Research has been undertaken at the University of Leicester for a number of years on large bearing errors (of up to around ±100°) which are frequently observed by HF direction finders when the signals are reflected from the high latitude ionosphere. In addition to gaining an understanding of the physical processes leading to these large errors, a major aim of the investigation is the development of a quality factor which may be assigned to individual bearing measurements. Such a quality factor should provide an indication, based on ionospheric propagation phenomena, of the reliability of a measured bearing as a indication of the great circle direction to the transmitter. Measurements of the bearings and signal characteristics of HF transmissions from Halifax, Nova Scotia and Iqaluit, North West Territories, have been made at a number of sites over a period of several years. This paper summarises, with supporting examples, the ionospheric propagation phenomena which lead to large deviations from the great circle for high latitude paths and relates these effects to various geophysical parameters. From this information, suggestions are given for the development of an appropriate HF-DF quality factor.