Guide device for use in the processing, in particular welding, of curved surfaces, in particular pipe surfaces. A guide device includes a flexible elongate body provided with a guide for processing means and a tensioning means for tensioning the flexible body around the curved surface. The flexible body is provided along its length with indicators arranged according to a determined pattern, this pattern being such that a determined position of the guide device along the flexible body can be inferred on the basis of the detection of the indicators.