An essential part of the space weather problem, important in the last decades, is the forecast of near-Earth space parameters, ionospheric and geomagnetic conditions on the basis of observations of various phenomena on the Sun. Of particular importance are measurements of magnetic fields as they determine the spatial structure of outer layers of the solar atmosphere and, to a large extent, solar wind parameters. Due to lack of opportunities to observe magnetic fields directly in the corona, the almost only source of various models for quantitative calculation of heliospheric parameters are daily magnetograms measured in photospheric lines and synoptic maps derived from these magnetograms. It turns out that results of the forecast, in particular of the solar wind velocity in Earth’s orbit and the position of the heliospheric current sheet, greatly depend not only on the chosen calculation model, but also on the original material because magnetograms from different instruments (and often observations in different lines at the same), although being morphologically similar, may differ significantly in a detailed quantitative analysis. A considerable part of this paper focuses on a detailed analysis of this particular aspect of the problem of space weather forecast.