@incollection{SorrowsHirtle99,
  author	= {M.E, Sorrows, and Hirtle, S. C.},
  editor	= {Freksa, C. and Mark, D. M.},
  year		= {1999},
  title		= {The nature of landmarks for real and electronic spaces},
  booktitle	= {Spatial information theory - Cognitive and computational foundations of geographic information science},
  pages		= {37--50},
  address	= {Berlin},
  publisher	= {Springer}
}