RPC Orthorectification

RPC Orthorectification Tutorial

The RPC Orthorectification workflow orthorectifies an image containing a Rational Polynomial Coefficient (RPC) sensor model or Replacement Sensor Model (RSM). Orthorectification geometrically corrects the data to remove distortions that occur during image capture.

See the following sections:

• Select Input Files
• Apply Ground Control Points
• Export the Orthorectified FIle

For a fully automated RPC orthorectification solution that uses a reference image to automatically generate GCPs, use the RPC Orthorectification Using Reference Image tool.

You can also write a script to perform RPC orthorectification using the RPCOrthorectification task.

Select Input Files

You will need one image with an RPC or RSM sensor model, along with a DEM file (for RPC files only). The following instructions pertain to files with RPC information.

Input Files

The input image must have associated RPC or RSM information.

For RSM, ENVI uses the Mensuration Services Program (MSP) and its elevation sources to calculate image-to-ground and ground-to-image coordinates. A separate classified DoD plug-in is required to use RSMs in ENVI.

RPC or RSM coefficients are required for the rational function expansion to convert ground coordinates into sensor coordinates. When you select a file for input, ENVI searches for the RPC or RSM information in one of the following ways:

• For GeoEye-1: An RPC filename consisting of the root name of the source data file appended with the extension .pvl.
• For IKONOS: An RPC filename consisting of the root name of the source image plus _rpc.txt.
• For OrbView-3: An RPC filename consisting of the root name of the source data file, but without the _image prefix, and appended by _metadata.pvl.
• For QuickBird: An RPC filename consisting of the root name of the source data file appended with the extension .rpb. Note that DigitalGlobe Level-1B data should be used with a rigorous orthorectification solution. While you can use RPC Orthorectification with Level-1B data, you will get more accurate results orthorectifying these data in ENVI's Rigorous Orthorectification tool.
• For WorldView data: An RPC filename consisting of the root name of the source data file appended with the extension .rpb. Note that DigitalGlobe Level-1B data should be used with a rigorous orthorectification solution. While you can use RPC Orthorectification with Level-1B data, you will get more accurate results orthorectifying these data in ENVI's Rigorous Orthorectification tool.
• For RapidEye Level 1B files: The RPC00B or RPC00A tag in the NITF metadata.
• For ENVI raster files: A header file (.hdr) that contains the string RPC INFO. If you have both standard map information and RPC INFO in the header, you need to remove the standard map information from the header to continue.
• For NITF: RPC image support data in the RPC00B or RPC00A Tagged Record Extensions (TREs). RSM image support data are contained in the NITF TREs.

Without elevation information from a DEM, RPCs only give an approximate geographic location. If you do not have a DEM file readily available, you can use the Global Multi-resolution Terrain Elevation Data 2010 (GMTED2010) DEM named GMTED2010.jp2 that is provided with your ENVI installation under the INSTALL_DIR/envixx/data folder. The RPC Orthorectification workflow uses this DEM by default (for RPC files only) unless you specify a different one. The GMTED2010 dataset has a mean resolution of 30 arc seconds. However, for best results, you should use a DEM raster with a higher resolution than GMTED2010. Many DEM products are available from the U.S. Geological Survey National Map and Download Platform or EarthExplorer.

Follow these steps to begin the workflow:

1. From the Toolbox, select Geometric Correction > Orthorectification > RPC Orthorectification Workflow.
2. Select an image with RPC or RSM information.
3. For RPC images, select a DEM file using the Data Selection dialog.
4. Enable the Geoid Correction option when using DEM data that contains orthometric heights, which are elevations above mean sea level (for example, GMTED2010 and National Elevation Dataset, NED). Disable this option for DEM data that contains height above ellipsoid (for example: ASTER Global DEM, DEMs resulting from stereo pair processing.) The RPC Orthorectification workflow performs geoid correction by using the Earth Gravitational Model (EGM) 1996 to automatically determine the geoid offset, which is displayed in meters next to the Geoid Correction option.
5. Click Next. The RPC Refinement panel appears, and the file opens in a new workflow view. If the selected file is displayed in an active view before you start the workflow, the display bands and image location are retained, as well as any brightness, contrast, stretch, and sharpen settings. The image location is not retained for pixel-based images or those with pseudo or arbitrary projections.

Apply Ground Control Points

In this step, you will use GCPs to improve the accuracy of the orthorectification.

Click the Load GCPs button and select a .pts file for input. See GCP File Formats for a detailed description of accepted formats.

Tip: Use the Generate GCPs From Reference Image tool to automatically generate GCPs, then click Load GCPs and open the resulting .pts file.

The GCPs are listed in the RPC Refinement panel and displayed in their corresponding image location.

• A icon means the GCP will be used to adjust the RPC model. These points are called adjustment GCPs and are marked with green crosshairs in the image display.
• A icon means the GCP will not be used to adjust the RPC model. These points are called independent GCPs and are marked with grey diamond symbols in the image display.
• A or icon means that the difference between the image-estimated ground coordinate and the GCP is greater than three times the RMSEx or RMSEy value. See Accuracy Assessment Background for details. This icon displays only if the Horizontal Accuracy is calculated from more than 10 GCPs. The difference between the green and grey icons is explained in the first two bullet points above.

The following are some tips for working with GCPs in the RPC Refinement panel:

• Select a GCP from the list to center the image over that location. With large images, you may need to use the various zoom tools available from the main toolbar to see the GCP marker.
• To change the status of an individual GCP from adjustment to independent (or vice-versa), right-click on the GCP name and select Change GCP Status. Or, change the status using the drop-down list provided in the Status field of the property table for that GCP.
• To change the status of all GCPs to adjustment GCPs, click the button. To change the status of all GCPs to independent GCPs, click the button.
• To delete a GCP, select the GCP name and click the Delete GCP button. To delete all GCPs in the list, click the Delete All GCPs button.
• To save the current list of GCPs to a shapefile, click the Save GCPs button and select an output location and filename. The shapefile will contain the following attributes for each GCP: identification number, name, map x/y coordinates, height above the ellipsoid, image x/y coordinates, and the associated image name. The image x/y coordinates are based on the upper-left corner of the first pixel (0,0).
• To edit GCP values, select a GCP from the table and edit the Name, Map X (longitude), Map Y (latitude), Height (height above ellipsoid), or Image X/Y values as needed, particularly when more accurate values are known. Your input image can be in any projection that ENVI supports, but map coordinates will display as Geographic WGS-84 in the Properties dialog.
• To add a new GCP, select the Symbol Annotation icon from the main toolbar and click in the image where you want the GCP to appear. A new GCP is added to the list. You can then edit the map and/or image coordinates as needed. These values are only estimates based on the image, so you should replace them with more accurate values.
• To interactively reposition a GCP (versus editing GCP values), choose the Select icon from the main toolbar. Within the image, select the GCP you want to move. A cyan-colored box appears around the GCP. Move the GCP to a new location.
• GCPs will be removed if you click Previous to go back to File Selection.

View Error Statistics

See Accuracy Assessment Background for details on how error statistics are calculated and a description of the options under the Statistics tab.

Error Overlay

If you have three or more GCPs, click the Show error overlay button to overlay a transparent color gradient that shows the relative error magnitudes of your GCPs.

Tip: Select Full Extent from the Zoom To drop-down list in the menu bar to see the entire scene. The error overlay is restricted to the area bounded by your GCP locations.

• Dark grey areas represent GCPs with negligible error magnitudes.
• Bright red areas represent GCPs with higher error magnitudes.
• White areas represent outlier GCPs, where the error magnitude is larger than a given threshold. See Outlier Thresholds for further details on setting this threshold.

The error overlay updates whenever you reposition GCPs in an attempt to minimize errors.

Error Vectors

Click the Show error vector button to view error vectors at each GCP location. This option lets you visualize the error magnitudes for each individual GCP, whereas the Show error overlay option provides a quick visual glance of how GCP errors are distributed across the scene.

Use the various zoom tools in the menu bar to zoom in to each GCP location. The cyan-colored vectors show the magnitude and direction of individual GCP errors. The red circles surrounding each GCP location represent the CE95 horizontal error value, described in Accuracy Assessment Background.

Set Advanced Options

1. The Output Coordinate System field lists the default projection (UTM) for the orthorectified image. To change it, click the Browse button and select a different coordinate system.
2. Enter an Output Pixel Size in the same units as the selected output projection. The default value is derived from the pixel size of the input image.
3. Select an Image Resampling technique from the drop-down list.
   • Nearest Neighbor: Uses the nearest pixel without any interpolation.
   • Bilinear: (default) Performs a linear interpolation using four pixels to resample.
   • Cubic Convolution: Uses 16 pixels to approximate the sinc function using cubic polynomials to resample the image.
4. Enter an Grid Spacing value. This value represents the grid spacing in output pixels, for which ENVI finds the corresponding pixels in the input images through an RPC-based transform. With a coarse grid, the RPC orthorectification is faster but less accurate. The default value is 10. A value of 1 is for a rigorous orthorectification when you have a high-resolution DEM and the study area has lots of terrain relief.
5. Enable the Preview check box to see a preview of the settings before you click OK to process the data. The preview is calculated only on the area in the view and uses the resolution level at which you are viewing the image. See Preview for details on the results. To preview a different area in your image, pan and zoom to the area of interest and re-enable the Preview option.

Export the Orthorectified Image

1. In the Export tab, select the type of Output File to create. ENVI and TIFF are the supported output formats.
2. Enter an output name and path for the orthorectified image.
3. Enable the Export Orthorectification Report option to create a text report that summarizes the details of the orthorectification process. The report lists the source files used, adjustment and independent GCPs, geoid offset in meters, error statistics when using GCPs, and output settings.
4. Click Finish to process the input and add the orthorectified image to the Layer Manager. When the export is complete, the workflow view closes. The original data and the export data display in the Image window view.