The invention relates to a device and to a method for adaptive three-dimensional derivation of a proximal isokinetic shell of a proximal flow convergence zone that forms in an observation area in a moving fluid, wherein the magnitude of the velocity of the fluid at each point of the proximal isokinetic shell is identical and equal to a velocity reference value, the method including: a) preparing locally distributed velocity measurements in a surrounding area of the observation area, the measurements representing at least one directional component of the local velocity of the fluid in a respective measurement direction, b) preparing an approximation surface as an initial proximal isokinetic shell) in such a way that the entire flow, at least substantially, in the flow convergence zone penetrates the approximation surface, c) establishing a plurality of approximation points on the approximation surface, d) determining the respective velocity measurements at the respective approximation points, e) calculating a corrected velocity (vkkJ) at each approximation point as a function of the velocity measurement indicated there, the respective measurement direction and the respective surface normal, f) determining a correction direction by comparing the corrected (vkkJ) velocity with the velocity reference value, g) optionally shifting the approximation points in the respective correction direction to new positions, h) acquiring a new approximation surface such that the same at least approaches the new positions of the approximation points, and i) repeating steps c) through h) until the change of the new approximation surface is sufficiently minimal compared to the old approximation surface.
