Example of the filamentary structures identified using the Hessian method with curvature thresholds λ₋ < 0 (top) and λ₋ < −1.8 Kdeg⁻² (bottom). The three maps represent: Left. H I intensity after the smoothing by a Gaussian kernel with the size of the derivative kernel. Center Second eigenvalue of the Hessian matrix, λ₋, calculated using Eq. 2. Left. Filament orientations, θ, calculated using Eq. 3. The radial velocity corresponds to a kinematic distance d = 12.7 kpc and Galactocentric radius R_gal = 16.1 kpc, as derived using the assumptions in App. B