An angular position encoder for a rotating shaft is disclosed, in particular for application on the rotor shaft in wind turbines. A materialized measure in the form of a roller chain is arranged around the circumference of the shaft. A scanning head in the form of a sensor is arranged at a distance from the materialized measure and scans the surface of the materialized measure as the shaft is rotating. A measurement value evaluation unit processes recorded measurement values to give positional and/or speed data on the shaft. A method for determination and measurement of the rotor speed or position for a rotating shaft by means of the angular position encoder is also disclosed which includes determination and measurement of the rotor speed or position, creating or mapping at least one reference pattern or map of the chain, and a comparing the pattern and an actual measurement value.