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Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Geometric CorrectionIt is vital for many applications using remotely sensed images to know the ground locations for points in the image. There are two similar processes that can help build the link between images and real world locations: image-to-map rectification and image-to-image registration.Image-to-map rectification: a process by which the geometry of an image is made planimetric with reference to a projected map.Image-to-image registration: a process which translate the geometry of one image to another (usually projected) image so that corresponding elements of the same ground area appear in the same place on the registered images. The image used as reference (with known projection and coordinates) is called the master image, and the image to be registered is called the subject image.It is important that the reference map or image is rendered in a standard map projection and coordinate systems.Map ProjectionMap projection is the process of systematic transformation of points on the Earth’s surface to corresponding points on a plane surface.CylindricalConicalPlanarCommonly Used Spheroid in Map ProjectionEllipsoids Date Semi-major axis Semi-minor axis EllipticityClarke 1866 6,378,206.4 6,356,583.6 1/294.98WGS72 1972 6,378,135 6,356,750 1/298.26GRS80 1980 6,378,137 6,356,752 1/298.257WGS84 1984 6,378,137 6,356,752 1/298.257…Many of the earlier US maps are based on Clarke 1866 ellipsoid which was determined by Sir Alexander Clarke in 1866. The World Geodetic System (WGS72 and 84) ellipsoids, determined from satellite orbital data are considered more accurate. GRS80 (Geodetic Reference System) ellipsoid is adopted by the International Association of GeodesyThe Global Coordinate Systemspherical coordinate system unprojected! expressed in terms of two angles (latitude & longitude) longitude: angle formed by a line going from the intersection of the prime meridian and the equator to the center of the earth, and a second line from the center of the earth to the point in questionlatitude: angle formed by a line from the equator toward the center of the earth, and a second line perpendicular to the reference ellipsoid at the point in questionlatitudepositive in n. hemispherenegative in s. hemispherelongitudepositive east of Prime Meridiannegative west of Prime MeridianOrigin of Geographic Coordinate SystemGlobal Coordinate SystemThe Universal Transverse Mercator Coordinate System60 zones, each 6° longitude wideStarting from 180 degrees eastwardzones run from 80° S to 84° Npoles covered by Universal Polar System (UPSTransverse Mercator Projection applied to each 6o zone to minimize distortionUTM Zone ProjectionUTM Coordinate ParametersUnit: metersZones: 6o longititueN and S zones: separate coordX-origin: 500,000 m east of central meridianY-origin: equatorUSA In The UTM ZonesState Plane Coordinate System• Each state has one or more zones• Zones are either N-S or E-W oriented (except Alaska)• Each zone has separate coordinate system and appropriate projection• Unit: feet no negative numbersMap Projections for State Plane Coordinate SystemN-S zones: Transverse Mercator ProjectionE-W zones: Lambert conformal conic projectionGeometric Correctionx’y’xyimagemapGCPGCP),(),(21yxfyyxfxGround Control PointsMaster x Master ySubject x Subject yx1 y1x2 y2 x3 y3… …x1 y1x2 y2 x3 y3… …Note: Coordinates must be in file coordinates (lines, samples).First order polynomial:ybxbbyyaxaax210210Second order polynomial:2524321025243210ybxbxybybxbbyyaxaxyayaxaaxThird order polynomial …Goodness of fit:niiiiiyyxxnRMSE122)()(1Unit of RMSE: pixelsImage Grids on Reference GridsThe output of geometric correction is a grid that exactly overlays the reference grid. Image-to-image registration: Reference grids already exist.Image-to-map rectification: Need to create a reference grid first. (1). Specify an origin (2). Translate map coordinates to image coordinates based on pixel size.Resampling Methods1. Nearest Neighbor: The DN values in the output grid takes from the pixel that is nearest in the input grid. The output grid maintains all the original DN values in the input grid. 2. Bilinear Interpolation:4124121kkkkkDDDNBVInverse distance weighted average of the four nearest pixels to the output pixel.3. Cubic Convolution:161216121kkkkkDDDNBVInverse distance weighted average of the 16 nearest pixels to the output


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UNC-Chapel Hill GEOG 577 - Geometric Correction

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