Rectifying Images - PDF document

Rectifying Images page 2 Before Getting Started You can print or read this booklet in color from MicroImages’ web site. TheYou can download an installation guide, sample data, and the latest versionThis process uses georeference control point information to perform simple recti-geographic coordinate system. The exercises cover the various options for con-resampling methods and geometric transformation models. Warping of distorted This booklet assumes that you have completed the exercises and ered again here. You will also find the concepts introduced in the tutorials and helpful in understandingimage rectification. Please consult those booklets for any review you need.distributed with the TNT products. If you do not have access to a TNT productsDVD, you can download the data from MicroImages’ web site. In particular, this and DATA data collections.fying and resampling raster images. Details of the process can be found in avariety of tutorial booklets, color plates, and Quick Guides, which are all availablefrom MicroImages’ web site. Pro and TNTmips FreeSystem) comes in three versions: the professional version of TNTmips (TNTmipsPro), the low-cost TNTmips Basic version, and the TNTmips Free version. Allthe same features. If you did not purchase the professional version (which re-TNTmips Free mode. The Automatic Resampling process is not available inTNTview or TNTatlas. All the exercises can be completed in TNTlite using the Rectifying Images Welcome to Rectifying Images / Automatic from thefor determining the cell size,the output raster. Pages 10-11 discuss the three optionsvalues in the output raster.pages 12-17. Vectorery. A correctly processed digital map is free ofprocess. These distortions can arise from tilt of thecauses. As a result the raw images do not have asimple “map-like” geometry, and accurate map rela-warping or “rubber sheeting”) changes or rectifiescontrol for the image. Depending on the geometrictransformation model you select, the process cannate system. Each input raster is processedseparately and each must be georeferenced. Con-all instances. Satellite imagery of low-relief areasmay have minimal internal distortions. The Displaysystems with reasonable registration. However, ifthese raster layers will be used together routinely,cantly speed up display times.To simply rescale, rotate, orFor the sake of brevity, andseparately, discussions of Rectifying Images page 4 on the Rasters panel ofthe Raster Resamplingwindow;use the standard File /Object Selection windowto select objects TM7,TM4, and Project File in dataGeoreference from theAutomatic Raster Resampling WindowThe Model menuthe method used to setoutput raster.The Orient menu offers dow allows you to select the input raster(s) forextents of the output raster, as well as the geometricing output cell values. You may have found that thetions for each menu. However, if you make a differentfault selection for the next session. Always checkeach of the parameter menus to confirm that yourdesired menu choice is selected.the output raster.for the output raster. Rectifying Images page 5 STEPSpress [Run...]use the standard File /Object selectionprocedures to name anew Project File and theoutput raster objects Rectifying to the Input Map ProjectionYou will probably use the Automatic Resamplingits georeference subobject. To do so, use the de-input). Horizontal lines of cells in the output rasterlel to the y coordinate axis. The input rasters used inthe input object’s georeference information; you canthe output product. The cell size, geographic ex-number of lines and columns in the output raster.7, G = 4, and B = System option to warp the Rectifying Images page 6 System...];in the CoordinateReference Systemwindow, on theStates / CaliforniaWarping to a New Map ProjectionSystem window, whichYou can also warp the input raster to a coordinategeoreferenced. Use the Coordinate Reference Sys-and datums. In this exercise the input raster set isWarping a raster to a different coordinate refer-changes in its geometry. If you warp the entire im-output raster. The warped and rotated image is em-the corners. (The blank areas are flagged in the null choose NAD27 /SPCS27 California zone3 (ftUS), then click [OK]choose Run in theRaster Resamplingwindow and name the Use of the Coordinate Reference Systemwindow is introduced in the CoordinateReference Systems tutorial. Rectifying Images page 7 Reference from the Cell object from the Project FileWarping to a Reference Raster system, with aprocess. Cell size and orientation are linked in thisinstance; choosing Match Reference from the Cell). With theseof the reference raster.You can use the Match Referenceto the exact extents of the reference raster. This is(as in this exercise). If the reference and input ras-area common to both.orientation of the output. run the resamplingprocess Rectifying Images page 8 Setting Output Extents and Raster SizeWe have covered several options for specifying theextents of the output raster, including Entire Input,Match Reference, and Overlap Reference. The finaloption is User Defined. When you select this optioncoordinate reference system. Click the Coordinatesto have a specific size in lines and columns. The Bythis capability. The line and column cell sizes areOutput rasters in State Plane 1927coordinate system, with extentsstill selected and available for use. However, theThe previously-selected State Plane coordinate You can use the Queue Job and Save JobTNT job processing system. See the Technical for details. Rectifying Images page 9 on the Rasters paneland select objectsPHOTO ProjectDATA data You can orient theThe vertical axis of an input raster’s map coordinateparallel to the local direction of true north. Thefour cardinal compass directions at the top. Theseready aligned with a map coordinate system, and youIn this example, the edges of the input raster set co-north at top. Reorienting it to the State Plane projec-degrees. Reorienting it with South at Top exactly on the Settings panel,choose Manual from theCell Size menuchoose GeographicSouth from the Orientmenurun the resamplingprocess PHOTOState Plane Coordinate mapgrid in yellow. Rectifying Images page 10 on the Rasters paneland select object PANCell Value Interpolation in ResamplingThe Automatic Resampling process uses severalsteps to create the transformed output raster. First,the proper extents and scale (cell size). Then a cellter. To do so, the geometric transformation iscolumn coordinates. The target output cell may belarger or smaller than an input cell, and it may over-lap several input cells. The output cell value musttherefore be calculated (interpolated) from someThe Method menu offers several options for inter-polating output cell values. The Nearest Neighbor,Bilinear, and Bicubic methods are illustrated in thediagram below and are discussed on the next page. output raster. Bold outline indicates Portion of original, distortedraster image. the Current Target Cell select Bilinear from the Rectifying Images page 11Resampling MethodsNearest NeighborBilinear Each output cell value in thefrom the closest input cell. Less computation is in-advantage for large input rasters. Preservation ofHowever, nearest neighbor resampling can cause fea-ture edges to be offset by distances up to half of theinput cell size. If the raster is resampled to a differ-duplication (smaller output cell size) or dropping An output cell value in the bilinear inter-input cells. This method produces a smoother ap- The bicubic method calculates an outputcells. The output value is a distance-weighted aver-age, but the weight values vary as a nonlinearfunction of distance. This method produces sharper,method. It is the preferred method when resamplingto a larger output cell size. Two variants are alsoproduced by the Automatic Classification process. CellPAN Rectifying Images page 12 Rectifying a Color Infrared AirslideTopographic map (object system. Note the nearly squarebounding the section. The from32 Project File in data in the32 Project Fileine the effects of different geometric transformationmodels in attempting to rectify this image.below). This indicates that the are curved outward slightly.suggesting that this is an effect of Rectifying Images page 13 on the Rasters paneland select objects from the32 Project File from the32 Project FileGeometric Transformation ModelsTo alter the geometry of the input raster, the Auto-reference system. The process compares the geo-model you have selected. The results are used todetermine numerical coefficients for coordinateThe Automatic Resampling process incorporates allof the geometric transformation models that are avail-quality of control point locations. Each transforma-tion model requires a minimum number of controlpoints for solution. The minimum number of con-the transformation. If additional control points aremation using least squares adjustment. Thisprocedure chooses the set of coefficients for whichnumber, accuracy, and distribution of the controlpoints and the choice of transformation model. Caresuccess in rectification. Position control points sothat they cover most of the image. Adjust controladjustment) for each control point. Appropriate usestransformation model thatobject’s georeferencesubobject. This choicepotentially allows differentapplied to different rasterused in the georeference Keep the current settings andproceed to the next page. Rectifying Images TranslateAffine Transformationschoose Affine from the raster to controlof the output image. TheThe affine transformation model projects coordinates plane (defined by theoutput coordinate system). An affine model can in-of the image. Rescaling can accomodate a separaterections. Any set of parallel lines in the source imageremain parallel in the output image. The affine modelThe affine model is appropriate when you need totified image) from its original coordinate system and(for example, UTM to State Plane coordinates). ItThe affine transformation rotated and rescaled the CIRof the section lines (the effects of more complex tilt Rectifying Images from the Model menuPlane Projective Model trapezoidal shape. Thedistortion found in the original image. The north and southsection lines are now nearly parallel. However, the subtlefrom relief displacement and/or lens effects still remains.this image because vertical relief is small (about 110 feet)compared to the image’s horizontal dimensions.The plane projective model transforms coordinatesbetween any pair of source and target planes, includ- planes. It employs a perspectivean aerial camera. For this reason, the plane projec-transformations found in the affine model. How-ever, the only lines that remain parallel in both theplanes. The plane projective model requires a mini- Rectifying Images page 16 Polynomial Models distortions in a raster image, as well as the linearously. Polynomial equations are used to relategeographic coordinate system. The control pointof curvature (concave or convex) in any direction.This model can correct for radial lens distortion oraltitude or satellite scenes of large areas. An Order 3in any direction, and an Order 4 polynomial allowsfor an even more complex fit. These models canOrder 2: 6 control pointsOrder 3:10 control pointsOrder 4:15 control pointspolynomials of differentorder. Input X coordinateGeographic X coordinateOrder 3PolynomialOrder 2Polynomial Polynomial transformations of thetest image produce very similarresults for all three orders, probablybecause of the small image area,dense network of control points, andsmall amount of terrain distortion.The Order 3 polynomial appears toproduce the best result (shownhere), comparable to the result fromthe plane projective transformation.Several of the section lines are stillbowed outward, an indication ofuncorrected terrain distortion. Rectifying Images page 17 choose Piecewise Affinefrom the Model menuPiecewise Affine Model photographs of high-relief areas cannot be corrected using the AutomaticResampling process. Rectification of these images to a map geometryelevation model (DEM). See the tutorial entitled The transformation models discussed previously allcompute a global best-fit solution for the entire im-age. They are best tailored to remove smoothly vary-ing distortions. Distortions that change significantlyover small areas are not corrected. In fact, a localdistortion that affects the positions of one or twocontrol points will influence the overall fit as muchas other correctly modeled points. As a result, thelocal distortion introduces a smaller component ofThe Piecewise Affine model offers an alternativeapproach. Each control point is assumed to be inment the image into a network of triangles. An affinetriangle. A single distorted control point locationonly affects the immediately surrounding triangles.At least six control points are required, but largerThe Piecewise Affine transformationboundaries. This method is also Rectifying Images page 18 Warping window, click from the from the output objectReprojecting Vector or CAD Objectsof the transformationthe Automatic ResamplingVector object after warping to32 map object.The Geometric Warping process permanentlyject to the selected coordinate reference system. Youcoordinate reference system to another, typically toa project. Although the TNTmips Display processent object. These distortions remain even afterto the vector object. For objects withric Warping process provides a subsetObviously, a better strategy is to georeference and rectify source imagerycreating a vector overlay. The vector object is then automatically object was created in theSpatial Data Editor by tracing the roads32 image. It was Rectifying Images Review and References reference system and cell size to facilitate spatial analysis, classification,change detection in multidate imagery, or other scene-to-scene compari-sons of individual cell values. This includes scanned topographic andto remove or reduce certain types of simple geometric distortions from aerialor satellite imagery, producing a more map-like image geometry.You do different images exhibit different types of distortion. The Mosaic process appliesa single selected geometric transformation model to rectify all georeferenced im-Successful rectification begins with careful georeferencing of the images. Youappropriate geometric transformation model. Use this model in the georeferenceChristensen, Albert H.J. (1996). The practice of piecewise fits with particular refer-Introductory Digital Image Processing: A Remote Sens- (2nd ed.). Chapter 6, Image Preprocessing: Radiometricand Geometric Correction. Upper Saddle River, NJ: Prentice-Hall. pp. 107-Novak, Kurt (1992). Rectification of digital imagery. Wolberg, George (1990). Digital Image Warping. Chapter 3, Spatial Transfor-mations. Los Alamitos, CA: IEEE Computer Society Press, p. 41-94.Wolf, Paul. R. and Ghilani, Charles. D. (1997). Adjustment Computations: Sta-tistics and Least Squares in Surveying and GIS. New York: John Wiley & Rectifying Images page 20 Advanced Software for Geospatial Analysis www.microimages.com MicroImages,Inc. data visualization, analysis, and publishing. Contact us or visit our web site for detailed prod-TNTmips Pro is a professional system for fully integrated GIS, imageanalysis, CAD, TIN, desktop cartography, and geospatial database management.TNTmips Freeals with small projects. You can download TNTmips Free from MicroImages’ web site.TNTeditTNTedit provides interactive tools to create, georeference, and edit vector, image,TNTview has the same powerful display features as TNTmips and is perfect forTNTatlasTNTatlas lets you publish and distribute your spatial project materials on CD orDVD at low cost. TNTatlas CDs/DVDs can be used on any popular computing platform. affine transformation.................................14bilinear interpolation.............................10,11cubic convolution interpolation.............10,11interpolation........................................4,10,11bilinear...........................................10,11cubic convolution.............................10,11to a reference raster..................................7nearest neighbor resampling...................10,11piecewise affine transformation...................17reference raster.............................................7 ECTIFICTION Rectifying Images page 1 TutorialImages ECTIFICTION