explain across track scanners (whiskbroom)
- Scanning lines are perpendicular to the
direction of flight - Records radiation from one side to one side
using an oscillating mirror - Arc below the aircraft between 90°-120°
what is field of view in remoe sensing
- The scanner records the energy within the
system’s instantaneous field of view (IFOV) - IFOV - the angle within which incident energy
is focused on the detector: β
β - determined by - instrument’s optical
system - size of its detectors
explain spatial resolution and its relationship to IFOV
- The ground observed when the IFOV
of a scanner is oriented directly
beneath the aircraft
𝐷𝐷 = 𝐻𝐻𝐻 × 𝛽𝛽 - The ground segment sensed at any
instant = ground resolution
explain spectral resolution and its relationship to noise and IFOV
- Spectral resolution – ability to discriminate fine
spectral signatures
– Large IFOV allow more energy -> improved
radiometric resolution
– Small IFOV – record fine spatial details - Spatial resolution and radiometric resolution are
inversely related - Large IFOV – large signal to noise ratio
– Proportion of recorded signal that is useable
information in respected with unwanted distortions - Term Resolution: loosely refers to GSD or IFOV
what are across track scanners subject to
Across-track scanners subject to
– Altitude and attitude variations
– Systematic geometric variations
what is tangential scale distortion
Tangential scale distortion
* Severe scale distortion perpendicular to the
flight direction
– Constant angular velocity of mirror ≠ constant
speed of scanner IFOV over terrain
– The ground distance cover per second increases
with departure from nadir
what is flight parameter distortion
Variation in flight trajectory affects relative
position of points recorded
what is 1d relief displacement
Objects are only side-viewed
– Vertical photo->displacement radial from principal point
– Across-track image->displacement at 90° from nadir
Single frame photo displacement Across-track scanner displacement
what is along-track scanner pushbroom
- Pushbroom scanners record multi- spectral
images along a swath beneath an aircraft - NO scanning mirror
- Linear array
– Multiple charged coupled devices - Each detector element senses the
energy of a single column of data.
Ground resolution determined by
the IFOV of a single detector
projected on the ground.
Across- vs Along-track scanning
- Advantages of along track scanning
– Longer detection time to measure energy
– Geometric integrity->fixed distance among detectors
– No geometric error induced by mirror rotation speed
– Smaller in size and lighter
– No moving parts - reliable and long life expectancy - Advantages of across-track scanning
– No need to calibrate multiple detectors
– Large range of spectral sensitivity - Linear array systems -> up to mid-IR
what are the geometric characteristics of pushbroom
- No scanning mirror -> uniformly spaced
ground resolution elements - No tangential scale distortion
what is exterior orientation
- Parameters that describe the position and
angular orientation of the coordinate system
of each photograph relative to the ground
coordinate system - Six parameters
– Three angles
– Three coordinates - Determining the exterior orientation
parameters for a photo -> georeferencing
what is georeferencing
- Establishes geometric relationships between
image and ground coordinates - Approaches for georeferencing frame camera
images:
– Indirect georeferencing: uses GCP and aero-
triangulation to compute exterior orientation param.
– Direct georeferencing: uses airborne GPS and IMU
values to compute exterior orientation parameters
what is collinearity condition
- Collinearity condition: the exposure station of
any photograph, any object point in the ground
coordinate system, and its photographic image lie on a straight line
what are the 3 steps of indirect georeferencing
1) Georeferencing (ESRI PRESS 2003)
To establish a relationship between page coordinates on a
planar map and known real-world coordinates.
2) Geometric Transformation (Chang)
The process of converting a map or an image from one
coordinate system to another by using a set of control
points and a transformation equation.
3) Image Resampling
A process of filling each pixel of a newly transformed image
with a value or a derived value from the original value
what is direct georeferencing
- Current preferred method for determining the
elements of exterior orientation - Airborne GPS and IMU calculate the position
and the angular orientation of each image - Direct georeferencing can eliminates the need
for GCP and aerotriangulation
what is Orthorectification
- Orthorectification - process of transforming
raw imagery to an orthogonal projection - Orthorectification transforms the central
projection of the photograph into an
orthogonal view of the ground
orthophoto vs true orthophoto
Product of orthorectification = orthoimage (photo)
– True orthophoto = orthophoto with orthogonal projection
what are Sources of image distortion
- Sensor orientation
- Topographic relief
- Earth shape and rotation
- Sensor orbit and attitude variations
- Systematic error associated with the sensor
what are Orthorectification algorithms
- Parametric approaches
– GCP
– Parameters: interior & exterior orientation para. - Nonparametric approaches
– “Rubber sheet” approach Schlapfer and Richter (2002)
– Large number of GCP -> reference image
– Algorithms - Polynomial transformations
- Projective transformations
what are Ground control points
- Ground reference data = ground control
- Ground control are physical points on the
ground with known positions - An important role of ground control is to
determine the exterior orientation parameters
of a photo at recording time
– Position
– Angular orientation
what is Parametric orthorectification
- Favorable for imaging spectrometry data
- Requires knowledge of
– Sensor model
– Orbits parameters - Achieved accuracy: 0.5 – 1.5 pixels
- Parametric models implemented in various
airborne scanners: AVIRIS
what is Interior orientation
- Reconstruct the bundle of rays: relationship between
image plane and projection center of a camera - Parameters:
– location of the principal point
– the focal length
– the radial symmetric lens distortion - Collinearity equation: image point, projection center,
and ground point are located on a straight line
what is Exterior orientation
- Parameters that describe the position and
angular orientation of the coordinate system
of each photograph relative to the ground
coordinate system - Six parameters
– Three angles
– Three coordinates
