Modelling the Earth
| The ability to locate an exact position on the Earth's surface is fundamental to many human activities. The presence of man-made satellites (and our aptitude in accurately determining their orbits) has led to the development of two scientific advances used to study the geosphere. Geodesy is the science of studying the exact shape and curvature of the Earth's surface and the position of landmarks on the surface. The orbits of satellites change slightly in response to gravitational variations in the Earth's Geoid (called an equipotential surface), and have allowed us to map subtle changes in the Earth's shape which influence our capability to map it accurately. The Earth is not a perfect sphere, and this causes problems when creating close-up maps of certain areas around the world. Flat maps that encompass a continent or the entire globe will often base their position calculations using the Earth as a perfect sphere, which is reasonable for such large areas. But when a county or city must be mapped, the calculations of position using a sphere introduce errors in the actual position of a landmark. Instead, geographers use spheroids (slightly-squashed spheres) to reduce these errors when mapping smaller areas. There are about a dozen or so important spheroids, and each one works better in certain places of the world than in others. As a consequence, different countries around the world employ different spheroids that work best for their regions. The art of air, ocean, and land navigation depends greatly on our increased understanding of the Earth's shape, and the ways in which our planet is mapped. |
Credit: Dylan Prentiss, Department of Geography, University of California, Santa Barbara
For more info: http://www.geog.ucsb.edu/~dylan/mtpe/geosphere/topics/rs/howis.html