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ENVI Tutorial: ImageGeoreferencing andRegistrationI m a g e G e o r e f e r e n c i n g a n d R e g i s t r a t i o n 2F i l e s U s e d i n t h i s T u t o r i a l 2B a c k g r o u n d 2O p e n i n g a n d D i s p l a y i n g G e o r e f e r e n c e d D a t a 4V i e w i n g M a p I n f o i n t h e E N V I H e a d e r 4D i s p l a y i n g t h e C u r s o r L o c a t i o n a n d V a l u e 5I m a g e t o I m a g e R e g i s t r a t i o n 6S t a r t i n g I m a g e R e g i s t r a t i o n a n d L o a d i n g G r o u n d C o n t r o l P o i n t s 6W o r k i n g W i t h G C P s 7W a r p i n g I m a g e s 8I m a g e t o M a p R e g i s t r a t i o n 1 0O p e n i n g a n d D i s p l a y i n g a S P O T I m a g e F i l e 1 0O p e n i n g a n d D i s p l a y i n g a L a n d s a t T M I m a g e F i l e 1 0S e l e c t i n g I m a g e - t o - M a p R e g i s t r a t i o n a n d R e s t o r i n g G C P s 1 0A d d i n g M a p G C P s U s i n g V e c t o r D i s p l a y o f D L G s 1 0R S T a n d C u b i c C o n v o l u t i o n W a r p i n g 1 2D i s p l a y i n g a n d E v a l u a t i n g R e s u l t s 1 21E N V I T u t o r i a l : I m a g e G e o r e f e r e n ci n g a n d R e g i st r a t i o nImage Georeferencing and RegistrationThis tutorial provides basic information about georeferenced images in ENVI and provides a startingpoint for conducting image-to-image and image-to-map registration using ENVI. It assumes that you arealready familiar with general ...
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ENVI Tutorial: Image Georeferencing and Registration
Image Georeferencing and Registration Files Used in this Tutorial Background Opening and Displaying Georeferenced Data Viewing Map Info in the ENVI Header Displaying the Cursor Location and Value Image to Image Registration Starting Image Registration and Loading Ground Control Points Working With GCPs Warping Images Image to Map Registration Opening and Displaying a SPOT Image File Opening and Displaying a Landsat TM Image File Selecting Image-to-Map Registration and Restoring GCPs Adding Map GCPs Using Vector Display of DLGs RST and Cubic Convolution Warping Displaying and Evaluating Results
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ENVI Tutorial: Image Georeferencing and Registration
Image Georeferencing and Registration
This tutorial provides basic information about georeferenced images in ENVI and provides a starting point for conducting image-to-image and image-to-map registration using ENVI. It assumes that you are already familiar with general image-registration and resampling concepts. This tutorial is designed to be completed in about 1 to 2 hours.
Files Used in this Tutorial ENVI Resource DVD:Data\bldr_reg
File bldr_sp.img (.hdr) bldr_tm.img (.hdr) bldr_tm.pts bldrtm_m.pts bldr_rd.dlg
File bldr_tm1.wrp (.hdr) bldr_tm2.wrp (.hdr) bldr_tm3.wrp (.hdr) bldr_tm4.wrp (.hdr) bldr_tm5.wrp (.hdr) bldrtm_m.img (.hdr)
bldrtmsp.img (.hdr)
Description Boulder SPOT georeferenced image subset Non-georeferenced Boulder TM data GCPs for TM-SPOT image-to-image registration GCPs for TM-Map registration Boulder roads DLG
Description Image-to-image result using RST and nearest neighbor Image-to-image result using RST and bilinear interpolation Image-to-image result using RST and cubic convolution Image-to-image result using 1st degree polynomial and cubic convolution Image-to-image result using Delaunay triangulation and cubic convolution Image-to-map result using RST and cubic convolution for the Boulder TM data Boulder TM/SPOT sharpening result using HSV sharpening, 10 meter pixels
Background ENVI provides full support for georeferenced images in numerous predefined map projections including UTM and State Plane. In addition, ENVI’s user-configurable map projections allow construction of custom map projections utilizing 30 standard projection types. In addition to the projections listed in Table D-8, ENVI also reads any projection type from a geodatabase or ESRI® layer file. See ArcGIS® Help for a full list of supported projections, or refer to the Tech Tip “ESRI® Projection Engine Reference v1.0” on the ITT Visual Information Solutions website. ENVI map projection parameters are stored in an ASCII text filemap_proj.txtthat you can edit or modify using ENVI map projection utilities. The information in this file is used in the ENVI header files associated with each image and allows simple association of a reference pixel location with known map projection coordinates. Selected ENVI functions can then use this information to work with the image in georeferenced data space.
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ENVI Tutorial: Image Georeferencing and Registration
ENVI’s image registration and geometric correction utilities allow you to reference pixel-based images to geographic coordinates and/or correct them to match base image geometry. Ground control points (GCPs) are selected using the Image and Zoom windows for both image-to-image and image-to-map registration. Coordinates are displayed for both base and uncorrected image GCPs, along with error terms for specific warping algorithms. Next GCP point prediction allows simplified selection of GCPs. Warping is performed using resampling, scaling and translation (RST), polynomial functions (of order 1 through n), or Delaunay triangulation. Resampling methods supported include nearest-neighbor, bilinear interpolation, and cubic convolution. Comparison of the base and warped images using ENVI’s multiple dynamic overlay capabilities allows quick assessment of registration accuracy. The following sections provide examples of some of the map-based capabilities built into ENVI. Consult ENVI Help for additional information.
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ENVI Tutorial: Image Georeferencing and Registration
Opening and Displaying Georeferenced Data
Before attempting to start the program, ensure that ENVI is properly installed as described in the Installation Guide that shipped with your software. 1. From the ENVI main menu bar, selectFile > Open Image File. 2. Navigate to theData\bldr_regdirectory, select the filebldr_sp.imgfrom the list, and clickOpen. The Available Bands List appears. 3. From the Available Bands List, select theGeoreferenced SPOTband from the list and click Load Bandto load the image into a new display group.
Viewing Map Info in the ENVI Header 1. In the Available Bands List, right click on theMap Infoicon and selectEdit Map Information. The Edit Map Information dialog appears.
This dialog lists the basic map information used by ENVI in georeferencing. The image coordinates correspond to a reference pixel used by ENVI as the starting point for the map coordinate system. Because ENVI knows the map projection, pixel size, and map projection parameters based on this header information and the map projection text file, it can calculate the geographic coordinates of any pixel in the image. You can enter coordinates in either map coordinates or geographic (latitude/longitude) coordinates.
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ENVI Tutorial: Image Georeferencing and Registration
2. Click on the projection field toggle button to display the latitude/longitude coordinates for the UTM Zone 13 North map projection. Note that ENVI makes this conversion on-the-fly. 3. Click theDDEGbutton then click theDMSbutton to toggle between degrees-minutes-seconds, and decimal degrees, respectively. 4. ClickCancelto exit the Edit Map Information dialog.
Displaying the Cursor Location and Value You can choose to display the location of your mouse cursor, screen value (RGB color), and the data value of the pixel underneath the crosshair cursor using the Cursor Location/Value dialog. When several display groups are open, this dialog specifies which display group’s location and value are being reported. 1. To display the cursor location and value, selectWindow > Cursor Location/Valuefrom either the ENVI main menu bar or the Display group menu bar. You can also double-click inside the Image window. 2. Move the mouse cursor over the Image, Scroll, or Zoom windows to display location and value information in the Cursor Location/Value dialog. Note that the coordinates are given in both pixels and georeferenced coordinates for this georeferenced image. Also note the relation between map coordinates and latitude/longitude. 3. From the Cursor Location/Value menu bar, selectFile > Cancelto close the dialog. 4. Keep the display group open for the next exercise.
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ENVI Tutorial: Image Georeferencing and Registration
Image to Image Registration
This section of the tutorial takes you step-by-step through Image to Image Registration. The georeferenced SPOT image will be used as the base image, and a pixel-based Landsat TM image will be warped to match the SPOT. 1. From the ENVI main menu bar, selectFile > Open Image File. 2. Navigate to theData\bldr_regdirectory, select the filebldr_tm.imgfrom the list, and clickOpen. The Available Bands List appears and the RGB image is automatically loaded into a new display group. 3. Double-click in the Image window to display the Cursor Location/Value tool. 4. Move the mouse cursor over the Image, Scroll, or Zoom windows to display location and value information in the Cursor Location/Value dialog. Note that the coordinates are given in pixels since this is a pixel-based image rather than a georeferenced image like the SPOT data you used in the previous exercise. 5. From the Cursor Location/Value menu bar, selectFile > Cancelto close the dialog.
Starting Image Registration and Loading Ground Control Points 1. From the ENVI main menu bar, selectMap > Registration > Select GCPs: Image to Image. The Image to Image Registration dialog appears. 2. In theBase Imagefield, selectDisplay #1(the SPOT image). In theWarp Imagefield, select Display #2(the TM image) 3. ClickOKto start the registration. The Ground Control Points Selection dialog appears.
4. You can add individual ground control points (GCPs) by positioning the cursor in the two images to the same ground location. In theBasefield, type753for the x location and826for the y location. Press theEnterkey after typing each value, to move the cursor in the SPOT image. In theWarpfield, type331for the x location and433for the y location to move the cursor in the TM image. 5. Examine the locations in the two Zoom windows and adjust the locations if necessary by clicking the left mouse button in each Zoom window at the desired locations. Note that sub-pixel
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ENVI Tutorial: Image Georeferencing and Registration
positioning is supported in the Zoom windows. The larger the zoom factor, the finer the positioning capabilities. In the Ground Control Points Selection dialog, clickAdd Pointto add the GCP to the list. Click Show Listto view the GCP list. Try this for a few points to get a feel for selecting GCPs. Note the list of actual and predicted points in the dialog. Once you have at least four points, the RMS error is reported. From the Ground Control Points Selection dialog menu bar, selectOptions > Clear All Pointsto clear all of your points. From the Ground Control Points Selection dialog menu bar, selectFile > Restore GCPs from ASCII. Navigate to theData\bldr_regdirectory, select the filebldr_tm.ptsfrom the list, and clickOpen. Previously saved GCP parameters are loaded into the dialog. Try positioning the cursor at a new location in the Image window containing the SPOT image. Click thePredictbutton to move the cursor position in the TM image to match its predicted location based on the warp model. The exact position can now be interactively refined by moving the pixel location slightly in the TM data. ClickAdd Pointin the Ground Control Points Selection dialog to add the new GCP to the list.
Working With GCPs The following image provides descriptions of the features and functions available in the Image to Image GCP List. ClickShow Listto view the GCP list. Click on individual GCPs in the Image to Image GCP List dialog and examine the locations of the points in the two images, the actual and predicted coordinates, and the RMS error. Resize the dialog to observe the total RMS Error listed in the Ground Control Points Selection dialog.
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ENVI Tutorial: Image Georeferencing and Registration
Warping Images Images can be warped from the displayed band, or multiband images can be warped all bands at once. For this exercise you will warp only the displayed band. 1. From the Ground Control Points Selection dialog menu bar, selectOptions > Warp Displayed Band. The Registration Parameters dialog appears. 2. Click theMethoddrop-down list and selectRST. 3. Ensure theResamplingdrop-down list has theNearest Neighboroption selected. 4. In theEnter Output Filenamefield, typebldr_tm1.wrpas the new filename and clickOK. The warped image is listed in the Available Bands List when the warp is completed. 5. From the Ground Control Points Selection dialog menu bar, selectOptions > Warp Displayed Band. The Registration Parameters dialog appears. 6. Click theMethoddrop-down list and selectRST. 7. Click theResamplingdrop-down list and selectBilinear. 8. In theEnter Output Filenamefield, typebldr_tm2.wrpas the new filename and clickOK. The warped image is listed in the Available Bands List when the warp is completed. 9. Repeat steps 5 – 8 using theRSTmethod andCubic Convolutionresampling, then name the output filebldr_tm3.wrp. 10. Repeat steps 5 – 8 using thePolynomialmethod andCubic Convolutionresampling, then name the output filebldr_tm4.wrp. 11. Repeat steps 5 – 8 using theTriangulationmethod andCubic Convolutionresampling, then name the output filebldr_tm5.wrp.
Comparing Warp Results Now you will use dynamic overlays to compare your warp results: 1. In the Available Bands List, click once to select thebldr_tm.imgfile, then selectFile > Close Selected Filefrom the menu bar. In the subsequent ENVI warning dialog, clickYesto close the associated display group. 2. In the Available Bands List, select thebldr_tm1.wrpfile, click theDisplay #drop-down button, selectNew Display, then clickLoad Bandto load the file into the new display. 3. Right-click in the Image window and selectLink Displays. The Link Displays dialog appears. 4. ClickOKin the Link Displays dialog to link the SPOT and the registered TM image. 5. Compare the SPOT and the TM images using the dynamic overlay by clicking the left mouse button in the Image window of the TM image. 6. Loadbldr_tm2.wrpandbldr_tm3.wrpinto new display groups and use the image linking and dynamic overlays to compare the effect of the three different resampling methods: nearest neighbor, bilinear interpolation, and cubic convolution. Note how jagged the pixels appear in the nearest neighbor resampled image. The bilinear interpolation image looks much smoother, but the cubic convolution image is the best result, smoother, but retaining fine detail.
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ENVI Tutorial: Image Georeferencing and Registration
Close thebldr_tm1.wrpandbldr_tm2.wrpdisplay groups (selectFile > Cancelfrom the associated Display group menu bars). Loadbldr_tm4.wrpandbldr_tm5.wrpinto new display groups, and use the image linking and dynamic overlays to compare tobldr_tm3.wrp(RST Warp). Note the effect of the three different warping methods: RST, 1st degree Polynomial, and Delaunay Triangulation on the image geometry. Use dynamic overlay to compare to the georeferenced SPOT data. To display the Cursor Location/Value tool, double-click in the Image window. Browse the georeferenced datasets and note the effect of the different resampling and warp methods on the data values. From the Display group menu bar, selectFile > Cancelto close the dialog. From the ENVI main menu bar, selectFile > Close All Filesto close all of the data files. Click Yeson the corresponding warning dialog.
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ENVI Tutorial: Image Georeferencing and Registration
Image to Map Registration
This section of the tutorial will take you step-by-step through an Image to Map registration. Many of the procedures are similar to Image to Image registration and will not be discussed in detail. The map coordinates picked from the georeferenced SPOT image and a vector Digital Line Graph (DLG) will be used as the base image, and the pixel-based Landsat TM image will be warped to match the map data.
Opening and Displaying a SPOT Image File 1. From the ENVI main menu bar, selectFile > Open Image File. 2. Navigate to theData\bldr_regdirectory, select the filebldr_sp.imgfrom the list, and clickOpen. The Available Bands List appears. 3. SelectGeoreferenced SPOTunderbldr_sp.img, and clickLoad Band. The georeferenced SPOT image appears in a new display group.
Opening and Displaying a Landsat TM Image File 1. From the ENVI main menu bar, selectFile > Open Image File. 2. Navigate to theData\bldr_regdirectory, select the filebldr_tm.imgfrom the list, and clickOpen. A Landsat TM RGB image is automatically loaded into a new display group.
Selecting Image-to-Map Registration and Restoring GCPs 1. From the ENVI main menu bar, selectMap > Registration > Select GCPs: Image to Map. The Image to Map Registration dialog appears. 2. From theDisplay #1drop-down menu, selectDisplay #2. You will warp the Landsat TM image (in Display #2) to match the georeferenced SPOT image. 3. EnsureUTMis selected as the projection. 4. Type13in theZonefield. 5. Enter10.0in theX/Y Pixel Sizefields. SPOT data have a spatial resolution of 10m. 6. ClickOKto start the registration. The Ground Control Points Selection dialog appears. 7. From the Ground Control Points Selection dialog menu bar, selectFile > Restore GCPs from ASCII. 8. Navigate to theData\bldr_regdirectory, select the filebldrtm_m.ptsfrom the list, and clickOpen. Previously saved ground control point parameters are loaded into the dialog. 9. In the Ground Control Points Selection dialog, clickShow List. The Image to Map GCP List dialog appears. Examine the base map coordinates, the actual and predicted image coordinates, and the RMS error.
Adding Map GCPs Using Vector Display of DLGs 1. From the ENVI main menu bar, selectFile > Open Vector File. 2. From the Files of type drop-down list, selectUSGS DLG.
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ENVI Tutorial: Image Georeferencing and Registration
Navigate to theData\bldr_regdirectory, select the filebldr_rd.dlgfrom the list, and clickOpen. The Import Vector Files Parameters dialog appears. In the Import Vector Files Parameters dialog, click theMemoryradio button, then clickOKto read the DLG data. The Available Vectors List dialog appears. Select theROADS AND TRAILS: BOULDER, COfile, then clickLoad Selected. SelectDisplay #1and clickOK. The Vector Parameters: Cursor Query dialog appears. Return to the Load Vector dialog. From The Available Vectors List dialog, select theROADS AND TRAILS: BOULDER, COfile, then clickLoad Selected. SelectNew Vector Window, and clickOK. This loads the vectors into a new vector window.
Click and drag the mouse in the vector window to activate a crosshair cursor. The map coordinates of the cursor will be listed at the bottom of the vector window. From the Display group menu bar in the TM image, selectTools > Pixel Locator. Type402in theSamplefield and418in theLinefield, then clickApplyto place the cursor on the road intersection. Note that sub-pixel positioning accuracy is again available in the Zoom window. In the Vector window, position the vector cursor at the road intersection at approximately 477593.74, 4433240.0 (40° 3' 3" N, -105° 15' 45" W) by clicking and dragging with the left mouse button and releasing when the circle at the crosshair intersection overlays the intersection of interest. Right-click in the vector window and selectExport Map Location. The new map coordinates will appear in the Ground Control Points Selection dialog.
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