Filters used in the example of convolution described in Sect. 3.2.1. Black pixels are set to 1, white pixels to −1, gray pixels to zero. The filter on the left is designed to pick vertical features, i.e., stripes that are aligned with meridians. The filter on the right picks features running along parallels. These filters are here shown with a diamond shape, but practically they are applied as 1D vectors to maps, following the algorithm sketched in Fig. 3. The unrolling into vectors is done clockwise, starting from the NW direction.