Left: schematic representation of the structure of the normal matrix in Eq. (9), with non-zero elements marked in grey. In this example the number of sources is N = 20, and the number of attitude points P = 15. There are five parameters per source; hence the 5 × 5 block structure in the upper-left part. The first source was observed at attitude points 1 and 7 the second source at 7 and 12, etc. Centre: the matrix obtained by collapsing the blocks in the normal matrix to single elements (actually I + K, where K is the adjacency matrix). Right: the non-trivial part E of the adjacency matrix.