Optimal map projections for analysing long-distance migration routes

Little attention has been given to the types of maps used in migration stud ies. However, we think that analyses of migration in two dimensions by appl ying different map projection rules may give valuable insights about biolog ical control programmes for long-distance migration. The Earth, being a glo be, cannot be projected on a two-dimensional plane without distorting one o r more of the following properties: distance, direction or area. Map projec tions which correctly preserve two of the above-mentioned properties do so only from one point. For analysing the routes of long-distance migrants the distance and direction are the most important factors due to high costs of travelling, either in the form of energy or time. When migrants are not in fluenced by topographical features they are expected to follow either ortho dromes or loxodromes. The orthodrome (great circle) is the shortest path be tween two points on the Earth's surface, the loxodrome (rhumbline) is the p ath of constant course between two points. Properties of several map projec tions and their usefulness for evaluating to what extent travelling paths o f migratory birds incorporate the effects of a spherical Earth are discusse d and illustrated. For studying orthodrome orientation principles the azimu thal projections are most relevant, mainly the gnomonic-, orthographic-, st ereographic- and azimuthal equidistant projections. Mercator and related pr ojections (oblique Mercator- and loximuthal projection) show migration in t he light of loxodromes and orientation along constant geographic or magneti c courses.