To demonstrate this effect, we have set up a ring shaped test source, added some noise and sampled it on a rectangular grid with some positional jitter (like it occurs in OTF due to variations in the startup time to accelerate the telescope to the scanning speed). We then fed the data into the triangulation command used in the Grenoble Software (GREG2 RANDOM_MAP) and into the Orthogonal Convolution scheme used by the latest Unix version (10/1998) of Robert Zylka's MOPSI (MAP RANDOM).
Figure 2 shows the results of this test. The triangulation (panel b) introduces strong striping artefacts because of the positional jitter and the noise while the Orthogonal Convolution map shows a result close to the expected structure. We strongly recommend to use such an algorithm to grid the OTF data. CLASS has a built-in gridding command called GRID. We have also have MOPSI and there are the gridding tasks GRID_EXTEND and GRID_SG in GRAPHIC. MOPSI is much faster than the other methods, but we recently discovered a problem with shift in the the velocity axis when writing the data from MOPSI to Gildas Data Format (GDF). This bug has NOT yet been fixed ! We therefore recommend using CLASS' GRID to reduce spectral line OTF data.