"Fast Mode" orthorectification with RPCs
The RPC (Rational Polynomial Coefficients or Rapid Positioning Coordinates) sensor model can be used to orthorectify data from IKONOS, QuickBird, OrbView-3, SPOT, and other sensors. The RPC orthorectification process combines several sets of input data to place each pixel in the correct ground location. The RPC Orthorectification tools can be found under Map->Orthorectification on the main ENVI menu. Because the process calculates each pixel location using elevation and geoid information, it can take a significant amount of time to process.
An alternative method to the standard RPC orthorectification is the "Fast Mode' orthorectification with RPCs. This method works by sacrificing accuracy for speed since the orthorectification processing time will be much faster, but the result will be slightly less accurate. However, accuracy loss should not be substantial, particularly when compared to the speed boost. Instead of solving the RPC equation for each pixel, the 'Fast Mode' RPC orthorectification will solve for a grid of points spaced throughout the image and triangulate a warp between this set of points. This is the same documented mechanism ENVI uses to perform reprojection.
Steps to perform the "Fast Mode" orthorectification with RPCs:
- Open an image. (QuickBird is a good example)
- Go to the ENVI Header Info dialog via File ->Edit ENVI Header or by selecting the file name in the Available Bands List and right-clicking to select Edit Header...
- From the Header Info dialog, select Edit Attributes->RPC Projection Emulation. It should automatically find the RPC data, if it is present, and pop up a dialog that allows you to turn on RPC projection emulation.
- Associate the desired DEM with the file from Edit Attributes->Associate DEM File.
- In the Header Info dialog, click OK.
- From the Main ENVI Menu, Select Map->Convert Map Projection, then select your input file. Select the output projection and click OK. Note: You may have to wait some time at this step for the next dialog to appear. This is the point where ENVI calculates the orthorectified points.
- In the Registration Parameters dialog, select your output parameters, if desired, and then Click OK.
Your output is the "fast mode" orthorectified product.
Review on 12/31/2013 MM