1. A method of navigating an autonomous surface treatment apparatus over a predetermined field of operation, comprising the steps of: dividing said predetermined field of operation into cells, each of which being adapted to be indicated as treated, untreated or occupied by an obstacle; determining, for a current cell in which the autonomous apparatus is located, a navigation route to an obstacle-free and untreated cell that requires the smallest amount of energy for moving the autonomous surface treatment apparatus thereto according to a predetermined energy cost function; and navigating the autonomous surface treatment. apparatus from the current cell to the obstacle-free and untreated cell according to the determined navigation route and updating the indication of that cell as treated, characterized by the steps of: defining a search algorithm based on the question whether there is an untreated cell with cost N, where N starts at 1 and counts upwards to create a number of cost levels based on specific movements of the autonomous surface treatment apparatus required for it to arrive at said untreated cell; building three lists around the currrent cell each list containing the coordinates of cells with the lowest cost and of the coordinates of cells of the two consecutive higher cost levels, but limited to cells adjacent to the current cell; processing the lists, one by one in cost-order, starting with the list having the lowest cost, wherein as a list is processed the cells are examined one by one to identify cells indicated as untreated; during processing of a list, adding cells to the two consecutive lists of higher cost by considering, for each cell under process, adjacent cells in the directions forward, left and right; after processing of a list and updating the two consecutive lists of higher cost, discarding the list under process and processing the list next to follow; repeating the search process until an untreated and unoccupied cell has been found;