A method comprising, performed by at least one apparatus : Obtaining at least one fingerprint sample (901-907) for at least one communication node , wherein each fingerprint sample (901-907) of the at least one fingerprint sample (901-907) comprises results of a measurement performed by a mobile device at a particular position on a signal of a communication node of the at least one communication node including at least a signal strength related value; and Determining , for at least one fingerprint sample (901-907) of the at least one fingerprint sample (901-907), whether the respective fingerprint sample fulfils (901-907) an inconsistency criterion, wherein the inconsistency criterion is based on the signal strength related value of the respective fingerprint sample (901-907) and a distance value indicative of an estimated distance between the mobile device and the communication node associated with the fingerprint sample (901-907); wherein the inconsistency criterion is further based on a value related to the signal strength of the transmitted signal of the communication node associated with the respective fingerprint sample (901-907) at the communication node ; the method comprising determining the value related to the signal strength of the transmitted signal of the communication node associated with the respective fingerprint sample (901-907) at the communication node based on the maximum value of a set of one or more signal strength related values associated with this communication node ; characterized in that said determining the value related to the signal strength of the transmitted signal of the communication node associated with the respective fingerprint sample (901-907) at the communication node is further based on an estimated value related to the altitude of the communication node . Obtaining at least one fingerprint sample (901-907) for at least one communication node , wherein each fingerprint sample (901-907) of the at least one fingerprint sample (901-907) comprises results of a measurement performed by a mobile device at a particular position on a signal of a communication node of the at least one communication node including at least a signal strength related value; and Determining , for at least one fingerprint sample (901-907) of the at least one fingerprint sample (901-907), whether the respective fingerprint sample fulfils (901-907) an inconsistency criterion, wherein the inconsistency criterion is based on the signal strength related value of the respective fingerprint sample (901-907) and a distance value indicative of an estimated distance between the mobile device and the communication node associated with the fingerprint sample (901-907); wherein the inconsistency criterion is further based on a value related to the signal strength of the transmitted signal of the communication node associated with the respective fingerprint sample (901-907) at the communication node ; the method comprising determining the value related to the signal strength of the transmitted signal of the communication node associated with the respective fingerprint sample (901-907) at the communication node based on the maximum value of a set of one or more signal strength related values associated with this communication node ; characterized in that said determining the value related to the signal strength of the transmitted signal of the communication node associated with the respective fingerprint sample (901-907) at the communication node is further based on an estimated value related to the altitude of the communication node . <EOS>