Remote sensing divided
into two words which are “remote” and
sensing, where by remote means away from or “at distance”
while sensing means “detecting a
property or characteristics”. Thus, the term remote sensing refers to the examination,
measurement and analysis of an object without being in contact with it. According
to the United Nations (UN) 1986 “the
term remote sensing means the sensing of earth’s surface from space by making
use of properties of electromagnetic wave emitted, reflected or refracted by
the sensed objects, for the purpose of improving natural resource management ,
land use, and the protection of the environment. According to NASA defined
“remote sensing as the instrument based technique used in the acquisition and
measurement of spatially organized (distributed data information on some
properties spectral, spatial or physical contact.Thus remote sensing can be
defined as an a science and an art of acquiring information about the earth
surface without actually being in contact with it, for example camera on
airplanes can take image on large area on the earth surface, allowing to see
much more than we can standing on the ground. Also other examples are balloons,
pigeon fled, and rocket.Spectral reflectance curve is a measure of how much
energy (as percentage) the surface reflects different amount of energy in
different portion of spectrum. These differences in reflectance make it
possible to identify different earth surface features or materials by analyzing
their spectral reflectance signature. Different surface features reflect and
absorbs sun’s electromagnetic energy in different ways. But the reflectance
properties of an object depend on the materials and its physical and chemical
state, the surface roughness as well as the angle of sunlight. Therefore,
spectral reflectance curve is the pictorial presentation of the signature which
corresponds to the electromagnetic spectrum. The spectral reflectance of any
surface is measured with the help of testing instrument called
spectrophotometer. The spectrophotometer used for sending a cluster of
electromagnetic radiation to a sample and then measures its spectral reflectance
by measuring amount of radiation reflected by the surface, with the help of
spectrophotometer one can easily obtain the spectral reflectance curve green
vegetation and ensure the best study of their quality, but also spectral
reflectance can be captured by other remote sensing sensors including those
mounted on aircraft and satellites. Each material on the earth surface has its
own reflectance, but major reflectance materials are soil, water and vegetation.
Soil spectral
reflectance, this increasing with the wavelength in the visible portion of the
spectrum and stay constant in the near infrared and shortwave infrared. The
presence of moisture in the soil will decrease its reflectance, this affect is
greatest in the water absorption band at 1.4, 1.9, 2.2, and 2.7 micrometer, Also, soil texture
such as course, silt, or clay affect the reflectance of the soil.
The reflectance characteristic of vegetation depends on the properties of leaves, including the orientation and structure of leaf canopy. The amount of radiation reflected for a particular wavelength depends on leaf pigmentation, thickness, and composition and the amount of water in the leaf tissues. In the visible portion of the spectrum the reflection of blue and red components of incident light is comparatively low, because these portion are absorbed by plant (mainly by chlorophyll) per photosynthesis, the vegetation reflects relatively more green light. The reflectance in the near-infrared radiation is highest, but the amount depends on leaf development and cell structure. In the shorter wavelength infrared range reflectance is mainly determined by the free water I leaf tissue, more free water result in less reflectance. Wavelength around 1.4 and 1.9 micrometer are called water absorption band.
Water
spectral reflectance, longer wavelength visible and near-infrared radiation is
absorbed more by water than shorter visible wavelength. This make water
typically looks blue or blue-green due to stronger reflectance at these shorter
wavelength and darks if viewed at red or near infrared wavelength. If there is
suspended sediments present in the upper layer of the water body, then this
will allow better reflection and brighter appearance of the water. Turbid water
has highest reflectance, water containing plants or algae has pronounced
reflectance peak for green light because of the chlorophyll present.
The following are the utilities of
spectral reflectance curves in remote sensing;
Spectral reflectance
curve used to indicate the kind of electromagnetic radiation in the
electromagnetic spectrum that are suitable for remote sensing, including those
radiation that support highest reflectance capability according to their
wavelength , frequency and energy. Where in spectral reflectance curve the
radiation of higher reflectance are visible radiation and infrared radiation,
and also I indicates how objects looks like under infrared compared to visible
parts of the spectrum, for example; At visible wavelength both snow and clouds
look very similar because both reflect a lot of light at all visible wavelength
and hence look white to us. However in the infrared part of the spectrum, snow
reflects very little and appears dark where clouds reflects a lot of radiation
and appear bright. So, apart from this difference in reflectance between
visible and infrared in spectral reflectance curve there are some other
electromagnetic radiation that does not support reflectance example, Gamma rays
and x-rays hence not suitable for remote sensing. So through spectral
reflectance curve make easy identification of electromagnetic radiation in the
electromagnetic spectrum that are suitable for remote sensing.
Spectral reflectance curve used to provide
repetitive information of an area suitable for remote sensing; including
different objects or features that return different amount of energy in
different bound of electromagnetic spectrum, and are influenced by different
factors like their structure, chemical and physical properties, surface
wavelength or radiant energy. Example in the vegetation. The health green
vegetation has a significant minimum reflectance in the visible portion of
electromagnetic spectrum that resulting to the pigment in plant leaves and
reflectance tend to increase dramatically in the near infrared and at the same
time stressed vegetation has lower reflectance in the infrared. Also other
object or features like soil reflectance curve indicates how reflectance is
affected by moisture content and soil texture, surface roughness, presence of iron
oxide and organic matter. In water also the reflectance curve indicates where
there is higher absorption and low reflectance. So due to the absorption of
radiation at this region indicates that all water bodies containing clean water
can be delaminated in remote sensing but turbid water have higher reflectance
in the visible region, hence it is suitable for remote sensing because can help
in monitoring the area of interest.
It is useful in
differentiating features or objects on the earth surface, through taking
reference from spectral reflectance curve can help image interpreters to
differentiate various objects found on the earth surface by considering
reflectance ability of materials. For example, an interpreter can determine both
manmade and natural features like forest, water bodies, and houses arrangement
by considering reflectance of those features, but when those features cannot be
seen clearly an interpreter can use other techniques to get information of an
area like by looking shape, shadow, texture and association between the
targeted features with others around them or an interpreter can g direct to
field for actual verification. Hence spectral reflectance curve is very useful
in remote sensing.
It provide information
about the type of plants and its health, by using spectral reflectance curve
can help to provide information about vegetation health especially for those who
are dealing with agriculture and forest. This can help image interpreters to
determine the health of various crops by looking chlorophyll presents in crops,
if the amount of chlorophyll is low means unhealthy crops or vegetation, for
example, during harvest of forest harvesters use sensed image to determine kind
of trees to harvested during that season.
Also there are some plant like soybeans
have spread out leaf clumps, corn has tall stalks with long narrow leaves and
thin also wheat has long thin central stems with a few small bent leaves on
short branches with head containing kernels.
It is useful in determine the biochemical
composition of the soil, soil spectral reflectance signature result from the
presence or absence as well as the position and shape of specific absorption
features of its constituents. Absorption are brought about the various
chemical/physical phenomena such inter-molecular and electronic process in the
atoms the visible and near infrared region (0.4 – 1.3 μm) are characterized by
broad spectral absorption features such as yellowish, ferrous iron absorption
features near (1 μm) and weaker absorption at 0.7 μm and 0.87 μm attributed to
red, ferric iron. Soil is mixture of a number of organic and inorganic
constituent so it’s not straightforward to evaluate the composition of soil
from their spectral signature. Many soil spectral signature match closely,
making it difficult to distinguish them. so this help environmentalist to
provide education about good use of resources as soil concerned.
Spectral reflectance
curve are useful in portraying the map, cartographers use their experience and
reference from reflectance of various materials to a map showing natural and
man made features present on the earth surface by comparing the features present
in sensed image with the reflectance ability of materials. For example, a
cartographer can differentiate layers in his or her drawing by painting green
represent green vegetation, blue represent water bodies and brown represent soil.
Hence spectral reflectance is important in making maps of cities, and local
villages.
Generally,
apart from the utilities of spectral reflectance curve, but also reflectance
curve have many weaknesses like, it requires skilled person to interpret remote
sensing image in relation to spectral reflectance of materials, so if a person
knowledge of comparing the reflectance curve and sensed image leads to errors.
Also various factors occurs on the atmosphere such scattering, absorption may
leads to wrong reading and recording of spectral reflectance of materials. So
it is better to correct those errors on the sensed image to reduce error before
further uses of the information in other sectors such as in agriculture, mining
and marine sectors.
REFERENCE
Drury , S. A (1990). A guide to remote sensing. Oxford University Press; Oxford
Hathout, S(2005). Introduction of remote sensing. Dar-al-maffa, Al-gamaiya ; Alexandria
Paul K (2012). The core of GIScience.ITC: University
of Twente. Netherland
Von der Dunk, Frans (2002).United Nations Principles on Remote Sensing and the User. Space, Cyber, and Telecommunications Law
Program Faculty Publications. 17.
https://www.nasa.gov/content/remote-sensing/
