A method of localising portable apparatus in an environment, the method comprising: obtaining a stored experience data set comprising a graph of connected nodes and edges, each said node comprising a representation image of a location within the environment under a particular environmental/visual condition, and the edges containing six degree of freedom transformations between the nodes; obtaining captured location representation image data provided by at least one sensor associated with the transportable apparatus; obtaining a current pose estimate of the transportable apparatus within the environment; using the current pose estimate to select a candidate set of the nodes from the stored experience data set that contain a potential match for the captured location representation data; the method characterised by: using the current pose estimate to obtain a set of paths from path memory data, each said path comprising a set of said nodes previously traversed in the environment when creating a said node based on the location representation image data captured under similar said environmental/visual conditions; using the set of paths to refine the candidate set of nodes, and comparing the captured location representation image data and the refined candidate set of nodes in order to identify a current pose of the transportable apparatus within the environment. obtaining a stored experience data set comprising a graph of connected nodes and edges, each said node comprising a representation image of a location within the environment under a particular environmental/visual condition, and the edges containing six degree of freedom transformations between the nodes; obtaining captured location representation image data provided by at least one sensor associated with the transportable apparatus; obtaining a current pose estimate of the transportable apparatus within the environment; using the current pose estimate to select a candidate set of the nodes from the stored experience data set that contain a potential match for the captured location representation data; the method characterised by: using the current pose estimate to obtain a set of paths from path memory data, each said path comprising a set of said nodes previously traversed in the environment when creating a said node based on the location representation image data captured under similar said environmental/visual conditions; using the set of paths to refine the candidate set of nodes, and comparing the captured location representation image data and the refined candidate set of nodes in order to identify a current pose of the transportable apparatus within the environment. using the current pose estimate to obtain a set of paths from path memory data, each said path comprising a set of said nodes previously traversed in the environment when creating a said node based on the location representation image data captured under similar said environmental/visual conditions; using the set of paths to refine the candidate set of nodes, and comparing the captured location representation image data and the refined candidate set of nodes in order to identify a current pose of the transportable apparatus within the environment.