RPC Orthorectification Tutorial

This tool performs a refined orthorectification by automatically generating ground control points (GCPs) from a reference image. This is a fully automated end-to-end solution if you have a reference image. Use the RPC Orthorectification workflow instead if you want to edit GCPs and review error statistics in an interactive environment.

You can also write a script to perform RPC Orthorectification using the RPCOrthorectificationUsingReferenceImage task.

Follow these steps:

  1. From the Toolbox, select Geometric Correction > Orthorectification > RPC Orthorectification Using Reference Image.
  2. Select any supported Input Raster that contains a community sensor model (CSM) or rational polynomial coefficient (RPC) sensor model.
  3. Select an Input Reference Raster. This is typically an orthorectified reference image with the same or slightly higher spatial resolution than the input raster. Here are some examples:
    • For the United States: U.S. Department of Agriculture National Agriculture Imagery Program (NAIP), Digital Orthophoto Quadrangles (DOQs), or High-resolution Orthoimagery. Download data from the U.S. Geological Survey National Map and Download Platform or EarthExplorer, then use the Seamless Mosaic tool to create a mosaic of individual tiles.
    • Controlled image base (CIB)

    Try to choose a reference image that is close to the year and season of the input image. Automatic GCP generation is based on image matching between the reference and source images, so the scene contents should not be vastly different. Automatic GCP generation is more robust if the reference image and input image have similar resolution. If the reference image has a much higher resolution than the input image (i.e., the ratio is greater than 2.5), consider down-sampling the reference image first.

  4. Select an Input DEM Raster. A DEM is only requred if the input raster has an RPC sensor model. 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 under the ENVI installation folder. 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.
  5. Set the DEM Is Height Above Ellipsoid radio button to Yes if the DEM is already expressed as the height above the ellipsoid and no geoid correction is required. Most DEM data contain orthometric heights, which are elevations above mean sea level (for example, GMTED2010 or National Elevation Dataset, NED). In these cases, keep the default selection of No.
  6. Enter the Requested Number of GCPs. The default value is 25. After ENVI generates and filters tie points (used to create GCPs) for radiometric and geometric criteria, the actual number of generated GCPs may be less than requested.
  7. Enter a Search Window Size. Increase the value if there is a big misalignment between the input raster and the reference raster.
  8. 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.
    • Click the From Dataset button to use the coordinate system of en existing raster dataset.
    • Click the Current View button to use the coordinate system established in the current view.
    • Click the Reset button to clear the Output Coordinate System field.
  9. Enter the Output Pixel Size in the X and Y direction in meters. The default value is derived from the pixel size of the input image.
  10. 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.
  11. 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.
  12. Enter an Output Raster filename and location for the orthorectified result.
  13. Enter an Output GCPs filename (.pts) and location for the GCPs that were generated.
  14. Enable the Display result check box to display the output in the view when processing is complete. Otherwise, if the check box is disabled, the result can be loaded from the Data Manager.

  15. To reuse these task settings in future ENVI sessions, save them to a file. Click the down arrow and select Save Parameter Values, then specify the location and filename to save to. Note that some parameter types, such as rasters, vectors, and ROIs, will not be saved with the file. To apply the saved task settings, click the down arrow and select Restore Parameter Values, then select the file where you previously stored your settings.

  16. To run the process in the background, click the down arrow and select Run Task in the Background. If an ENVI Server has been set up on the network, the Run Task on remote ENVI Server name is also available. The ENVI Server Job Console will show the progress of the job and will provide a link to display the result when processing is complete. See ENVI Servers for more information.

  17. To see a model-based version of this tool that shows how the tool is constructed from individual tasks, click Open in Modeler.

  18. Click OK.