An apparatus, comprising means comprising at least one processor, and at least one memory including computer program code, the at least one memory and computer program code configured to, with the at least one processor, cause the performance of the apparatus, the means configured for performing: - determining horizontal progression of a mobile object (3, 4) in an underground tunnel from a preceding position estimate or an initial position of the mobile object, - determining a horizontal position of the mobile object on the basis of a floor model of the tunnel and the estimated horizontal progression of the mobile object, the determining the horizontal position of the mobile object comprising: - establishing a set of location estimate particles associated with a position value, the set of location estimate particles being representative of horizontal position estimate options for the mobile object, - updating positions of the particles on the basis of the determined horizontal progression of the mobile object, - fusing the updated positions of the particles with the floor model, wherein the fusing comprises: • comparing the updated positions of the particles to a sub-set of floor point positions of the floor model, • filtering out particles that have no floor points within a threshold distance, • assigning, for each particle, a position of a closest floor point within a threshold distance; and - generating a three-dimensional position indicator on the basis of the horizontal position of the mobile object and on the basis of a three-dimensional model of the tunnel, wherein the three-dimensional position indicator is generated on the basis of a sub-set of three-dimensional floor point locations within a threshold distance from the determined horizontal position. - determining horizontal progression of a mobile object (3, 4) in an underground tunnel from a preceding position estimate or an initial position of the mobile object, - determining a horizontal position of the mobile object on the basis of a floor model of the tunnel and the estimated horizontal progression of the mobile object, the determining the horizontal position of the mobile object comprising: - establishing a set of location estimate particles associated with a position value, the set of location estimate particles being representative of horizontal position estimate options for the mobile object, - updating positions of the particles on the basis of the determined horizontal progression of the mobile object, - fusing the updated positions of the particles with the floor model, wherein the fusing comprises: • comparing the updated positions of the particles to a sub-set of floor point positions of the floor model, • filtering out particles that have no floor points within a threshold distance, • assigning, for each particle, a position of a closest floor point within a threshold distance; and - establishing a set of location estimate particles associated with a position value, the set of location estimate particles being representative of horizontal position estimate options for the mobile object, - updating positions of the particles on the basis of the determined horizontal progression of the mobile object, - fusing the updated positions of the particles with the floor model, wherein the fusing comprises: • comparing the updated positions of the particles to a sub-set of floor point positions of the floor model, • filtering out particles that have no floor points within a threshold distance, • assigning, for each particle, a position of a closest floor point within a threshold distance; and • comparing the updated positions of the particles to a sub-set of floor point positions of the floor model, • filtering out particles that have no floor points within a threshold distance, • assigning, for each particle, a position of a closest floor point within a threshold distance; and - generating a three-dimensional position indicator on the basis of the horizontal position of the mobile object and on the basis of a three-dimensional model of the tunnel, wherein the three-dimensional position indicator is generated on the basis of a sub-set of three-dimensional floor point locations within a threshold distance from the determined horizontal position.