1. FUNDAMENTALS OF REMOTE SENSING
DR. G.A.B. YIRAN

2. Structure of Course
First part of our meetings each morning deals with theory
Second part focuses on a practical examples using remote sensing data
Practicals: Use ENVI
widely-used
good range of functionality
relatively easy to use (GUI)
Assessment
exam (45%), lab work (40%), attendance/participation (5%) and Seminar presentation (10%)
Lab work write-up submission date – last day after seminar

3. What is remote sensing?
Remote Sensing is the science and art of acquiring information about material objects, area, or phenomenon, without coming into physical contact with the objects, or area, or phenomenon under investigation.
For GI Science, the objects are earth resources and human properties that can be detected
Students should read on historical development of remote sensing

4. Why remote sensing? Advantages: Repeated reliable measurements
Global coverage & access
Frequent access
Non-intrusive
Digital downlink
Worldwide archive (back to 1973)
Fast and cheaper when dealing with large areas
Disadvantages:
Historically low resolution (now 50 cm-commercial)
Still expensive
Atmospheric Conditions (clouds, haze, precipitation)
Distribution networks
Timeliness

5. Principles of remote sensing
Detection and discrimination of objects or surface features means detecting and recording of radiant energy reflected or emitted by objects or surface material.
The energy can be solar, heat, sound, etc.
We largely use solar energy for earth surface, sound for objects below the oceans
The energy must come from a source, incident on the objects and reflected into a recording system or the objects emit the energy and recorded by the system
In both cases, the energy must pass through a medium
Earth objects reflect or emit energy differently and this is key for remote sensing.

6. Principles of remote sensing
A = Energy Source
B =Intervening Atmosphere
C=Interaction with the Target
D= Sensor Recording
E=Transmission, Reception, and Processing
F=Interpretation and Analysis
G=Application
Assignment: explain this diagram in detail

7. Energy – atmosphere – earth interactions
Assignment:
Explain the energy interactions in the diagram
Explain the various types of scattering of energy
Scattered
reflected
energy
Transmission
processes

8. Electromagnetic radiation (EMR)
For remote sensing, the most important source of EMR is the sun. students should look for other sources.
Types of electromagnetic radiation: gamma rays, X-rays, UV, visible light, Infrared, microwaves and radio and TV waves.
These energy types put together and distributed continuously in order of increasing wavelength constitute the electromagnetic spectrum
Thus the electromagnetic spectrum is the full range of radiation distributed continuously in order of increasing wavelength

9. Electromagnetic spectrum

10. Theories of EMR
There are two basic theories that describe EMR – the wave theory and the particle theory
In the wave theory, EM energy is considered to propagates at the speed of light through space in a sine wave form.
The distance between two successive wave crests or troughs is called wavelength (λ)
The number of times a wave cycle passes through a fixed point over a given period of time is called frequency (f) measured in Hertz (Hz)
Speed of light c = frequency (f ) x wavelength (λ), i.e. c = f λ.

11. Theories of EMR
In the particle theory, EM energy is composed of discrete units called photons or quanta
The energy possessed by a photon Q= hf; h = Planck's constant
But c = f λ. This means that f = c/ λ
Therefore energy Q= h(c/ λ). Note c and h are constants
This means that energy is inversely proportional to wavelength and thus, the shorter the wavelength the higher the energy.
This theory is very useful in quantifying the amount of energy measured by the sensors.

12. Atmospheric windows
The part of the electromagnetic spectrum that can pass through the atmosphere with little interference is called atmospheric window
The constituent particles in the atmosphere absorb the radiation
As shown in the diagram portions that have high transmissions are the windows

13. Some terminologies
Spectral reflectance is the ratio of reflected energy to incident energy as a function of wavelength
The value of reflectance of objects averaged over different, well- defined wavelength intervals comprise the spectral signature of the objects or features
It is used to distinguish different objects because different materials have different spectral reflectance characteristics.
Irradiance refers to the light energy per unit time (power) impinging on a surface, normalized by the surface area, and is typically specified in watts per square meter (W/m2).
Radiance is simply how much light the instrument sees from the object being observed (i.e. the reflected or emitted energy that is able to pass through the atmosphere and reaches the sensor)

14. Spectral bands Landsat TM bands
The EM spectrum is divided into narrow wavelength intervals called a band or channel
The energy detected are stored in bands
Bands are numbered from 1 till the number of bands that can be recorded by a sensor system

15. Usefulness of the bands
Blue - useful for atmosphere and deep water imaging, and can reach depths up to 150 feet (50 m) in clear water.
Green - useful for imaging vegetation and deep water structures, up to 90 feet (30 m) in clear water.
Red - useful for man-made objects, in water up to 30 feet (9 m) deep, soil, and vegetation.
Near infrared - useful primarily for vegetation studies.
Mid-infrared – useful for vegetation, soil moisture content, and some forest fires.
Mid-infrared - useful for soil moisture, geological features, silicates, clays, and fires.
Thermal infrared captures emitted instead of reflected radiation to study objects (e.g. geological structures, thermal differences in water currents, and fires, and for night studies)

16. Typical spectral response pattern of vegetation, soil and water

17. PLATFORMS & SENSORS
Platform:      the vehicle carrying the remote sensing device
- ground-based, airborne (aircraft), spaceborne (satellite)
Sensor:        the remote sensing device recording wavelengths of energy – photographic film, scanner, etc.
e.g. Aerial photography – platform is the plane and sensor is the camera
Satellite image example:
Platform: Landsat (1, 5, 7 etc..)
Sensor: Multispectral Sensor (MSS) or Thematic Mapper (TM),

18. Satellite orbits Sun-synchronous polar orbits
Most earth imaging satellites are polar-orbiting, meaning that they circle the planet in a roughly north-south ellipse while the earth revolves beneath them. Therefore, unless the satellite has some sort of "pointing" capability, there are only certain times when a particular place on the ground will be imaged
global coverage, fixed crossing, repeat sampling
typical altitude 500-1,500 km
example: Terra/Aqua, Landsat
Non-Sun-synchronous orbits
Tropics, mid-latitudes, or high latitude coverage, varying sampling
typical altitude 200-2,000 km
example: TRMM, ICESat
Geostationary orbits
regional coverage, continuous sampling
over low-middle latitudes, altitude 35,000 km
example: GOES

19. The earth is divided into path and rows
Landsat path
Landsat Rows

20. Types of remote sensing
Passive: source of energy is either the Sun or Earth/atmosphere
Sun
- Wavelengths: μm
Earth or its atmosphere
- Wavelengths: 3 μm-30 cm
Active: source of energy is part of the remote sensor system
Radar
- wavelengths: mm-m
Lidar
-wavelengths: UV, Visible, and near infrared

21. Sensor systems
Sensor systems are of two types: imaging and non-imaging
Non-imaging sensors
- Sounders and altimeters for measurement of high accuracy topographic profiles
- spectrometers or spectroradiometers for measurement of high spectral resolution along track lines or swaths
- radiometers, scatterometers and polarimeters for high accuracy intensity measurements and polarization changes measurements along track lines or wide swaths
imaging sensors
- Imaging Sensor systems are of two types: Framing and scanning
framing systems
- images of the target are taken frame by frame (e.g. photographic film and return beam videcon)
Scanning systems
- across track scanners (Whiskbroom) and along track scanners (push broom)

22. Scanner types
Whiskbroom (mirror/ cross-track): a small number of sensitive diodes for each band sweep perpendicular to the path or swath, centred directly under the platform, i.e. at 'nadir'  e.g. LANDSAT MSS /TM
Pushbroom (along-track): an array of diodes (one for each column of pixels) is 'pointed' in a selected direction, nadir or off-nadir, on request, usually 0-30 degrees (max.), e.g. SPOT HRV

23. Diagram of Scanner types

24. Image resolution
Spatial Resolution: represent the smallest size of an object that can be detected.
IFOV is the angular cone of visibility of the sensor and determines the area on the Earth's surface which is "seen" from a given altitude at one particular moment in time (i.e. size area viewed = distance of sensor multiplied by IFOV)
Pixel size should not be confused with spatial resolution. A pixel with spatial resolution of 30m has a size of 30m x 30m (i.e. 900m2)
The equivalent of the “area viewed” on the ground is resolution cell
For a uniform feature to be detected, it has to be equal to or larger that the resolution cell.
A scene is the entire area seen by all the detectors. All satellite images are delivered in scenes. Size of a scene depends on sensor type and also IFOV
Spectral resolution: describes the ability of a sensor to define fine wavelength intervals. It also refers to the number of spectral bands. The finer the spectral resolution, the narrower the wavelength range for a particular channel or band.

25. Image resolution
Radiometric resolution: describes its ability to discriminate very slight differences in energy
It is determined by the sensitivity of a sensor system to the magnitude of the electromagnetic energy
The radiometric resolution correspond to the number of bits used in recording the energy levels. It is refers to the number of divisions of bit depth and represented by the DNs (e.g. 4bits = 16; 8bits = 256; 16bits = , etc.)
The radiometric characteristics describe the actual information content in an image.
Temporal resolution: refers to the length of time it takes for a satellite to complete one entire orbit cycle. That is, the absolute temporal resolution of a remote sensing system to image the exact same area at the same viewing angle a second time.
Note, some satellite systems are able to point their sensors to image the same area between different satellite passes separated by periods from one to five days. Thus, the actual temporal resolution of a sensor depends on a variety of factors: the satellite/sensor capabilities, the swath overlap, and latitude.

26. Examples of Remote sensor types
Name
Abbreviation
Moderate-resolution Imaging Spectroradiometer
MODIS
Airborne Visible/Infrared Imaging Spectrometer
AVIRIS
Advanced Spaceborne Thermal Emission and Reflection Radiometer
ASTER
Landsat Thematic Mapper 5
TM5
Landsat Enhanced Thematic Mapper 7
TM7
Landsat Data Continuity Mission (Landsat 8)
LDCM
Satellite Pour l'Observation de la Terre 5
SPOT5
Satellite Pour l'Observation de la Terre 1-4
SPOT1-4
Satellite Pour l'Observation de la Terre Vegetation
SPOT Vegetation
RapidEye
Students should search for more

27. Photo Interpretation

28. Types of aerial Photos There are 4 types of aerial photos:
Vertical Photos – These are photos taken with the camera axis vertical as much as possible. They look straight down. That is, the camera axis is close to the nadir or lies exactly on it in the ideal case
Oblique Photos– these are photos taken with the camera axis inclined at an angle to the ground. That is, the camera axis is tilted away from the nadir. Oblique photos often include the horizon
Orthophotos – These are vertical photos which have been geometrically corrected to be used as maps. They have coordinates written on them and are also called orthomaps.
Combinations – these are made by combining several photos

29. Vertical and oblique photos
Vertical Photo
Oblique Photo

30. Photo scale determination

31. Photo scale

32. What is Aerial photo interpretation
It is the identification and communication of features on the photograph or the art of examining photographic images for the purpose of identifying objects and to evaluate their significance.
N.B: Aerial photographs are permanent records of data and form a suitable basis for investigation of various projects.

33. Why photo interpretation
The interpreter can examine and evaluate larger areas in a short time without going to the site
Saves time because no site visiting is required during preliminary investigation
Aerial photographs are free from human errors and therefore results of photo-interpretation are more reliable and accurate.
The results compiled by junior staff can be checked by senior and experienced staff for accuracy
It is more economical as no field work is involved

34. Principle of photo-interpretation
The fundamental principle is the study of the characteristics of photographic images which enable their easy recognition and accurate identification

35. Characteristics or elements of photo-interpretation
TONE: refers to the relative brightness of features on the photographs.
For a black & white photo, it varies between black and white.
Bright features – features that reflect more light e.g. exposed dry soil (cleared land for farming, feeder road), aluminum roofs, withered crops, crops ready for harvesting,
Dark features – features reflecting less light, healthy vegetation, wet soil, thatch roofs, roofs with dark colours, water bodies,

36. Tone
Bright
Dark

37. Characteristics or elements of photo-interpretation
SHAPE: refers to the form, configuration or outline of the objects.
Shape can be
Linear - roads, canals, rivers, etc
rectangular/polygon - plot of land, house, filed, lake, etc.
circular – pond, silos,
tanks, etc.
regular – man-made
features
irregular – natural features

38. Characteristics or elements of photo-interpretation
SIZE: refers to the dimensions or area occupied by the object.
NB: The size of objects on a photo should be considered in relation to the photo scale.
Small size – ponds, residential buildings, rocks,
Large size – lakes, factories, farms, forest, etc.
the size of house will be
smaller than a farm

39. Characteristics or elements of photo-interpretation
PATTERN: relates to the spatial arrangement of objects on the ground.
Planned area – regular siting of buildings inter-woven with roads e.g. manet, regimanuel, trasacco
Unplanned area – haphazard/irregular e.g. Christian village, kiseman, nima

40. Characteristics or elements of photo-interpretation
SHADOW: the lengths of shadows of various objects are proportional to their heights. Thus shadows are useful in determining relative heights of objects. However, shadows can obscure other important features in areas where they are cast and therefore must be kept minimum. For this reason, most photographs are taken around noon (i.e. 12pm ±2)
LOCATION/SITE: refers to the geographic location of objects.
This helps in the identification of certain features that can be found at particular locations. e.g. mangroves can be located at poorly drained lowland, Settlement on gentle slope, quarry/surface mining on mountains or hills,

41. Characteristics or elements of photo-interpretation contd
ASSOCIATION: considers the occurrence of certain features in relation to others.
A linear occurrence of vegetation could be associated with a river
An isolated building on a farms could be a storage facility
Roads with settlement and farms and vice versa
Canals with dams and farms

42. Characteristics or elements of photo-interpretation contd
TEXTURE: Texture determines the visual smoothness or roughness of image features.
Rough texture – trees, rocks, buildings,
Smooth texture – bareland, surface of road and water bodies, grass,
Rough texture
Smooth texture

44. Instruments for photo-interpretation
Lens/pocket stereoscope:
A lens stereoscope consist of two single lenses of focal length 250mm mounted on a frame.
Eye base = 65mm
The magnifying power of the lenses is 2.
The stereoscope is small
folding legs
it is portable.
cheap.
Lens
legs

45. Mirror Stereoscope
Mirrors inclined at 45° to the plane of the photographs with their reflecting surfaces facing each other.
The mirrors reflect the rays from the photograph to the prisms which in turn reflect them to the two convex lenses fitted at the top of the instrument.
The refractive power of the lens makes the outgoing rays parallel.
The entire stereoscopic area of the photograph can be viewed without moving the instrument or the photographs.
The lenses have no magnifying power but binoculars can be fitted to the lenses to provide a magnification of 2 to 4 power.
The mirror stereoscope is expensive and heavy and cannot easily be carried about.

46. lenses
Mirror cover
Prisms
Mirrors
Mirror cover
Binoculars