9/9/2020

Introduction
• For some remote sensing instruments, the distance between
the target being imaged and the platform, plays a large role
in determining the detail of information obtained and the
total area imaged by the sensor.
CIV4261 Geographical Information • Sensors onboard platforms far away from their targets,
typically view a larger area, but cannot provide great detail.
Systems and Remote Sensing Compare what an astronaut onboard the space shuttle sees
Level: 4 of the Earth to what you can see from an airplane.
Lecturer 10: Image resolution • The astronaut might see your whole province or country in
one glance, but couldn't distinguish individual houses. Flying
over a city or town, you would be able to see individual
buildings and cars, but you would be viewing a much smaller
area than the astronaut.
By Ir. Gilbert NDUWAYEZU • There is a similar difference between satellite images and
airphotos.

Learning objectives Image characteristics

• For some remote sensing instruments, the distance between the
1. Explain the relationship and confusion between spatial target being imaged and the platform, plays a large role in
resolution and image scale. determining the detail of information obtained and the total area
2. Explain the influence of: response curve, EM range, imaged by the sensor.
spatial resolution, date/time, EM band width, • Sensors onboard platforms far away from their targets, typically
radiometric sensitivity (resolution and dynamic range), view a larger area, but cannot provide great detail.
A/D conversion on EO measurements. • Image resolution is what defines the level of details that can be
3. State the main image data characteristics and the “extracted” from the image and can be expressed with four
‘trade-offs’ between them (costs, time, spatial, spectral different types of resolution:
and temporal). 1. Spatial resolution
2. Spectral resolution
3. Radiometric resolution
4. Temporal resolution

1
 9/9/2020

Spatial (image) resolution Spatial resolution

• Spatial resolution of the sensor and refers to the smallest spatial • Spatial resolution of passive sensors (we will
area from which information is obtained look at the special case of active microwave
OR sensors later) depends primarily on their
Instantaneous Field of View (IFOV).
• The size of the smallest possible feature that can be detected.
• The IFOV is the angular cone of visibility of
• This corresponds to size of a pixel. the sensor (A) and determines the area on
the Earth's surface which is "seen" from a
given altitude at one particular moment in
time (B)
• The size of the area viewed is determined by
multiplying the IFOV by the distance from
the ground to the sensor (C).
• This area on the ground is called the
resolution cell and determines a sensor's
maximum spatial resolution.
High Resolution Low Resolution

Pixel Spatial Resolution of optical space Systems

The cells are sensed one after another along the line.
In the sensor, each cell is associated with a pixel that is tied to a
microelectronic detector

Pixel is a short abbreviation for Picture Element

a pixel being a single point in a graphic image

Each pixel is characterized

by some single value of radiation
(e.g., reflectance) impinging on
a detector that is converted by
the photoelectric effect into
electrons

2
 9/9/2020

Spectral resolution Multi spectral remotely sensed images

• Spectral resolution is defined by the band widths (wavelength
interval size) and number of bands of the response recording
system.
• Many remote sensing systems record energy over several
separate wavelength ranges at various spectral resolutions.
• These are referred to as multi-spectral sensors.
• Advanced multi-spectral sensors called hyperspectral sensors,
detect hundreds of very narrow spectral bands throughout the
visible, near-infrared, and mid-infrared portions of the
electromagnetic spectrum.
• Landsat - (E)TM: 7 spectral channels/bands with band widths
from 70 nm to 200 nm (band 1 - 5, 7) and 2200 nm (band 6)
• Hyperspectral Sensors (imaging spectrometers): Up to 256
contiguous bands with 5 nm to 20 nm band width.

Radiometric resolution Radiometric resolution

• The radiometric characteristics describe the actual information
content in an image.
• Every time an image is acquired on film or by a sensor, its
sensitivity to the magnitude of the electromagnetic energy
• Digital number (DN) The recorded digital read-out of an
determines the radiometric resolution.
electronic detector. It is the quantized sampled value of the
• The radiometric resolution of an imaging system describes its electrical signal which is generated by the detector. The DNs
ability to discriminate very slight differences in energy. correspond to photon energy incident upon the detector and
• The finer the radiometric resolution of a sensor, the more radiances at the detector, but have not a meaningful physical unit.
sensitive it is to detecting small differences in reflected or In 8 bits recording, the DNs are in the range [0, 255].
emitted energy. • Binary expressed as 8-bits (0-255), 11-bits (0-2047), 12-bits (0-
• Radiometric resolution refers to the effective bit-depth of the 4095) or 16-bits (0-65535)
sensor response the number of gray levels to be recorded. • Landsat ETM: max. of 256 gray values (8 bits)
• Hyperspectral Sensors: Up to 65536 gray values (16 bits)

3
 9/9/2020

Radiometric resolution
Variation in Radiometric Resolution

256 Grey Levels 2 Grey Levels

Temporal resolution
Example: radiometric characteristics
• Temporal resolution is defined by the
time (i.e. days) that passes between
imagery collection periods (repeat
cycle)
• The smallest period (in time) between
two images of the same area.
• This refers to the length of time it takes
for a satellite to complete one entire orbit
cycle (revisit period).
• Landsat 4, 5, 7 : 16 days
• SPOT 4, 5: 25 days in nadir mode
• Sentinel-2: 10 days one satellite; 5 days
two satellites

4
 9/9/2020

Data acquisition repeatability Criteria for sensor selection

• Accuracy
• Required object
resolution
• Costs of primary data
(Mapping the entire • Repeatability of data
world with IKONOS acquisition
would take some 5 years)
• Availability of
sensors/images
• Interpretability

Approx. Image costs and covered area Summary

Classification of sensors
• Depending of the platform (ground, airborne, spaceborne)
• Depending of the type of energy (active, passive)
• Depending of the camera (analoque, digital)
Sensors for aerial image acquisition and Image characteristics
• Spatial, spectral, radiometric, temporal resolution
Criteria for sensor selection
• Spatial resolution, overlap, cost, repeatability, availability,
stereo, interpretability

5
 9/9/2020

Thanks for your attention