1. A method, comprising: forming a curtain comprising a coating material; forming a coated web by coating a moving primary web with the coating material, wherein the curtain and the primary web meet at a contact line; projecting a primary pattern on the curtain, wherein the primary pattern comprises a first pointer feature; capturing a first image comprising a first sub-image of the first pointer feature; detecting the position of the first sub-image in the first image; providing distance information by comparing the detected position of the first sub-image with a reference position; detecting a temporary presence of an object in front of the curtain; and providing surveillance data indicative of the temporary presence of the object during the coating such that the surveillance data is associated with at least one portion of the coated web.