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We study a fragment of a large quiescent filament observed on May 29, 2017 bythe Interferometric BIdimensional Spectropolarimeter (IBIS) mounted at the DunnSolar Telescope. We focus on its quiescent stage prior to its eruption. Weanalyse the spectral observations obtained in the H$\alpha$ line to derive thethermodynamic properties of the plasma of the observed fragment of thefilament. We used a 2D filament model employing radiative transfer computationsunder conditions that depart from the local thermodynamic equilibrium. Weemployed a forward modelling technique in which we used the 2D model toproducesynthetic H_alpha line profiles that we compared with the observations.We then found the set of model input parameters, which produces syntheticspectra with the best agreement with observations. Our analysis shows that onepart of the observed fragment of the filament is cooler, denser, and moredynamic than its other part that is hotter, less dense, and more quiescent. Thederived temperatures in the first part range from 6,000 K to 10,000$ K and inthe latter part from 11,000 K to 14,000 K. The gas pressure is 0.2-0.4dyn/cm}^{2} in the first part and around 0.15 dyn/cm}^{2} in the latter part.The more dynamic nature of the first part is characterised by the line-of-sightvelocities with absolute values of 6-7 km/s and microturbulent velocities of8-9 km/s. On the other hand, the latter part exhibits line-of-sight velocitieswith absolute values 0-2.5 km/s and microturbulent velocities of 4-6 km/s.

2D non-LTE modelling of a filament observed in the H_alpha line with the DST/IBIS spectropolarimeter