DFDY filter

The DFDY filter is a standard linear filter which works in a 5 by 1 environment. It calculates the first derivative in y-direction (df/dy) per pixel. The values in the matrix are:

Gain factor = 1/12 = 0.0833333

When using the DFDY filter on a DEM:

• positive values in the output map mean that the terrain goes up from bottom to top (South to North);
• negative values mean that terrain goes down.

Tip:

To find the real first derivative (per meter), the output values have to be divided by the pixel size of the map. You can use the MapCalc expression:

See also:

Filter types : linear filters

Filter types: gradient or derivative filters (technical information)

Filters : user-defined linear filters