Gradients¶
The gradient calculation calculates different derivatives of the data. It allows for the calculation of a conventional horizontal derivative, a derivative ratio (ratio of x and y derivatives), a vertical derivative and a total horizontal gradient. The algorithms are based on Cooper and Cowan (2006, 2007). The input dataset must be in a projection, not in geographic/geodetic coordinates.
Based on the selected filter, different options become available:
Directional derivative: Calculate a horizontal derivative in the direction of the specified Azimuth, with 0° being E-W, 45° being NW-SE, 90° being N-S and -45° being NE-SW. Features perpendicular to the azimuth are highlighted.
Derivative Ratio: Calculate the ratio of x and y derivatives using the following equation (Cooper and Cowan, 2007)
where n is the Strength Factor that controls the strength of the filter. A value of 1 is recommended as a starting point. The Azimuth works the same way as for the Directional Derivative.
Vertical Derivative: Calculates the first vertical derivative of a data set.
Total Horizontal Gradient: Combines the horizontal derivatives to enhance features of any orientation (Cooper and Cowan, 2006).

References¶
Cooper,G.R.J., Cowan, D.R. 2007. Enhancing linear features in image data using horizontal orthogonal gradient ratios. Computers and Geosciences, 33, p.981-984