An orthorectified image (or orthophoto) is one where each pixel represents a true ground location and all geometric, terrain, and sensor distortions have been removed to within a specified accuracy. Orthorectification transforms the central perspective of an aerial photograph or satellite-derived image to an orthogonal view of the ground, which removes the effects of sensor tilt and terrain relief. Scale is constant throughout the orthophoto, regardless of elevation, thus providing accurate measurements of distance and direction. Geospatial professionals can easily combine orthophotos with other spatial data in a geographic information system (GIS) for city planning, resource management, and other related fields.

The SPEAR Orthorectification Wizard allows you to orthorectify images using rational polynomial coefficients (RPCs), elevation and geoid information, and optional ground control points (GCPs). RPCs and elevation information do not provide enough details to build a rigorous model representing the path of light rays from a ground object to the sensor. Use Rigorous Orthorectification for a more rigorous solution. That tool requires an additional license for the ENVI Photogrammetry Module; contact your ENVI sales representative to obtain a license.

  1. From the Toolbox, select SPEAR > SPEAR Orthorectification. The SPEAR Orthorectification Wizard displays the File Selection panel.
  2. Click Select Image to Ortho, choose a file, then click OK. The input image must contain RPCs and can be in any format readable by ENVI.
  3. To optionally process only a portion of the scene, click Select Subset. A small Select Spatial Subset dialog appears.
  4. Click Spatial Subset. The standard Select Spatial Subset dialog appears. When finished, click OK to return to the File Selection panel.
  5. Select an elevation source. In flat areas or when only general correction is needed, select Z Value. Use Auto Retrieve to retrieve the mean elevation for the area covered by the input image. Click Select Elevation File to select a file with elevation data. The elevation file must contain map information and can be in any format readable by ENVI.
  6. By default, output files are saved to the same directory and use the same rootname as the input file, minus any extension. Output files are appended with a unique suffix. To change the directory and/or root filename, click Select Output Root Name.
  7. Click Next. The Select Method dialog appears.
  8. Select Normal or Fast mode. Normal mode is more accurate and calculates an orthorectified coordinate for every pixel in the image. Fast mode calculates an orthorectified coordinate for pixels laying on a grid with user-defined spacing, then triangulates the position of all of the points within each grid cell. In relatively flat areas, Fast mode will produce accurate results with faster processing.
  9. Optionally, collect GCPs to refine the georeferencing accuracy of the orthorectified image.
  10. Click Next. The Orthorectification Parameters dialog appears.
  11. Click Show Ortho Params to optionally modify the following default parameters:
    • X Pixel Size and Y Pixel Size values default to the resolution of the input file.
    • The Image Resampling Method determines the pixel values in the input image when it is converted from its current orientation into the new orientation. The choices are Nearest Neighbor, Bilinear, and Cubic Convolution. The default is Bilinear.
    • The Image Background allows you to set a value for pixels in the output image that are outside the boundary of the source image.
    • DEM Resampling defines the method to be used to resample the DEM to the resolution of the input or base image.
    • DEM to Geoid Offset defines the geoid offset value. The geoid closely approximates mean sea level, and the geoid offset accounts for the difference between mean sea level and the WGS-84 ellipsoid. DEMs are typically referenced to mean sea level, while SPEAR Orthorectification requires height above the ellipsoid. For DEMs referenced to mean sea level, accurate orthorectification requires a single geoid offset for the geographic area covered by the image. For example, at 40°N, 105°W, the geoid height is 17 meters below the WGS-84 ellipsoid. For this region, enter a value of -17. See the UNAVCO web site for a geoid height calculator.

    • Grid Spacing is available for Fast mode. Fast mode calculates an orthorectified coordinate for pixels laying on a grid with this spacing value, then triangulates the position of all of the points within each grid cell.
    • Click Next. The Examine Results dialog appears.

      The orthorectified image is displayed. If GCPs were selected, the base image will be displayed and geographically linked to the orthorectified image so that you can examine the georeferencing accuracy.

  12. When you are finished examining your results, click Finish to exit the Wizard.