Examples of travel time maps of supergranulation (f-mode, center-to-annulus geometry, annulus size 13.4 Mm, outward minus inward travel times) computed from compressed and uncompressed Dopplergrams. Top left: uncompressed data, top right: quantization (two velocity bins), bottom left: JPEG compression (quality = 5), bottom right: 2 × 2 subsampling combined with JPEG compression with a quality of 10. The travel times clearly show the supergranulation pattern. They correspond to the divergence of horizontal flows, with negative values indicating outflows and positive values showing inflows.