An adjustable base structure supports a person in an upright attitude on uneven terrain and comprises a plate and multiple legs independently pivotally connected to the plate. When in an operative mode, the legs diverge from each other and from the plate and rest on the terrain to orient the plate horizontally. A cable connects the legs and maintains them in the operative mode. A bushing on the plate receives a post of a carrier that is attached to a seat. When the carrier post is in the bushing, the person can sit upright regardless of the contour of the terrain. A dome-shaped pad is joined to each leg for swiveling in a manner that accommodates the terrain and that supports the adjustable base structure in soft soil. Different lengths of the cable are storable inside one of the legs to adjustably limit the amount of pivoting of the legs.