A system, comprising: a display; and a processor, configured to: obtain multiple local activation times (LATs) at different respective measurement locations on an anatomical surface of a heart, compute respective directions of electrical propagation at one or more sampling locations on the anatomical surface, by, for each sampling location of the sampling locations: selecting a respective subset of the measurement locations for the sampling location, constructing a set of vectors, each of at least some of the vectors including, for a different respective measurement location in the subset, three position values derived from respective position coordinates of the measurement location and an LAT value derived from the LAT at the measurement location, and computing the direction of electrical propagation at the sampling location based on a Principal Component Analysis of a 4x4 covariance matrix for the set of vectors, and indicate the directions of electrical propagation on the display. a display; and a processor, configured to: obtain multiple local activation times (LATs) at different respective measurement locations on an anatomical surface of a heart, compute respective directions of electrical propagation at one or more sampling locations on the anatomical surface, by, for each sampling location of the sampling locations: selecting a respective subset of the measurement locations for the sampling location, constructing a set of vectors, each of at least some of the vectors including, for a different respective measurement location in the subset, three position values derived from respective position coordinates of the measurement location and an LAT value derived from the LAT at the measurement location, and computing the direction of electrical propagation at the sampling location based on a Principal Component Analysis of a 4x4 covariance matrix for the set of vectors, and indicate the directions of electrical propagation on the display. obtain multiple local activation times (LATs) at different respective measurement locations on an anatomical surface of a heart, compute respective directions of electrical propagation at one or more sampling locations on the anatomical surface, by, for each sampling location of the sampling locations: selecting a respective subset of the measurement locations for the sampling location, constructing a set of vectors, each of at least some of the vectors including, for a different respective measurement location in the subset, three position values derived from respective position coordinates of the measurement location and an LAT value derived from the LAT at the measurement location, and computing the direction of electrical propagation at the sampling location based on a Principal Component Analysis of a 4x4 covariance matrix for the set of vectors, and selecting a respective subset of the measurement locations for the sampling location, constructing a set of vectors, each of at least some of the vectors including, for a different respective measurement location in the subset, three position values derived from respective position coordinates of the measurement location and an LAT value derived from the LAT at the measurement location, and computing the direction of electrical propagation at the sampling location based on a Principal Component Analysis of a 4x4 covariance matrix for the set of vectors, and indicate the directions of electrical propagation on the display.