To assist in targeting of landers and to enable comparison of new elevation data to previous topography models, we have re-determined the position of the 6.1-mbar atmospheric pressure surface on Mars with respect to surface topography from the Mars Orbiter Laser Altimeter (MOLA). The 6.1-mbar surface lies, on average, 1600 m below the geoid-defined zero level of MOLA topography, which occurs at an average pressure of 5.2 mbars at LS=0¿. The elevation of the 6.1-mbar surface is expected to vary by 1.5--2.5 km over the Martian year due to the seasonal exchange of CO2 between the atmosphere and polar caps. For average Martian atmospheric conditions, the pressure at the Mars Surveyor '98 landing site is expected to be ~5.0 mbars during the lander science phase of the mission assuming an elevation of 1600 meters above the zero level defined by MOLA topography. ¿ 1998 American Geophysical Union