Hello everyone and welcome to this 6th lecture In this course there is introduction to remote
sensing and in this particular topic we will be discussing the interaction mechanism of
electro magnet radiation with the objects which are present on the ground especially
vegetation bare soil water bodies and their response spectral response curves which we
will see how this curves of different objects different wavelengths they behave
For example when there is a when this solar radiation interacts with the target and example
here of a vegetation then there might be reflection from the vegetation there might be some aborptions
of solar energy by the plants and there might be transmission So all these three things
will keep occurring whereas special leaves specially if they are having chlorophyll content
then they will have a different response in visible part compared to the infrared channel
and so on As we can see in this small spectral curves of three types of objects which are
shown here and these are more or less standard spectral curves wavelengths on the x axis
and reflection in percentage is given in the y axis
But we see that the bare soil shows the maximum reflection around 1.4 to 2.6 micrometer and
whereas the vegetation which is very important to notice that suddenly around 0.7 it jumps
and it shows the very high reflection compare to other parts of spectrum Whereas the water
only shows in a very small part of EM spectrum some reflection and then there is a sort of
complete absorption
So all these reflection absorption and transmission phenomena will occur together and different
objects in different parts of EM spectrum will behave differently as we are seeing here
we will see these things in much more in details Radiation that is not absorbed or scattered
in atmosphere can reach and interact with the earth surface and remember that when the
solar radiation is coming some part of some portion of that is absorbed within the atmosphere
but remaining part may reach to the earth surface and can interact with the objects
which are present there And the three forms of interaction can take place when the energy
strikes or it is the incident upon the surface and these three are absorption which we have
discussed transmission and reflection
So the total incident energy will interact with that surface in one or more these three
ways The proportions of each will depend on the wavelength of energy that is very very
important and the material and the conditions of the features Condition of the features
means if the vegetation is very healthy having high chlorophyll content it will behave differently
but the same vegetation if it is not having this drying up or having some disease it will
behave differently in different part of EM spectrum That is why it is mention is the
condition of feature
Now this radiation or interaction there might be two ways that one is this reflection and
there might be a reflection by the man made surfaces or some natural surfaces and which
we call as a specular reflection and there might be a defuse reflection maybe because
of vegetation maybe bare soil some rocks and other thing These will affects the signatures
or their presence in satellite bends images get different bands Two types of reflection
which are observed may leave which are two extreme ends the way energy interact and the
reflected by one is specular reflection maybe from the roads maybe from it come water surface
and so on Whereas the defuse reflection maybe from the vegetation in other places
So specular reflection the maximum energy is reflected back whereas defuse reflection
some and it is scattered all around So when the surface is smooth we get specular or mirror
like reflection where all or almost all of the energies are directed from the surface
is in the single direction and whereas defuse reflection occurs when the surface is rough
and the energy is reflected almost uniformly in all directions And most of the earth features
which are not really perfect specular reflectors or neither defuse reflectors
Most of the features falls in between of these two categories and whenever a particular target
reflects (())(5:49) or defusely or somewhere in between depends on the surface roughness
of the feature in comparison to the wavelength of incoming radiation
And the example here is the wavelength as much smaller than the surface variation on
the particle size that make the surface defuse reflection will dominate For example fine
green sand specially present in the desert areas could appear fairly smooth to long wave
micro waves but would appear quite rough to the visible wavelength So it depends the same
surface but depend on the wavelength in which wavelength or in which part of EM spectrum
you are looking or which band of the satellite image you are looking The same object will
look very smooth or in a larger wavelength but may look rough in visible wavelengths
Similarly the leaves say plant leaves chlorophyll strongly absorbs radiation in the red part
and the blue wavelengths but reflects green wavelength that is why we see in the visible
part that is why we see vegetation healthy vegetation generally green because the red
portion the blue portion are absorbed whereas the green is reflected Leaves appears greenest
to most of us in summer when chlorophyll content is at maximum depending on the season of different
areas in the country or world all over whereas in autumn when there is a less chlorophyll
content so there is less absorption and proportionally more reflected of red wavelength and making
leaves appear either yellow or red and yellow is a combination of red and green wavelength
So when chlorophyll is less then the green is not reflected and whereas the red is reflected
or yellow or green is reflected and that makes us yellow
Similarly in this in this healthy vegetation eye act as a excellent defuse reflectors of
near infrared bands infrared wavelengths and that is why you know though in our eyes its
different but in infrared radiation in infrared bands the vegetation is having maximum reflection
so our eyes were sensitive to basically near infrared trees would appear extremely if at
all these our eyes would have been but we are having sensors near infrared and therefore
we get very high reflection and infact measuring and monitoring near infrared reflection is
one way that scientist can determine how healthy or unhealthy vegetation maybe
And this people exploit specially for crop estimations or health of a forest area and
so and so forth that if you are getting very high reflection through infrared channels
of an area which is completely vegetated or having some agricultural crop you can assess
that area is enjoying the growth of plants or good chlorophyll content present in the
leaves but if a this near infrared reflection is less than there might be some problem with
the plants
As you can see that these are the plants seen through the infrared channels and they are
showing very high reflection whereas the roof of a house or a window are showing very low
reflection So this property of plants we exploit in our remote sensing data and we use the
combination to create certain products which we can apply for different kind of applications
Now we take the water then longer wavelength visible in near infrared radiation is absorbed
more by water than the shorter visible wavelength As you saw earlier in spectral curve of water
in the shorter wavelengths there is some reflection by the water otherwise in the longer wavelength
it is completely absorbed And that you can see here also that this is completely absorbed
water typically looks blue green dues to stronger reflection at this shorter wavelengths and
darker if viewed in red and infrared so whenever we see infrared images in which the water
bodies is present generally a water is pure it will completely appear black or dark because
there is no reflection in that part of EM spectrum
Whereas we look the visible part of shorter wavelengths we might see some reflection of
water especially in the blue portion of em spectrum or blue or green that is why we see
generally water though you know that pure water doesn't have any color but these colors
are coming because of reflection in the different part of EM spectrum and if there are suspended
particles present in this upper layer of water body when this will allow better reflectivity
and brighter appearance of the water So if it is not pure like here suspended particles
are shown here then you will get a larger reflection in this parts and same maybe little
more in the red and infrared maybe less so this kind of changes will occur when we are
having suspended sediments or we also call it turbidity present in the water bodies
And this is the example is shown here this is delta which is shown here and Ganges in
Brahmaputra basically are coming here Sundarban delta and they are bringing sediments and
mixing with this Bay of Bengal and what is happening because of its presence of suspended
particles and high and due to that high reflection especially in the blue part of EM spectrum
and therefore in a land set TM near near real color images we see a blue portions and that
is directly indicating high concentration of a sediments present whereas you don't have
much sediments present then hardly your having reflection and therefore there true color
images they are appearing as black
So this is how the interpretations can be made of satellite images whereas the green
is of course vegetation and there are water bodies in land water bodies which may not
be having suspended particles and therefore they are appearing black So the quality of
water can also be assessed through such interpretations of satellite data
As I have already mentioned that suspended sediments which shown here which can be easily
confused with the shallow but clear water but these two phenomena appear very similar
that means that if water conditions are very shallow then the bottom of say there is a
lake it is very shallow then there the bottom of the lake will also reflect So one should
not get confused with that kind of thing Shallow the less column of water may also reflect
whereas the same time the suspended particles can also reflect So one has to ground check
whether this is because of the suspended particles or shallowness of a lake or a water body
And in normal condition one may find the algae and other things which are present in the
water which will again because algae might be having chlorophyll so chlorophyll in algae
absorbs more blue wavelengths and reflect green making the water appear more green in
color when algae is present And this type of growth of algae is specially in sea water
and detection of this algae is used to find certain type of fish which may preferred that
kind of algae
So this is how remote sensing can be implied to find a certain type growth of algae and
then associated fish might be there and sometimes the undulations or ruggedness or topography
of the water surface that is if it is not standstill water or rough or depending on
the rough or smooth or floating material etc can also lead to complication for water reflected
interpretation do potential problem of specular reflection and other influences on the color
and brightness
This curve spectral curves of three main very standard curve which we have seen already
in a small portion that generally water will reflect only in the shorter wavelength near
0.4 to 2.7 0.1 or 2 and then it is completely absorbed So only in the first part of visible
part of EM spectrum and maximum you would see around blue that is why pure water in
color composite may appear as blue Whereas bare soil it increases very smoothly and in
mid infrared reaches to the maximum and therefore you don't see much reflection of bare soil
in early part of EM spectrum specially visible near infrared
Whereas the vegetation is having altogether different there is a absorption of vegetation
but starting from 0.7 to about 1.2 or 1.3 it is having maximum peak Where you get the
maximum reflection by healthy vegetation green vegetation and if there is a vegetation not
healthy then there might be a shift curve in a and curve may shift towards the left
so we also use this to find out whether vegetation is healthy enjoying good health chlorophyll
content is normal or abnormal if there is not high reflection which one is getting near
infrared
So by measuring the energy that is reflected or emitted by the target on the earth surface
over variety of different wavelengths we can build up a spectral response for that object
These are the standard generalize spectral curves of three different types of earth objects
which you are seeing here By comparing these response pattern of different features like
here soil vegetation in water we may to distinguish between them As you can see that all three
different objects are showing completely different signatures or curves or behavior in this part
of EM spectrum specially in visible near infrared and mid infrared
As we have seen like water and vegetation may reflect somewhat similarly in visible
wavelength but are almost always seperable in infrared so in particle channel or band
you may not differentiate certain objects two objects but if you start looking the different
channels you will find a completely different signatures or there pixel values completely
different in different channels Spectral response can be quite where even vary on same target
type and can also vary with the time especially talking abortions the vegetation so the same
vegetation may reflect differently in different season of a year
So understanding of these factors which influence the spectral response or features or interest
are critical and they makes once we start understanding these spectral curves of different
features especially in mineral exploitation or in other mapping then we can make our interpretation
much more reliable
Here few more curves have been also added and then corresponding bands or channels of
different few satellites are also shown mainly spot and land set So let me go one by one
When you are having clear water lake.. lake water which is standstill Clear means you
don't have the suspended particles you may not be having any algae or any chlorophyll
content then the maximum reflection will come around 0.6 micrometer and then there is a
total absorption in beyond this range Whereas if water is turbid that means there is (())(19:55)
there are suspended particles are present then the reflection may continue even upto
2.9 or very close to 1 even in near infrared part
Similarly in case of wet soil or dry soil this behavior of this soil depending on the
content of water they may show completely different reflection curves As we can see
here that initially they are showing like dry soil will always have more reflection
as compared to wet and there are some portions where you are having maximum difference So
two examples are shown 5% water dry soil and which is be consider 20% wet is and as soon
as soil becomes wet it is showing a less reflection in almost between 0.4 to 2.4 micrometer
Throughout this visible infrared and mid infrared vision whereas vegetation if it is healthy
vegetation it will show very high reflection between 0.7 to 1.1 or 1.2 micrometer and in
between also there are some other peaks are there but otherwise this is the standard one
So initially Landsat 1 was design we also call ERTS and the sensors MSS at that time
these bands were did bands had the same width in the continuous fashion So like 0.4 to 0.5
0.5 to 0.6 likewise but later on it was realized that such approach is not good and therefore
wherever for whichever phenomena which we were looking to discriminate or the objects
which we wanted to discriminate very easily so then these bands and there width were changed
Like in case of land set TM bands as you can see that the land set 1 and 2 are in the blue
and green part 3 is in the red part then 4 is having much more larger width as compared
to 3 and 3 is having very thin band compared to 1 2 or 4 and then because of in between
nothing was kept because if 1 is looking the vegetation then the band 4 is the best in
that sense If 1 is looking for water bodies and other things then 1 and 2 or even 3 upto
3 are good
So then the large gap in EM spectrum was left and then the 5th one was kept here to see
the differentiation and specially another peak of vegetation and then again the band
7 was kept here to catch the peak of wet soil dry soil as well as the vegetation But in
between 5 and 7 the large gap was left Band 6 which is missing here falls in the thermal
infrared part that is why it is not shown here Whereas in case of spot which was a (())() satellite
multi spectral bands which are quite similar to the land set TM bands but there is some
shifting of band 1 instead of starting from 0.4 or 0.45 it really started at 5.5 to 0.6
and very small gap between band 1 and 2 but there is some gap between band 2 and 3
So later on people realized that based on these spectral curves instead of having continuous
bands and having uniform thickness bands it is better to have bands based on these spectral
curves where we can discriminate or record these different objects which are present
on the earth in a different way rather than having uniform slicing and same size or location
or continuous slicing So that was a intelligent move later on once people realize through
the experience of land set MSS
So this brings to the end of this particular topic about the how this solar radiation and
tracks in different parts of EM spectrum with different objects which are present on the
surface of the earth We mainly focused on the water bodies bare soils and vegetation
and how this absorption phenomena and atmospheric windows which are available and how different
satellites and their sensors have been designed and kept at different locations so we can
capture all the important features or objects which are present on the surface of the earth
Thank you very much