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The determination of the magnetic filed vector in solar filaments is possibleby interpreting the Hanle and Zeeman effects in suitable chromospheric spectrallines like those of the He I multiplet at 10830 A. We study the vector magneticfield of an active region filament (NOAA 12087). Spectropolarimetric data ofthis active region was acquired with the GRIS instrument at the GREGORtelescope and studied simultaneously in the chromosphere with the He I 10830 Amultiplet and in the photosphere with the Si I 10827 A line. As it is usualfrom previous studies, only a single component model is used to infer themagnetic properties of the filament. The results are put into a solar contextwith the help of the Solar Dynamic Observatory images. Some results clearlypoint out that a more complex inversion had to be done. Firstly, the Stokes $V$map of He I does not show any clear signature of the presence of the filament.Secondly, the local azimuth map follows the same pattern than Stokes $V$ as ifthe polarity of Stokes $V$ were conditioning the inference to very differentmagnetic field even with similar linear polarization signals. This indicationsuggests that the Stokes $V$ could be dominated by the below magnetic fieldcoming from the active region, and not, from the filament itself. Those andmore evidences will be analyzed in depth and a more complex inversion will beattempted in the second part of this series.

Spectropolarimetric analysis of an active region filament. I. Magnetic and dynamical properties from single component inversions