Geometry of the basket-weaving problem. Usually two (quasi) orthogonal maps are observed in so-called on-the-fly-mode (OTF) where the telescope constantly drives across the area. Each of these scan lines is visualized by an arrow (red: first coverage in longitudinal direction, blue: second lateral coverage). At the crossing points d_j,i the contribution of the true brightness temperature should be the same for both coverages, while baseline (offsets) may differ. The distance between two recorded samples (dumps) on a scan line is often smaller than the separation of the scan lines. Depending on the projection and coordinate system chosen, the scan lines may not be plane-parallel.