Describing the accessibility, by public transport, of a given place is of particular importance today. The speed and multiplicity of urban public transport have made many places far more accessible than they were at the beginning of the 20th century. Descriptions of the accessibility of a given place, expressed in terms of travel time, have been carried out since the 19th century, when the first map for journeys to London by public transport was published. Since then, many "accessibility maps" have been produced and the theoretical principles for their creation drawn up. However, the influence that particular methods for interpolating travel times have on the accuracy and appearance of such maps has not been analysed. The purpose of this article is an analysis of interpolation methods, using a tool such as GIS, for estimating the accessibility of any location. The research was conducted by the Military University of Technology in Warsaw (MUT), drawing up an isochrone map that showed travel times by public transport to the MUT campus. According to the results, the lowest mean absolute percentage error (MAPE) and mean absolute error (MAE) were shown applying the IDW method, in which small isochronic islands did not appear. The highest estimation errors occurred with the nearest neighbour method, and the least beneficial arrangement of isochronic islands appeared using the spline method.