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Generate GCPs from Reference Image

Generate GCPs from Reference Image

Use this tool to automatically generate ground control points (GCPs) for an input raster by matching and using the geographic coordinates of a reference image. The elevation values of the GCPs are calculated from a DEM raster. The input raster must have an RPC spatial reference.

You can use the resulting GCPs in ENVI applications such as RPC Orthorectification, Image-to-Map Registration, DEM Extraction, and Rigorous Orthorectification. If you want a fully automated RPC Orthorectification solution, use the RPC Orthorectification Using Reference Image tool.

You can also write a script to generate GCPs from a reference image using the ENVIGenerateGCPsFromReferenceImageTask routine.

Follow these steps to generate GCPs:

  1. From the Toolbox, select Geometric Correction > Generate GCPs from Reference Image.
  2. Select any supported Input Raster that contains an RPC spatial reference.
  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. It must cover at least the spatial extent of the input raster. 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 Harris\ENVIxx\Data folder. The GMTED2010 dataset has a mean resolution of 30 arc seconds. A 7.5-arc second version is also available from our website. 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. Enter an Output GCPs filename (.pts) and directory.
  9. Click OK. ENVI writes the GCP file (.pts)to the specified directory.

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