth environment and outlook
12
Remote sensing applied 33
to desertification monitoring List of acronyms
and abbreviations
22
A few examples of remote 34
sensing used at various scales To go deeper…
3Re mote sensing, a support
to the study and monitoring
of the Earth environment
arth observation technologies play a major part
in the study, modelling and monitoring of envi-
ronmental phenomena, at various spatial andE temporal scales, and on an objective, exhausti-
ve and permanent basis. These technologies therefore
open the way for the implementation of early warning
systems, and capacitate policy- and decision-makers to
set out relevant strategies for sustainable development.
Various national and international programmes for space-
based Earth observation (LANDSAT, SPOT, IRS, ERS,
ADEOS, RADARSAT, ENVISAT, TERRA, METEOSAT, MSG,
etc.) have been implemented as soon as 1960 and are still
ongoing. They evidence the degree of priority that States
(among which France plays a prominent part) attach to
this technology. Up to now, the progress achieved
(satellite design, measurement instruments, etc.) offers
increasing capabilities to study and monitor our envi-
ronment as well as global change.
Remote sensing: both a science and a technology
Remote sensing is defined as “all the knowledge and tech-
niques used to determine the physical and biological cha-
racteristics of an object by measuring it without physical
1 thcontact with it” (From Journal Officiel , December 11 ,
1980).
Instead of this quite broad definition, remote sensing is
usually understood as a tool that allows to study
Global view of the Earthphenomena involving only electromagnetic waves,
© NASA-MODIS
mainly detected and recorded by sensors onboard Source: NASA “Visible Earth” website,
2 consulted on January 11th, 2005. http://visibleearth.nasa.govplanes or satellites . Remote sensing is consequently a
way to define an object or group of objects on the Earth
surface from its particular features:
A spectral signature, i.e. a characteristic electroma-
gnetic signal or set of electromagnetic signals in specific
Remote sensing is both a technology and a science thatmore or less narrow wavelength(s) of the electromagnetic
enables to observe and analyse our environment and sub-range;
sequently to define, monitor and assess policies for natural A temporal variation in this spectral signature;
resource management. Satellite-based remote sensing is A determined spatial distribution of this object;
currently one of the only tools that allow to collect One or several relations of this object with the other
detailed information (quite) anywhere on Earth, quicklyobjects that surround it, i.e. the so-called “neighbouring
and objectively, regularly and repetitively, thus enabling toobjects”.
monitor environmental events (pollution, forest fires, earth-
quakes, floods, desertification, etc.). It also allows to deriveVegetation, lands, rivers, water-covered areas, buildings,
applications in many fields such as agriculture, forestry,and generally speaking, any element located on the Earth
hydrology and water resources, oceanology, geology, map-surface and interacting with an electromagnetic
ping, town planning, cadastre, as well as strategic radiation, are considered as objects.
information (most of remote sensing techniques were first
1 Bulletin of official announcements issued by the French Republic (translator’s note).
developed for military purposes).
2 This use is not universal; for instance, our Russian colleagues have a much
wider definition of remote sensing.
4 Remote sensing, a tool to monitor and assess desertificationAerospace remote sensing appeared in the 60’s, but was
really developed at an international scale with the NASA
(National Aeronautics and Space Administration) LANDSAT
programme in 1972. A second key date was indisputably
the launching of the SPOT satellite by France (with
Swedish and Belgian contributions) in 1986.
A number of programmes and satellites have followed
since then, and the design of satellites together with the
conception and variety of measurement instruments have
been substantially improved, thus enabling to collect a
great variety of highly accurate top-quality data. Space-
based remote sensing already offers considerable capa-
bilities, but many studies remain to be undertaken in order
to further enhance its use.
Brief history Remote sensing principles: the basics
Remote sensing was born with the first aerial black and Remote sensing uses the physical properties of objects,
white photograph taken by Nadar from a balloon above commonly called targets, to collect information on their
Paris in 1858. However, aerial photography, that allows to nature and define them. It supposes an interaction bet-
obtain a global vision of our environment, was actually ween the energy that is transmitted by electromagnetic
developed during World War I. At first limited to the visi- radiation coming from a natural (e.g. the sun) or artificial
ble range (wavelength [λ] between violet [0.4 µm] and (e.g. microwave emission) source, and the target. This
red [0.8 µm]), photography was then extended to the near energy is then sensed by an observing system, the sensor
infrared radiation (λ between 0.8 µm and 1 µm). From the (embarked onboard a satellite), that records it and trans-
60’s, its until then military use was broadened to civilian mits it to a receiving station, then transforming this signal
applications such as vegetation study. into a digital image. Electromagnetic radiation interacts
From World War II, airborne remote sensing techniques with the atmosphere a first time when it passes through
were enhanced, especially through the development of from the source to the target, and then in the opposite
new instruments such as radars (the first imaging radars direction, from the target to the sensor. These interactions
were made in England in order to improve the accuracy induce modifications in the electromagnetic signal, which
of night bombing). are used to characterise the object observed at ground.
Remote sensing, a support to the study and monitoring of the Earth environment 5Basic physical foundations
Electromagnetic spectrum and radiation sources
The electromagnetic spectrum is divided into different
ranges, from short to long wavelengths. Space-based
remote sensing only uses part of the electromagnetic spec-
trum, on technological grounds and also because the
atmosphere is not “translucent” in all the wavelengths.
These ranges are mainly the following ones: visible
(λ between 0.4 µm and 0.8 µm), near infrared (λ between
30.8 µm and 1.1 µm), middle infrared , thermal infrared
(λ between 10 µm and 12 µm, which is the radiation emit-
ted under the form of heat by the Earth surface), and
microwave range (radar remote sensing). There are two Artist view of an ENVISAT satellite
© ESA-DENMAN Productionsmain types of remote sensing, passive remote sensing
and active remote sensing (radar). Passive remote
sensing resorts to passive sensors that measure the
natural radiation reflected by objects on the Earth The three types of electromagnetic radiation:
surface, whereas with active remote sensing, the system reflected, emitted and backscattered radiation
both emits and receives an electromagnetic signal.
Signal reflected by objects on the Earth surface
Electromagnetic radiation may be transmitted by When solar radiation hits the ground, it is in part reflected
different sources: to the atmosphere off the Earth surface and objects at
ground. Signal reflection depends on the nature and
The sun (visible, near and middle infrared ranges): properties of the surface and on its wavelength. On
sensors record the solar energy reflected by objects on the perfectly smooth surfaces, the whole solar energy is
Earth surface. reflected in a single direction, whereas on rough surfaces,
The ground (thermal and microwave fields): remote it is reflected in every direction (as usually occurs). In such
sensing receivers record the energy emitted by the Earth case, the solar flux reflected mainly corresponds to the
from its surface temperature. visible and near infrared ranges. Recordings are only pos-
A so-called artificial source, i.e. an active sensor sible during daytime and if the atmospheric transmission
(e.g. lasers and microwave radars). of electromagnetic radiation is good. With active remote
sensing (radar), the energy reflected in the direction of
Disturbances caused by the atmosphere on radiation the sensor is said to be backscattered.
Solar radiation, emitted or backscattered by objects at
ground, is subjected to alterations or disturbances Energy emitted by objects
(refraction, absorption, scattering, proper emis- With passive remote sensing, sensors measure the energy
sion) of various kinds when its passes through the atmo- directly emitted by objects, in the thermal infrared as well as
sphere. Indeed, the atmosphere allows electromagnetic microwave ranges. This energy is related to the temperature
radiation to pass through in specific spectral bands only, and surface state of objects. Contrary to the former case, the
the so-called “atmospheric windows”. Atmospheric signal emitted can be measured night and day.
influence must therefore be considered by modelling, in
order to compute fluxes measured by space-based Energy backscattered by objects
4sensors . This concerns active remote sensing: the observing
system includes both a transmitter (artificial source) and
a receiver, usually located at the same place. The electro-
3 Middle infrared is usully limited to a wavelength range in which thermal magnetic radiation that is emitted in the direction of the
emission is not significant (λ < 5 µm). target interacts with its surface and is scattered in every
4 A computer displays each digital value of an image as light intensity.
direction. Part of the energy is consequently reflected in
6 Remote sensing, a tool to monitor and assess desertificationRadiation – atmosphere –
target – interactions
the direction of the sensor: this is the backscattered signal. Sensors
The basic principle of a radar is transmission and recep- They are measurement instruments that allow to collect
tion of pulses, which makes it sunlight-independent. and record data on objects observed on the Earth surface
Radars consequently allow night and day recordings, and (in one or several given wavelengths) and to transmit them
are particularly useful in cloudy areas (microwaves do not to a receiving system. There are passive sensors that only
depend on meteorological conditions), where it is often record the solar radiation reflected or the own radiation
difficult to collect data in the visible or near infrared emitted by objects, and active sensors that both emit and
ranges. receive the energy reflected by the target. Sensors are
characterised by their:
Elements of remote sensing systems Spatial resolution: It corresponds to the size of the
smallest element (Pixel) detectable on the Earth surface.
A remote sensing system is a whole combination that The sharpness and details that can be distinguished in a
includes a platform, one or several sensors, and various remotely sensed image are a function of spatial resolution.
means of controlling the system and of processing the Spectral resolution: It is defined as the width or wave-
data collected. length range of the part of the electromagnetic spectrum
the sensor can record and the number of channels the
Platforms sensor uses.
They are aerial (plane or balloon) or spatial (satellite) vehi- Ground swath: It is the surface observed at ground
cles that embark tools (sensors) to measure and record (the targeted scene).
data collected on objects observed at ground. A satellite
may be sun-synchronous or geostationary. Because of their high altitude (36,000 km), sensors of
geostationary satellites observing large surfaces cannot
supply detailed images of our planet. On the contrary,
sensors onboard lower orbiting satellites (for instance,
sun-synchronous satellites, from 750 km to 900 km
height) provide detailed images but on smaller areas.
Control and receiving facilities
A satellite remote sensing system is always associated with
a mission (or programming) centre that regularly defines
the tasks to be performed by the satellite, a control
centre to pilot the satellite, data receiving and recording
stations, one (or several) data pre-processing centre(s),
and structures for data dissemination (distribution /
marketing). Pre-processing centres (that are often
Sun-synchronous orbiting satellite
(in white: ground track combined with receiving stations) supply standard
and swath) products of easier use.
Remote sensing, a support to the study and monitoring of the Earth environment 7Characteristics of the main current and future operational sensors and satellites – A non-exhaustive list
Satellite Panchromatic Spatial Spectral Ground Derived products
Multiband resolution resolution Swath (non-exhaustive list)
Very high spatial resolution satellites
SPOT 5 Panchromatic 2.5 m and 5 m Optical 60*60 km Maps (geological, soil, land cover, vulnerability
Multiband 10 m 60*120 km maps), satellite image maps, informative plans
(river systems, road and railway networks), Digital
Terrain Models (DTMs)
IKONOS 2 Panchromatic 1m Optical 11*11 km Maps, satellite image maps, informative plans, DTMs
Multiband 4 m
QUICKBIRD Panchromatic 0.60 and 0,7m Optical 16.5 km
Multiband 2.4 and 2.88 m
ORBVIEW 3 1 and 4 m Optical 8*8 kmmative plans, DTMs
HELIOS 2A 30 cm Confidential Defence
Pléiades Panchromatic 0.7 m Optical 21 km Maps, satellite image maps, infor
(2008-2009) Multiband 2.8 m
EROS A Panchromatic 1 - 1.8 m Optical 12.5*12.5 kmmative plans, DTMs
ROCSAT-2 2 - 5 m 24*24 km
Multiband 8 - 20 m
IRS-P6 5.8 m Optical 24 to 70 km Maps, satellite image maps, informative plans
23 m 140 km
60 - 70 m 740 km
RADARSAT-1 3 to 100 m Radar 20 to 500 km Informative plans, DTMs, maps (soil moisture,
flooded area maps)
Medium spatial resolution satellites
ERS 1,2 25 m Radar 100 km Coherence products that allow to derive land cover
(especially in tropical areas) and geological maps.
Soil moisture, flooded area maps. DTMs
SPOT 1, 2, 3 Panchromatic 10 m Optical 60*60 km Maps, satellite image maps, DTMs, informative plans
and 4 Multiband 20 m 60*80 km
LANDSAT 7 (ETM) Panchromatic 15 m Optical 185*170 kmmative plans
Multiband 30 m
LANDSAT 4, 5 30 and 80 m Optical 185 km Maps, satellite image maps, informative plans
ENVISAT (ASAR) 10 to 1,000 m Radar 15*5 km to Maps (geological, topographical, soil moisture,
405*405 km flooded area, marine pollution, coastal dynamics,
glaciology maps), informative plans, DTMs
TERRA (ASTER) Multiband 15 to 90 m Optical 60 kmmative plans, DTMs
Low spatial resolution satellites
SPOT Multiband 1 km Optical 2*2 km Synthesis products (daily, ten-day syntheses),
(VEGETATION) NDVI (Normalised Difference Vegetation Index)
METEOSAT Multiband 2.25 and 4.5 Optical Hemisphere Meteorological, oceanographic
km and geophysical products
MSG (Meteosat Multiband Optical Hemisphere Meteorological, oceanographic
Second 1 and 3 km
Generation)
ENVISAT (MERIS) Multiband Optical 1,150 km Products derived from ocean colour measurements
300 m (carbon cycle, fishing area management, coastal
area management…)
SMOS Radar 1,000 km Maps (soil moisture, ocean salinity maps)
(Feb. 2007) 35 and 50 km
PARASOL Multiband Optical 2,400 km Radiative budget maps, observation
6*7 km of clouds and aerosols
Correspondence between satellite image resolution and map scale:
1,000 m -> 1/1,500,000 30 m -> 1/80,000 20 m- > 1/50,000 10 m- > 1/24,000 5 m -> 1/12,000 1 m -> 1/2,000
8 Remote sensing, a tool to monitor and assess desertification

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