  We compare magnetic field measurements taken by the FIELDS instrument on
Parker Solar Probe (PSP) during its first solar encounter to predictions
obtained by Potential Field Source Surface (PFSS) modeling. Ballistic
propagation is used to connect the spacecraft to the source surface. Despite
the simplicity of the model, our results show striking agreement with PSPs
first observations of the heliospheric magnetic field from 0.5 AU (107.5 Rs)
down to 0.16 AU (35.7 Rs). Further, we show the robustness of the agreement is
improved both by allowing the photospheric input to the model to vary in time,
and by advecting the field from PSP down to the PFSS model domain using in situ
PSP/SWEAP measurements of the solar wind speed instead of assuming it to be
constant with longitude and latitude. We also explore the source surface height
parameter (RSS) to the PFSS model finding that an extraordinarily low source
surface height (1.3-1.5Rs) predicts observed small scale polarity inversions
which are otherwise washed out with regular modeling parameters. Finally, we
extract field line traces from these models. By overlaying these on EUV images
we observe magnetic connectivity to various equatorial and mid-latitude coronal
holes indicating plausible magnetic footpoints and offering context for future
discussions of sources of the solar wind measured by PSP.
