Remote Sensing Remote Sensing

Image Rectification and Image Rectification and RestorationRestoration

Image Rectification and Image Rectification and RestorationRestoration

► Geometric correctionGeometric correction► Radiometric correctionRadiometric correction► Geometric restorationGeometric restoration

1. Geometric Correction1. Geometric Correction

► For raw image rectificationFor raw image rectification► For multi-date images registrationFor multi-date images registration► For multi-resolution images or data For multi-resolution images or data layers registrationlayers registration

► Systematic distortion vs. random Systematic distortion vs. random distortiondistortion

Skew CorrectionSkew Correction

http://rst.gsfc.nasa.gov/Intro/Part2_15.html

Coordinate transfer

Pixel value resampling

Ground Control Points (GCP)Ground Control Points (GCP)► Features with known locations on a map Features with known locations on a map (X,Y coordinates). These are the (X,Y coordinates). These are the “ground control points”“ground control points”

► The same features can be accurately The same features can be accurately located on the images as well (column, located on the images as well (column, row numbers)row numbers)

► The features must be well distributed The features must be well distributed on the map and the imageon the map and the image

► Highway intersections are commonly used Highway intersections are commonly used ground control pointsground control points

Finding UTM Finding UTM coordinates coordinates

on a mapon a map

Coordinate TransformCoordinate Transform ► Coordinate transform equations relate Coordinate transform equations relate geometrically correct map coordinates to the geometrically correct map coordinates to the distorted image coordinates distorted image coordinates

x = ax = a00 + a + a11X + aX + a22YY y = by = b00 + b + b11X + bX + b22Y Y x,y: column, row numberx,y: column, row numberX,Y: coordinatesX,Y: coordinates

► Root Mean Square Error (RMSE) Root Mean Square Error (RMSE) = √(= √(x)x)22 + ( + (y)y)22

Calculate RMSE for all control pointsCalculate RMSE for all control points

ResamplingResampling► The purpose is to assign pixel values The purpose is to assign pixel values to the empty pixels in the rectified to the empty pixels in the rectified matrix outputmatrix output

► Superimpose the rectified output Superimpose the rectified output matrix to the distorted image matrix to the distorted image

► The digital number (DN) of a pixel in The digital number (DN) of a pixel in the output matrix is assigned based on the output matrix is assigned based on the DN of its surrounding pixels in the DN of its surrounding pixels in the distorted imagethe distorted image

Re-sampling MethodsRe-sampling Methods► Nearest neighbor resamplingNearest neighbor resampling► Bilinear interpolationBilinear interpolation► Cubic convolution resamplingCubic convolution resampling

Nearest Neighbor ResamplingNearest Neighbor Resampling► The DN of a pixel in the output matrix is The DN of a pixel in the output matrix is assigned as the DN of the closest pixel in assigned as the DN of the closest pixel in the distorted image the distorted image

► AdvantagesAdvantagessimple computationsimple computationmaintain the original valuesmaintain the original values

► DisadvantageDisadvantagespatial offset up to 1/2 pixelspatial offset up to 1/2 pixel

Bi-linear InterpolationBi-linear Interpolation► Distance-weighted average of DN values of the Distance-weighted average of DN values of the closest 4 pixelsclosest 4 pixels

► AdvantageAdvantageoutput image is smoother than the nearest output image is smoother than the nearest neighbor methodneighbor method

► DisadvantageDisadvantagealters the original DN valuesalters the original DN values

Cubic Convolution ResamplingCubic Convolution Resampling► Uses DN values of the closest 16 Uses DN values of the closest 16 pixels, adjusted by distancepixels, adjusted by distance

► AdvantageAdvantagesmooth output imagesmooth output image

► DisadvantageDisadvantagealters the original DN valuesalters the original DN values

When to RectifyWhen to Rectify

► Rectify before image classificationRectify before image classification► Rectify after image classificationRectify after image classification

2. Radiometric Corrections2. Radiometric CorrectionsRadiometric responses differ by Radiometric responses differ by ► datesdates► sensor typessensor types► imagesimages

► Causes:Causes:- Illumination- Illumination- Atmospheric conditions- Atmospheric conditions- View angle or geometry - View angle or geometry - Instrument response- Instrument response

Radiometric CorrectionsRadiometric Corrections

► Sun elevation correction Sun elevation correction ► Atmospheric correctionAtmospheric correction► Conversion to absolute radiance Conversion to absolute radiance

Sun Elevation CorrectionSun Elevation Correction

DNDN► ----------------------------------------------------------------

Sin (Sun elevation angle)Sin (Sun elevation angle)

► Assuming the terrain is flatAssuming the terrain is flat

Spring / Fall

Satellite

Summer

Winter

Zenith

Equator

Tangent plane

Solar Elevation Angles

Atmospheric CorrectionAtmospheric Correction

► Haze compensation Haze compensation The DN value of an object (e.g., a The DN value of an object (e.g., a deep clear water body) with 0 deep clear water body) with 0 reflectance = Lpreflectance = Lp

► Subtract the DN from the entire bandSubtract the DN from the entire band

Absolute IrradianceAbsolute Irradiance ► Conversion of DN values to absolute Conversion of DN values to absolute radiance values radiance values

► It is necessary when compare It is necessary when compare different sensors, or relate ground different sensors, or relate ground measurements to image datameasurements to image data

► L = (LL = (Lmaxmax- L- Lminmin)/255 * DN + L)/255 * DN + Lminmin

3. Geometric Restoration3. Geometric Restoration ► StrippingStripping► Line-dropLine-drop► Bit errorsBit errors

StripingStriping ► Malfunction of a Malfunction of a detectordetector

► Use gray scale Use gray scale adjustment to adjustment to correct the correct the stripsstrips

Line DropLine Drop ► using average of the above and below using average of the above and below lines to fill the dropped linelines to fill the dropped line

Bit ErrorBit Error ► Salt and Salt and pepper effect pepper effect due to random due to random errorerror

► Use 3x3 or 5x5 Use 3x3 or 5x5 moving window moving window average to average to remove the remove the noisenoise

ReadingsReadings ► Chapter 7Chapter 7

Earth-Sun Distance Earth-Sun Distance CorrectionCorrection

EE00 Cos Cos00

► E = ------------E = ------------ dd2 2 ► Irradiance is inversely related to the Irradiance is inversely related to the square of the earth-sun distancesquare of the earth-sun distance

► E - normalized solar irradianceE - normalized solar irradiance► EE00 - solar irradiance at the mean Earth-sun - solar irradiance at the mean Earth-sun distancedistance

► 00 - sun angle from the zenith - sun angle from the zenith► d - Earth-sun distanced - Earth-sun distance

Atmospheric CorrectionAtmospheric Correction

ETET► LLtottot = --------- + Lp = --------- + Lp reflection of targetreflection of targetE - irradiance on the targetE - irradiance on the targetT - transmission of atmosphereT - transmission of atmosphereLp - scattered path radiationLp - scattered path radiation