Transcript Concept on Remote Sensing by G.C.Nayak 18-02-2008

Digital Photogrammetry
by
G.C.Nayak
01-06-2010
Point of Discussion
• Approch to Photogrammetry:
Integrated with RS
• Process Involved
• Issues Involved in each Process
What do we mean by;
Photogrammetry:
Photo (light) + gram (write) + metron
(measurement)
defined as
the art, science and technology
of
obtaining reliable information
about
physical objects and environment
through the process
of interpreting, measuring and recording
From photographs
Remote Sensing:
Remote (distance) + Sensing (gathering
information / making inference)
Defined as the Science &Technology
of
making inference
about material / objects
from
measurements made at a distance
without
coming into physical contact
with
the objects under study.
Examples:
- How we see by our Eye
- Photographs Taken from
Ground (Terrestrial) or
Above (Aerial)
- Satellite Image taken from space
In General we mean by;
Photogrammetry- concern with Aerial Photos
Remote Sensing- concern with Satellite imageries
But both are the Products of Remote Sensing
Therefore
Photogrammetry as we mean
can be dealt
as a part of Remote Sensing
Only
Classes are different
Let us go back to the Definition of RS
What is being Sensed in RS ?
Reflected & Emitted Light energy
from the Object of interest
is sensed and recorded by
our Eye, Camera or Satellite Sensors
In the Sensing Process Elements
Involves are:
• The energy – EMR
• Sensor – The equipment which sense
• Platform – Where the sensor is placed
Depending on the above elements
Aerial Photographs &
Satellite image are
Products of two different class
How these can be classified?
i. Depending on the EMR used:
Which part of the EMS is sensed,
Example: Visible, IR, Thermal,
Microwave, Xray etc.
Measuring Light: Bands
• Human eyes only ‘measure’ visible light
• Sensors can measure other portions of
EMS
Bands
Depending on the Sensor used:
i. Active Sensor
i. Passive Sensor
Depending on Plat form:
i.
Terrestrial:
ii. Air borne:
iii. Space borne:
If Space born
Geostationary
Sunsynchronous
The process of remote
sensing
12
65
28
33
76
C
E
A
D
A
B
A. Radiation and the atmosphere C. Energy recorded and converted
by sensor
B. Interaction with target
E. Interpretation and analysis
D. Reception and processing
For mapping purpose we use;
Aerial photographs having overlaps obtained
from
Airborne camera which sense
visible & a part of NIR of EMR
Product format:
- Analogue / Hard copy from frame camera
- Digital image from Digital camera
Satellite image from
Space borne sun synchronous satellite which
sense
visible and IR part of EMR
Product format:
Digital image in bands
Aerial photo vs. Satellite image for
spatial data extraction
Because of low flying ht. Aerial photo
capable of providing:
- High spatial resolution image/
photographs
- Stereo viewing capability for precise ht.
measurements
With the development in space technology
satellite is also capable of providing :
- High spatial resolution image
-Stereo viewing capability, though not yet
attend the precise level that of aerial photo
Technology involved in data extraction:
i. Photogrammetry
By using the geometry of a pair of
overlapping stereo image for measurements
in 3D
ii. Image Processing
By processing a single digital image for
interpretation and data extraction
Aspects involved in data extraction:
Quantitative – measurement of coordinates
(XYZ), length, area, shape
Photogrammetric solution precisely does this
i.
ii. Qualitative – This is about interpretation,
recognition, identification
Spatial resolution and contrast of the image
influence the both aspects
By photogrammetry method:
Requirements:
- Overlapping image for stereo view required for
3d viewing & measurement
- Ground Control Points( XYZ) for establishing
positional relationship between photo & ground
How Stereo Overlap Aerial image obtained?
By having an overlap in successive photos
Overlap Region
60% Overlap
Stereo Pair
Forward Overlap
Before information is extracted
The conditions as it was during are the
photography is recreated
A relationship is established the photo / image
and the ground system
The whole process to find the solution is
called Orientation
Then data are collected in 3d or 2d environment
Evolution of Photogrammetric
technique
Evolution of Photogrammetric
technique;
Phase →
Input
Solution
Data
Acqusition
Out put
Phase –I
Analogue
Analogue /
Hard copy
Analogue
Mech./Opt.
Manual
Phase –II
Analytical
Analogue /
Hard copy
Phase –III
Digital
Digital
Math.
Math.
Manual
Analogue
Analogue /
Digital
Manual /
Automatic
Digital
Digital Photogrammetry is also called
Soft copy Photogrammetry
• Mathematics is the same as in traditional
Photogrammetric technology.
• The images (diapositives) are replaced by
scanned aerial images / image acquired in
digital form .
• Measurements and image viewing are on
a computer screen instead of through high
quality optics.
In Digital photogrammetry input is Digital Image
obtained through scanning hard copy photo or
direct from Digital camera
The main work flow involed;
Input ( digital image) – output of digital camera or
through scanning of photographs
Solution – Through process called Aerial
Triangulation
Generation of DEM – for surface
Generation of Orthophoto – Equivalent of Maps for
2D digitisation
3d Data capture
Prducts – Input for GIS , Cartographic products
(Maps)
MAIN WORK FLOW
INPUT
Photo Coordinates
PROCESSING
Triangulation
OUT PUT
EO Parameters – Establishment of relationship
between Image and Ground
PRODUCTS
DEM& Contour , Ortho Photo , Feature Extractions
Establishment of Relationship
Established through Six Elements of Exterior
Orientation
X,Y, Z ,,  ,
Z Axis
X,Y,Z ,,  ,
These 6 parameters define
location and orientation of
each exposure station/
perspective center
DURING TRIANGULATION THE FOLLOWING
UNKNOWNS
ARE SOLVED AS UNDER:-
- EXTERIOR ORIENTATION (EO) PARAMETERS
(X0,Y0,Z0,)
- COORDINATES OF IMAGE POINTS IN GROUND TERMS
EO PARAMETERS
-X0,Y0,Z0- ARE THE COORDINATES OF THE EXPOSURE
STATIONS IN GROUND TERMS
- ,, ARE THE ORIENTATIONAL PARAMETERS
OF THE CAMERA
Once that solution is found;
• Further products are obtained by extracting
the information
Products:
2D / 3D Features, DEM , Ortho images
Issues to be addressed during
Process;
• Project Parameters
• Scanning
• AT
• DEM Generation
• Ortho image Generation
• 2D / 3D data Capturing
Project Parameters;
• Camera / Sensor Model
Frame / Digital / Sensors
• Coordinate System
Horizontal and Vertical Datum / Projection
System
Scanning
• What should be the Scanning Resolution?
We can not achieved better then the
Resolving Power of the used frame
camera expressed in line pair / mm (lpm).
Ideal case it is
50 lpm equivalent to 10 micron
Practically achievable as best
40 lpm equivalent to 12.5 micron
Or
25 lpm equivalent to 20 micron
Aerial Triangulation (AT)
- Number & pattern of GCP
AS principle remain same, only technology
has changed.
Thus the established pattern of GCP in
Analogue & Analytical method holds good
- Coordinate System
- Number of Iteration
- Convergence
- Statistical Weights
Image Points, GCPs, etc.
AT Continued…..
- Accepted norm of accuracy in X,Y,Z
Required accuracy level depending on the
output scale and purpose
+/- 1pixel in x,y and =/-1.5 pixel term in z
is achivable
Tie Point Measurements
• For a solution of number photographs only
GCPs are not sufficient
• Thus Tie points are measured in all
overlapping images
• Issues
Automatic or Manual
Number and Distribution per images
If automatic what are the parameters
Parameters for Auto Tie Point
• This employs Image matching technique
Correlation size
Search window size
Correlation Tolerance
DEM GENERATION & ORTHO RECTIFICATION
- Once the Triangulation is over, xyz coordinates of any point can be
extracted.
- During DEM generation
- Mass points are generated through out the overlapping area.
- Taking mass points TIN is generated
- Grid is generated with the defined grid size. Out put is an image
file where pixel value is the height.
Generation of contour:
- From DEM contours are generated.
USING DEM DISTORTIONS DUE TO RELIEF & TILT ARE
RECTIFIED – OUT PUT IS ORTHOPHOTO ie. Transforming
from A perspective projection to orthogonal projection
What is Digital Elevation Model (DEM) ?
A surface generated from sample points
having x,y,z
What is orthophoto?
Bringing from central
Proj. to Ortho. Proj.
Error due to relief is
corrected using DEM
Process called
ortorectification
Issues to be addressed for DEM
generation:
- Purpose of DEM
For Orthophoto or Contour generation or any other
surface analysis
The purpose will influence the mass point / break
line collection for DEM generation
- Resolution of Grid ( Raster DEM)
How data will be used / integrated will influence the
grid size
Generally it is 2-3 time denser than observed
density
Working procedure is TIN to Grid
Issues to be addressed for Ortho
Image generation:
• Error will be more as we move away from
the nadir
• Resolution Depends on
Scan Resolution
It should be resample to 1.2 time or more
Output scale
DEM Resolution
Feature Extraction:
Planimetric :
In 2D environment;
-From Orthophoto using feature code
Feature vertex comprises of only x,y
DEM contains surface height information
which is used in GIS for analysis
If required Contour is generated from it
In 3D environment;
- Features are digitised in stereo viewing with
feature codes
- Stereo viewing by alternative or Simultaneous
display
- Feature vertex comprise of x, y & z
- Features such as stream, ridges etc are
introduced as break lines for generating DEM
- For DEM mass points are collected with
operator intervention
Issues to be addressed while
collecting features:
- 2D or 3D
Purpose Specific
- Scale of out put
- Zoom level of viewing
- Features to be collected
- Geometry of a feature (Point, line or
Polygon)
- Feature class
Digitisation Mode;
- All Features, Natural & Cultural (Man
made) ,are Digitised / traced manually
- Height features through DEM is Automatic
- Interpretation and entering of vertex is
subjective on the part of Operator
Out turn of Feature collection;
- As Features are collected (traced) Manually
The out turn is almost same as in Analogue
- Irrespective of Scale and type of terrain the
number Data points entered by the operator
remain same
So optimum zoom level and density of points
as per requirement should be maintained
Various experience indicate 3000 -4000 data
points per man days in 3D data captures and
In 2D it is 2 – 2.5 times
Scope of Automation;
- It is complex issue to adopt an algorithm to
interpret the image while recognizing features
as human mind in coordination with eye does
- Development is in the process in computer
vision involving Artificial Intelligence (AI) using
Image geometry , position and dimension
Example ; Automatic Building Extraction
Approach for purchasing Software
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Modular Approach for required Functionality
Image/Frame Manager Module
Ground Control/Tie Point Measurement Module
Aerial Triangulation Module
DEM/DTM Generation Module
Orthophoto Generation Module
Stereo Editing Module:
Minimum hardware specifications
Any questions?