A measuring system comprising a fixed measurement unit (101), a data processing unit (130) and a mobile unit (120). The mobile unit comprises a planar base (111), a reflector (118), an elevation element (119) fixed to the base and the reflector, and attaching the reflector to a fixed position in respect of the base. The mobile unit comprises also mobility means (112) for moving the base along a surface (113) such that the spatial orientation of the base (114) substantially corresponds with the spatial orientation of the currently underlying part of the surface. In addition the mobile unit comprises tilt measuring means (123) for determining a deviation between the spatial orientation of the base and a plane perpendicular to the ambient gravitational force, and tilt elimination means for eliminating the effect of the determined deviation. Measurement results are thus more accurate.