New data from the POLAR and HEO (high earth orbit) 95-034 satellites are used in a preliminary study to estimate the position of the open/closed (OC) field line boundary using energetic particle and plasma observations. In this study, the OC boundary positions were compared in several different coordinate systems and the GSM and eccentric dipole coordinates were found to be equally effective in organizing the HEO 95-034 data. No coordinate system appeared to organize the POLAR data best. The HEO 95-034 and POLAR data were examined for interplanetary field and plasma dynamic pressure control of the dayside OC boundary. The HEO 95-034 data showed a clear response to solar wind dynamic pressure with the OC boundary being at lower latitude with increasing pressure. The HEO 95-034 data also showed some response to the IMF BZ, hinting that the OC boundary would move to lower latitude with increasingly negative BZ but be unchanged for positive BZ. The POLAR data showed no dependence on either the solar wind dynamic pressure or BZ in this initial study. The data were relatively sparse; about a month of observations for each satellite.