Spatial power spectra of the flow divergence at disk center (averaged power for T = 120 days). Black: uncompressed data; blue: quantization (n = 2); green: JPEG (quality = 10); red: large time lag between consecutive velocity maps (ΔT = 45 min). The thickness of the lines shows the 1σ scatter of the mean power. All curves exhibit a peak at kR_⊙ ≈ 120 resulting from supergranulation. At high wavenumbers, the additional noise introduced by the compression is visible. A large time lag between consecutive velocity maps does not remove all the noise from granulation. Quantization and JPEG lead to noise on a pixel-by-pixel scale, since JPEG is worse than quantization. At supergranulation scales, the influence of quantization and JPEG compression is negligible.