Forward or Inverse This field is a pull-down box which allows the user to select whether a conversion using this conversion grid is performed in a forward or inverse fashion. To calculate an inverse transformation, NADCON performs a forward conversion, reverses the sign, and then performs the transformation in the reverse direction. NADCON then takes the output coordinate from the conversion and runs it back through the forward conversion to see if the original point is output. If not, NADCON uses the new coordinate, performs the same calculations with it, and repeats this process (up to ten iterations) until the correct point has been established. NADCON determines this when it can perform a transformation in both directions and get the same answer (within .000000001 geographic seconds). |