  We present observations and magnetic field models of an intermediate filament
present on the Sun in August 2012, associated with a polarity inversion line
that extends from AR 11541 in the east into the quiet sun at its western end. A
combination of SDO/AIA, SDO/HMI, and GONG H alpha data allow us to analyse the
structure and evolution of the filament from 2012 August 4 23:00 UT to 2012
August 6 08:00 UT when the filament was in equilibrium. By applying the flux
rope insertion method, nonlinear force-free field models of the filament are
constructed using SDO/HMI line-of-sight magnetograms as the boundary condition
at the two times given above. Guided by observed filament barbs, both modelled
flux ropes are split into three sections each with a different value of axial
flux to represent the non-uniform photospheric field distribution. The flux in
the eastern section of the rope increases by 4$\times$10$^{20}$ Mx between the
two models, which is in good agreement with the amount of flux cancelled along
the internal PIL of AR 11541, calculated to be 3.2$\times$10$^{20}$ Mx. This
suggests that flux cancellation builds flux into the filament's magnetic
structure. Additionally, the number of field line dips increases between the
two models in the locations where flux cancellation, the formation of new
filament threads and growth of the filament is observed. This suggests that
flux cancellation associated with magnetic reconnection forms concave-up
magnetic field that lifts plasma into the filament. During this time, the free
magnetic energy in the models increases by 0.2$\times$10$^{31}$ ergs.
