ENVI Tutorial
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E N V I T u t o r i a l :O r t h o r e c t i f y i n g I m a g e r yu s i n g R a t i o n a l P o l y n o m i a lC o e f f i c i e n t s ( R P C s )O r t h o r e c t i f y i n g U s i n g R a t i o n a l P o l y n o m i a l C o e f f i c i e n t s ( R P C s ) 2F i l e s U s e d i n T h i s T u t o r i a l 2I n t r o d u c t i o n t o R P C O r t h o r e c t i f i c a t i o n 3V i e w I m a g e s 4R u n t h e O r t h o r e c t i f i c a t i o n 5E x a m i n e t h e O r t h o r e c t i f i c a t i o n R e s u l t s 61E N V I T u t o r i a l : O r t h o r e ct i f yi n g I m a g e r y u si n g R a t i o n a l P o l yn o m i a l C o e f -f i ci e n t s ( R P C s)O r t h o r e c t i f y i n g U s i n g R a t i o n a l P o l y n o m i a lC o e f f i c i e n t s ( R P C s )This tutorial demonstrates ENVI's orthorectification tools that use rational polynomial coefficients(RPCs). You will orthorectify an IKONOS image of La Jolla, California, USA, courtesy of SpaceImaging. You will then compare the orthorectified image to the uncorrected image and examine thedifferences.F i l e s U s e d i n T h i s T u t o r i a lENVI Resource DVD: D a t a \ o r t h oF i l e D e s c r i p t i o nc o n u s _ u s g s . d e m USGS digital elevation model (DEM) for the area of theIKONOS imagep o _ 1 0 1 5 1 5 _ m e t a d a t a . t x t Metadata file for the IKONOS imagep o _ 1 0 1 5 1 5 _ p a n _ 0 0 0 0 0 0 0 _ r p c . t x t Text file containing RPCsp o _ 1 0 1 5 1 5 _ p a ...
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ENVI Tutorial: Orthorectifying Imagery using Rational Polynomial Coefficients (RPCs)
Orthorectifying Using Rational Polynomial Coefficients (RPCs) Files Used in This Tutorial Introduction to RPC Orthorectification View Images Run the Orthorectification Examine the Orthorectification Results
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ENVI Tutorial: Orthorectifying Imagery using Rational Polynomial Coef-ficients (RPCs)
Orthorectifying Using Rational Polynomial Coefficients (RPCs) This tutorial demonstrates ENVI's orthorectification tools that use rational polynomial coefficients (RPCs). You will orthorectify an IKONOS image of La Jolla, California, USA, courtesy of Space Imaging. You will then compare the orthorectified image to the uncorrected image and examine the differences. Files Used in This Tutorial ENVI Resource DVD:Data\ortho File Description conus_usgs.dem USGS digital elevation model (DEM) for the area of the IKONOS image po_101515_metadata.txt Metadata file for the IKONOS image po_101515_pan_0000000_rpc.txt Text file containing RPCs po_101515_pan_0000000.tfw TIFF world file containing initial georeferencing information po_101515_pan_0000000.tif (.hdr) IKONOS image data in TIFF format
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ENVI Tutorial: Orthorectifying Imagery using Rational Polynomial Coef-ficients (RPCs)
Introduction to RPC Orthorectification Orthorectification is a process of making the geometry of an image planimetric, or map-accurate, by modeling the nature and magnitude of geometric distortions in the imagery. These distortions are caused by topography, camera geometry, and sensor-related errors. Orthorectification is a logical step when precise positional accuracy and uniform scale are required throughout an image. After orthorectifying an image, you can measure or precisely locate features in the image, collect information for a GIS, or combine the image with other orthorectified images for sophisticated analyses. ENVI's orthorectification tools rectify data from specific pushbroom sensors (currently ASTER, CARTOSAT-1, FORMOSAT-2, GeoEye-1, IKONOS, OrbView-3, QuickBird, RapidEye, SPOT, and WorldView-1), using an RPC model. Data from each of these sensors typically include an ancillary RPC file generated from ephemeris data, which ENVI uses to perform the orthorectification. The following are required input for RPC orthorectification: The image to rectify l RPC model (not required for SPOT data) l Elevation information l Offset between mean sea level and the gravitational potential surface (known as the geoid), so the l elevation can be correctly interpreted If approximate geolocation information is not available for the source image, the rough location of l the image on the earth’s surface must be computed to provide a location base needed for the RPC transformation. The input image must be linked to the RPC coefficients contained in an ancillary text file. These coefficients are required for the rational function expansion to convert ground coordinates into sensor coordinates. In the case of opening an IKONOS image, which you will use for the following exercise, ENVI searches for an RPC filename consisting of the root name of the source image plus_rpc.txt.
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ENVI Tutorial: Orthorectifying Imagery using Rational Polynomial Coef-ficients (RPCs)
View Images 1. Fromthe ENVI main menu bar, selectFile > Open Image File. A file selection dialog appears. 2. NavigatetoData\orthoand selectpo_101515_pan_0000000.tif. ClickOpen. 3. Inthe Available Bands List, select theGray Scaleradio button, selectBand 1, and clickLoad Band. Although the image has map information associated with it, orthorectification is still required since the coordinates reported for any given point in the image are likely to have significant positional inaccuracy.
4. ADEM is optional input, but it enhances the accuracy of the orthorectification. From the main ENVI menu bar, selectFile > Open External File > Digital Elevation > USGS DEM. 5. Selectconus_usgs.demand clickOpen. A USGS DEM Input Parameters dialog appears. 6. Enterortho_dem.datfor the output filename and clickOK. 7. Inthe Available Bands List, clickDisplay #1and selectNew Display. 8. Inthe Available Bands List, selectDEM Imageand clickLoad Band. The elevation for this area ranges from sea level to 245 m. This significant topographic variation is sure to introduce geometric inaccuracies into the IKONOS image. The DEM and the IKONOS image do not have the same map projection or pixel size. However, you do not have to reproject or resample the two images; ENVI’s orthorectification tool accounts for their differences.
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ENVI Tutorial: Orthorectifying Imagery using Rational Polynomial Coef-ficients (RPCs)
Run the Orthorectification 1. Fromthe ENVI main menu bar, selectMap > Orthorectification > IKONOS > Orthorectify IKONOS. A file selection dialog appears. 2. Selectpo_101515_pan_0000000.tifand clickOK. An Orthorectification Parameters dialog appears. 3. ImageResampling is the method for determining pixel values in the IKONOS image during the orientation. The default method is Bilinear, which provides moderately smooth results. The Cubic Convolution option provides smoother results, while the Nearest Neighbor option does change the original pixel values. The Nearest Neighbor option leads to a relatively choppy appearance, but it is the only valid option if you intend to perform analyses on the orthorectified image. For this tutorial, selectBilinear. 4. Backgroundrefers to the value assigned to the border pixels in the orthorectified image. Leave the value at0. 5. InputHeight specifies whether a DEM or a fixed elevation value will be used for the entire image. Because you have a DEM (the more accurate option), leave the DEM option selected. 6. ClickSelect DEM. A Select Input DEM Band dialog appears. 7. SelectDEM Imageunderortho_dem.datand clickOK. 8. DEMResampling is the method used to determine pixel values for an internally calculated version of the DEM image with the same orientation and pixel size as the IKONOS image. Again, use the defaultBilinearmethod. 9. Geoidoffset is the height of the geoid above mean sea level in the geographic area covered by the image. Most DEM images provide information about the elevation above mean sea level for each pixel. Orthorectification, however, requires information about the height above the ellipsoid for each pixel. To convert from the DEM mean sea level values to height above the ellipsoid, you must add the geoid height to the DEM. Enter aGeoid offsetvalue of-35. This means the ellipsoid is about 35 m above mean sea level in this area. Many institutions that perform photogrammetry have their own software for determining geoid heights, or you can obtain software from NOAA, the National Geospatial Intelligence Agency (NGA), USGS, or other sources See the following URL for a geoid height calculator:http://www.ngs.noaa.gov/cgi-bin/GEOID_ STUFF/geoid99_prompt1.prl. 10. Theright side of the dialog has parameters related to the extent and pixel size of the output image. The default values are calculated from the georeferencing information in the original IKONOS image. These values are appropriate for this example. You could also optionally change the projection for the output orthorectified image by clicking Change Proj. 11. Enterikonos_ortho.datin theOrthorectified Image Filenamefield. 12. ClickOKto begin the orthorectification process, which can take several minutes. After processing is complete, the orthorectified image is added to the Available Bands List.
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ENVI Tutorial: Orthorectifying Imagery using Rational Polynomial Coef-ficients (RPCs)
Examine the Orthorectification Results 1. Displaythe orthorectified image in Display #2, which currently contains the DEM image. 2. Comparethe original IKONOS image to the orthorectified image by selectingTools > Link > Link Displaysfrom a Display group menu bar and clickingOKin the Link Displays dialog. 3. Clickinside an Image window to toggle between the two images. Notice the subtle difference in geometry, especially in the upper-right corner of the two images:
4. Whenyou are finished, selectFile > Exitfrom the ENVI main menu bar.
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