The Mystery of 31 New Inn by R. Austin Freeman

Few people, I imagine, will agree with him. To most readers, and certainly to the kind of reader for whom an author is willing to take trouble, complete realism in respect of incidents and methods is an essential factor in maintaining the interest of a detective story. Hence it may be worth while to mention that Thorndyke’s method of producing the track chart, described in Chapters II and III, has been actually used in practice. It is a modification of one devised by me many years ago when I was crossing Ashanti to the city of Bontuku, the whereabouts of which in the far interior was then only vaguely known. My instructions were to fix the positions of all towns, villages, rivers and mountains as accurately as possible; but finding ordinary methods of surveying impracticable in the dense forest which covers the whole region, I adopted this simple and apparently rude method, checking the distances whenever possible by astronomical observation.
—from this Book