Let $M$ be a compact orientable surface equipped with a volume form $��$, $P$ be either $\mathbb{R}$ or $S^1$, $f:M\to P$ be a $C^{\infty}$ Morse map, and $H$ be the Hamiltonian vector field of $f$ with respect to $��$. Let also $\mathcal{Z}_��(f) \subset C^{\infty}(M,\mathbb{R})$ be set of all functions taking constant values along orbits of $H$, and $\mathcal{S}_{\mathrm{id}}(f,��)$ be the identity path component of the group of diffeomorphisms of $M$ mutually preserving $��$ and $f$. We construct a canonical map $��: \mathcal{Z}_��(f) \to \mathcal{S}_{\mathrm{id}}(f,��)$ being a homeomorphism whenever $f$ has at least one saddle point, and an infinite cyclic covering otherwise. In particular, we obtain that $\mathcal{S}_{\mathrm{id}}(f,��)$ is either contractible or homotopy equivalent to the circle. Similar results hold in fact for a larger class of maps $M\to P$ whose singularities are equivalent to homogeneous polynomials without multiple factors.

13 pages, 3 figures, corrections in the abstract