Large indoor spaces such as office complexes and shopping malls have become an important part of our daily lives and research into indoor space is increasingly attracting attention from academia and industry. Indoor space differs from outdoor space in several important aspects such as layout, topology, constraints, or access restrictions. Techniques that are successful for outdoor spaces, for example GPS positioning, do not work well in indoor environments and require new approaches. Similarly, giving directions in outdoor spaces usually requires only two dimensions, whereas indoor navigation often involves the third dimension as office buildings and shopping malls typically have several floors.