As the basic principles of EDM instruments have changed little since the third edition of 1990, there was no need for significant changes. This edition differs from its predecessor in that it contains corrections of a number of errors and misprints, totally revised tables in Appendices D, E and F and a new note in Section 2. 4. 3 on the introduction of the new temperature scale in 1990. The author is indebted to the many readers who reported the many small errors and misprints. T. Black, H. Buchanan, R. Da-Col, R. Kochle, P. H. Lam, 1. Nolton, 1. R. Pollard and A. Quade were particularly helpful. All known errors have been corrected. The assistance pro vided by most manufacturers (or their agents) with the updating of the tables with the instrument data was greatly appreciated. Sydney, February 1996 1. M. RUEGER v Preface The book has evolved from the author's continuing teaching of the subject and from two editions of a text of the same title. The first edition was published in 1978 by the School of Surveying, Universi ty of New South Wales, Sydney, Australia. Like its predecessors, this totally revised third edition is designed to make the subject matter more readily available to students proceeding to degrees in Survey ing and related fields."