Truncation error maps for test case A1 at selected points in the simple iteration scheme (SI, left column) and conjugate gradient scheme (CG, right column). See Fig. 2 for further explanation of the diagram layout. Each map shows the difference between the result in the current iteration and the (presumably converged) solution obtained after 250 (SI) or 100 (CG) iterations. Although the parallaxes have converged more or less to their final values after iteration 76 (SI) or 20 (CG), as shown in Fig. 4, there are then still spatially correlated truncation errors at the  ± 0.6   μas level in both solutions.