EarthView -- a "planet browser"

Paul Hansen and TonVan Sant<sup>1

1</sup>Geosphere
146 Entrada Drive
Santa Monica, California

Email: geosphere@aol.com


There have been many proposals for a "digital Earth" system. The most prominent being that put forward by Al Gore and being developed by NASA. In such a scheme, the internet has sites which contain data that is geo-referenced; i.e., the data describes some aspect of the Earth at some location. In the case of satellite imagery and terrain elevation data, it is possible for a digital Earth system to include a 3D graphics representation of the Earth or parts of it, and provide a real-time fly-thru and viewing of the data.

EarthView is a software system that does this by pre-processing the image and terrain data into a grid of multi-resolution tiles, and quickly calculating which tiles are required for a particular view. The term "multi-resolution" means that some tiles contain data at higher or lower resolution than other tiles; for example, a view of Earth from a great distance in space would use lower resolution tiles than a closer view. The contents of a single low-resolution tile will require 4 tiles at the next higher resolution level, since it will appear to be twice as wide and twice as high, and more image detail will be seen.

A unique feature of this software is the way that the north and south polar regions are handled. Standard cylindrical (Mercator) projection has the drawback that the polar regions are "stretched" more as latitude increases, while it has the benefit that latitude lines are horizontal and longitudinal lines are vertical. EarthView can take advantage of this benefit from the equator to the 45th parallel (north and south), but from there to the poles converts the data into separate regions with the pole in the center. This is also somewhat distorted, but when these regions are mapped to a globe in computer graphics, the result is a high-quality simulation that avoids mapping problems and saves diskspace by reducing the "polar stretch".

In this mapping, the entire globe is represented in six portions, four around the equator, and two polar. Within each of these 90 degree regions there is then a recursive subdivision into 4,16, 64, etc. subtiles, down to the desired resolution. The EarthView software can manage these tiles at extremely high speed on today's low-cost computers, so that everyday citizens can view a digital Earth with the sophistication that was only recently available to expensive business, government and military installations.