Digital (Line Central)

1. From the Type drop-down list in the Build RPCs dialog, select Digital (Line Central).
2. Enter a Focal Length (mm) value for the camera or sensor. This field is required.
3. Enter Principal Point x0 (mm) and Principal Point y0 (mm) coordinates, which are usually available from the camera calibration report. The default value is 0 for both fields.
4. Enter X Pixel Size (mm) and Y Pixel Size (mm) values. These are required fields.
5. Select a Sensor Line Along Axis option. Each sensor line has one projective center.
• X: The sensor line direction is along the image x-axis.
• Y: The sensor line direction is along the image y-axis.
• Set the required Polynomial Orders for XS, YS, ZS, Omega, Phi, and Kappa.
• 0: The parameter is constant for the entire image.
• 1: The parameter has a linear relationship with the y camera coordinates, for example: XS(i) = a₀ + a₁y_i

• 2: The parameter is modeled using a second-order polynomial, for example: XS(i) = a₀ + a₁y_i + a₂y_i²

  The default value is 1 for all six exterior orientation parameters. The higher you set the polynomial order, the more GCPs you must select in the image. Usually, a second-order polynomial is only needed to model a scenario where there is a nonlinear variation of the exterior orientation between sensor lines, which implies an unstable flight path. Experiment with different polynomial orders to select the optimal modeling strategy.

6. Click Select GCPs in Display. A Select GCPs in Display dialog appears. See Build Exterior Orientation for the remaining steps.
7. If you want to further improve the RMS error of the exterior orientation model, click Select GCPs in Display again. You can add more GCPs or delete GCPs with large errors.
8. When you are finished modifying the GCPs, click Recalculate Exterior Orientation in the Build RPCs dialog.
9. Click OK. The Scene Elevation in Meters dialog appears.
10. The Minimum Elevation and Maximum Elevation fields are initially populated with the range of global elevation values in world_dem (found in the data directory of your ENVI installation path). If you know the elevation range of your scene, you can enter new Minimum Elevation and Maximum Elevation values. These values represent the height above the WGS-84 ellipsoid for the geographic region that the image covers.

11. Click OK. After processing is complete, an ENVI Message dialog appears: “RPCs have been calculated for this file, and the header has been updated.” Click OK.

    Once RPCs are computed, the RPC information is added to the input file header so that you can use the file with DEM Extraction.