A method comprising, performed by at least one apparatus (100, 300, 310, 400, 411, 412): Obtaining at least one fingerprint sample for at least one communication node (340, 440), wherein each fingerprint sample of the at least one fingerprint sample comprises results of a measurement performed by a mobile device (300, 411, 412) at a particular position on a signal of a communication node (340, 440) of the at least one communication node (340, 440) including at least a signal strength related value; and Determining, for at least one fingerprint sample of the at least one fingerprint sample, whether the respective fingerprint sample fulfils an inconsistency criterion, wherein the inconsistency criterion is based on the signal strength related value of the respective fingerprint sample and a distance value indicative of an estimated distance between the mobile device (300, 411, 412) and the communication node (340, 440) associated with the fingerprint sample; wherein the inconsistency criterion is further based on a value related to the signal strength of the transmitted signal of the communication node (340, 440) associated with the respective fingerprint sample at the communication node (340, 440); the method comprising determining the value related to the signal strength of the transmitted signal of the communication node (340, 440) associated with the respective fingerprint sample at the communication node (340, 440) based on the maximum value of a set of one or more signal strength related values associated with this communication node (340, 440); characterized in that said determining the value related to the signal strength of the transmitted signal of the communication node (340, 440) associated with the respective fingerprint sample at the communication node (340, 440) is further based on an estimated value related to the altitude of the communication node (340, 440). Obtaining at least one fingerprint sample for at least one communication node (340, 440), wherein each fingerprint sample of the at least one fingerprint sample comprises results of a measurement performed by a mobile device (300, 411, 412) at a particular position on a signal of a communication node (340, 440) of the at least one communication node (340, 440) including at least a signal strength related value; and Determining, for at least one fingerprint sample of the at least one fingerprint sample, whether the respective fingerprint sample fulfils an inconsistency criterion, wherein the inconsistency criterion is based on the signal strength related value of the respective fingerprint sample and a distance value indicative of an estimated distance between the mobile device (300, 411, 412) and the communication node (340, 440) associated with the fingerprint sample; wherein the inconsistency criterion is further based on a value related to the signal strength of the transmitted signal of the communication node (340, 440) associated with the respective fingerprint sample at the communication node (340, 440); the method comprising determining the value related to the signal strength of the transmitted signal of the communication node (340, 440) associated with the respective fingerprint sample at the communication node (340, 440) based on the maximum value of a set of one or more signal strength related values associated with this communication node (340, 440); characterized in that said determining the value related to the signal strength of the transmitted signal of the communication node (340, 440) associated with the respective fingerprint sample at the communication node (340, 440) is further based on an estimated value related to the altitude of the communication node (340, 440).