Digital Photogrammetry by

G.C.Nayak01-06-2010

Point of Discussion

• Approch to Photogrammetry:

Integrated with RS

• Process Involved

• Issues Involved in each Process

What do we mean by;Photogrammetry:Photogrammetry: Photo (light) + gram (write) + metron Photo (light) + gram (write) + metron (measurement)(measurement)defined asdefined asthe art, science and technology the art, science and technology of of obtaining reliable information obtaining reliable information about about physical objects and environment physical objects and environment through the process through the process of interpreting, measuring and recording of interpreting, measuring and recording From photographsFrom 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 distancewithout 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 PhotosRemote Sensing- concern with Satellite imageries

But both are the Products of Remote Sensing

Therefore Photogrammetry as we mean can be dealtas a part of Remote SensingOnlyClasses 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

A. Radiation and the atmosphere

B. Interaction with target

C. Energy recorded and converted by sensor

D. Reception and processing

E. Interpretation and analysis

A

A

B

D

E

C

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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 ProcessingBy processing a single digital image for interpretation and data extraction

Aspects involved in data extraction:

i. Quantitative – measurement of coordinates (XYZ), length, area, shape

Photogrammetric solution precisely does this

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?

Stereo Pair

Overlap Region

60% Overlap

By having an overlap in successive photos

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 → Phase –I

Analogue

Phase –II

Analytical

Phase –III

Digital

Input Analogue /

Hard copy

Analogue / Hard copy

Digital

Solution Analogue

Mech./Opt.

Math. Math.

Data Acqusition

Manual Manual Manual / Automatic

Out put Analogue Analogue / Digital

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 Axis

Y AxisZ A

xis

X,Y,Z,,, 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 analysisThe 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) ManuallyThe 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

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?