4.RS Geometric Correction 2014

8/10/2019 4.RS Geometric Correction 2014

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S Sivanantharajah

Lecture 06

2014/11/22

RS Image Processing

Geometric Correction

Fundamentals of Remote Sensing

Image Processing

Preprocessing

Atmospheric correction


Image enhancement

Processing

Image classification

Data Merging & GIS integration


Image Pre-processing/ Image

Restoration Pre-processing operations, referred to

as image restoration and rectification,

are intended to correct for sensor- and

platform-specific radiometric and

geometric distortions of data.

Pre processing functions are normally

carried out prior to the main dataanalysis and extraction of information.


Image Preprocessing

Radiometric Correction

include correcting the data for sensor irregularities

and unwanted sensor or atmospheric noise, and

converting the data so they accurately represent the

reflected or emitted radiation measured by the

sensor.


include correcting for geometric distortions due to

sensor-Earth geometry variations, and conversion of

the data to real world coordinates (e.g. latitude and

longitude) on the Earth's surface.


Why Geometric Correction?

To allow an image to overlay a map.

To warp an image to eliminate

distortion. caused by terrain, instrument

wobble, earth curvature, etc.

To change the spatial resolution of an

image.

To change the map projection.


Geometric Corrections

Geometric corrections include correcting for

geometric distortions due to sensor-Earth

geometry variations, and conversion of thedata to real world coordinates (e.g. latitude

and longitude) on the Earth's surface.

Sources of distortions are

Variation in the altitude

Altitude & Velocity of the sensor platform

Earth curvature

Atmospheric refraction

Relief displacement and

Nonlinearities in the sweep of a sensors IFOV


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Geometric aspects of image data

2D Approaches

Geometric distortions

Georeferencing

Geocoding

3D Approaches

Monoplotting

Orthoimage production

stereoplotting


2D Approach - Geometric Correction

Flow of Geometric correction

8

Input Image

(2) Determination of

Parameters

(1) Selection of Model

Output Image

(3) Accuracy Check

(4) Interpolation & Resampling

Ground ControlPoints (GCPs)

Well balanced distribution

Enough points

High Accuracy

Well defined targets/Features


2D Approaches

Georeferencing & GeocodingGeoreferencing

Calculation of the

appropriate transformation

between an image and a

map projection system.

Geocoding

Georeferencing with

additional resampling the

image so that the pixels areexactly positioned within the

terrain coordinate system.


Image system & Map projection system

Transformation from image system tomap projection system.

1

Points known in both system

Ground Control Points (GCP)

2

System of equations

3

Solved by Georeferencing software


Georeferencing

Georeferencing is a matter of coordinatetransformation

Coordinate system for the Image

Coordinate system for the Map


Geometric .

Geocoding:This step involves resembling

the image to obtain a new image in which

all pixels are correctly positioned within the

terrain coordinate system.

Resampling is used to determine

the digital values to place in the

new pixel locations of the

corrected output image.


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Geocoding


Ground Control Points (GCPs)

Road intersections, river bends, distinctnatural features, etc.

GCPs should be spread across image

Requires a minimum number dependingon the type of transformation

Some say that it is better to haveclusters of GCPs

Must choose a map projection for GCPcoordinates.

Second Ground Control Point

Ground Control Points (GCPs) Ground Control Points (GCPs)


Accuracy with respect to Number &

Distribution of points


Mathematical Transformations

1stOrder

Linear Transformations/ Affine transformation/ first

order transformation X = a0+ a1x + a2y

Y = b0+ b1x + b2y

where

X , Y are the Rectified coordinates (output)

x,y are the source coordinates (input)

Requires minimum of 3 GCPs

Use for small, flat areas


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Mathematical Transformations (cont.)

2ndOrder


Use for larger area where earth curvature is a factor

Use where there is moderate terrain

Use with aircraft data where roll, pitch, yaw arepresent

3rdOrder


Very rugged terrain

Typically want at least 3x the minimum number ofGCPs


Root Mean Square Error (RMS error)


Accuracy of the Transformation

The method used involves computing Root Mean

Square Error (RMS error) for each of the ground

control point.

RMS error is the distance between the input (source

or measured) location of a GCP and the

retransformed (or computed) location for the same

GCP.

RMS error is expressed as distance in the source

coordinate system.

An RMS error of 1 means that the reference pixel is1 pixels away from the retransformed pixel.

Georeferencing & Geocoding


Image Resampling or

Intensity Interpolation

Once an image is warped, how do you assign DNs

to the new pixels? Since the grid of pixels in the source image rarely

matches the grid for the reference image, the pixels

are resampled so that new data file values for the

output file can be calculated.

This process involves the extraction of a brightness

value from a location in the input image and its

reallocation in the appropriate coordinate location in

the rectified output image.


Resampling Techniques

Nearest NeighborAssigns the value of the nearest pixel to the

new pixel location Bilinear

Assigns the average value of the 4 nearestpixels to the new pixel location

Cubic Convolution

Assigns the average value of the 16nearest pixels to the new pixel location


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Resampling

The resampling process calculates the

new pixel values from the original digital

pixel values in the uncorrected image.There are three common methods for

resampling.

Nearest neighbour, bilinear interpolation,

and cubic convolution.


Nearest Neighbour Nearest neighbourresampling uses the digital

value from the pixel in the original image which is

nearest to the new pixel location in the corrected

image.

This is the simplest method and does not alter theoriginal values, but may result in some pixel

values being duplicated while others are lost.

This method also tends to result in a disjointed or

blocky image appearance.


Bilinear interpolation

Bilinear interpolationresampling takes a

weighted average of four pixels in the original

image nearest to the new pixel location.

The averaging process alters the original pixel

values and creates entirely new digital values in

the output image.


Cubic convolution

Resampling goes even further to

calculate a distance weighted average of

a block of sixteen pixels from the

original image which surround the new

output pixel location.


Which distortions/type of Images

can be handled by 2D approaches

Perspective of the sensor optics Some of it Forward motion of the platform Yes

Platform attitude altitude Yes

Platform attitude, altitude Yes

Terrain relief No

Curvature and rotation of the earth Yes


3D Geometric Aspects


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3D approaches to account for geometric distortions

Monoplotting is a feature extraction

procedure from an aerial photo that

incorporates corrections for Relief

Displacement (= georeferencing +) X,Y,Z

Orthoimaging is a geocoding procedure

that incorporates corrections for relief

displacement

(= geocoding +) orthophoto


Monoplotting


Stereo Model

A stereo model is a construct of overlappingphotos/Satellite images

Measurements made in a stereo model utilizethe phenomenon of parallax

A stereo model enables parallax measurementand X, Y, Z measurements

Analogue and analytical plotters were used inthe past

Nowadays Digital PhotogrammetricWorkstations are used

Stereovision is made possible throughspecialized monitors + spectacles


Stereo Plotting

To get a geometrically correct model, we must

Determine the relationship between the digitalimage system and the photo/camera system;for this interior orientation we need the position of the principal point

the principal distance (c)

Determine the relative tilts of the twophotographs (around 3 axes); this is called therelative orientation

Bring the model a known scale and level it with

respect to the terrain system; this is called theabsolute orientation


Stereo Model


2D versus 3D

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4.RS Geometric Correction 2014