OPTIMAL MAP PROJECTIONS FOR ANALYSING LONG-DISTANCE MIGRATION ROUTES

Little attention has been given to the types of maps used in migration studies. However, we think that analyses of migration in two dimensions by applying different map projection rules may give valuable insights about biological control programmes for long-distance migration. The Earth, being a globe, cannot be projected on a two-dimensional plane without distorting one or more of the following properties: distance, direction or area. Map projections 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 influenced by topographical features they are expected to follow either orthodromes or loxodromes. The orthodrome (great circle) is the shortest path between two points on the Earth's surface, the loxodrome (rhumbline) is the path of constant course between two points. Properties of several map projections and their usefulness for evaluating to what extent travelling paths of migratory birds incorporate the effects of a spherical Earth are discussed and illustrated. For studying orthodrome orientation principles the azimuthal projections are most relevant, mainly the gnomonic-, orthographic-, stereographicand azimuthal equidistant projections. Mercator and related projections (oblique Mercatorand loximuthal projection) show migration in the light of loxodromes and orientation along constant geographic or magnetic courses.