mobile GIS and field mapping application running on Windows Mobile/Pocket PC/Smart Phone
There is a concept of a navigable "Scene". The expance of a Scene is dependent on the source of the imagery and the imagery's projection system. For data in the Geographic projection system, the expanse is the entire globe. Effectively there is one Scene.
For UTM data, there are sixty Scenes, each covering approximately six longitude degrees.
Projection Methods
You are dealing with "Projection Systems" anytime you are dealing with a map or a globe. For our purposes within the TerraService, the earth is a sphere. Points on it are referenced by Longitude and Latitude "lines". They are not lines really, but complex curves. Maps are flat drawing of earth. Satellite or aerial imagery, in its raw state, is more-flat-than-its round. It is mathematically impossible to present a sphere on a flat surface without distorting something. Cartographers have dealt with this issue for centuries by developing different methods, known as map projections, for representing portions of earth with minimal, or at least predictable, distortions.
The USGS Cartographers selected the Universal Transverse Mercator (UTM) projection system for many of their high resolution map products, for example the USGS Topographic Map theme. The USGS chose to project the USGS Digital Orthoquadrangle aerial imagery into the UTM. There are several variants of the UTM projection system, the USGS aerial imagery and USGS topographic map data within TerraServer are in the UTM NAD 83 projection. (NAD 83 stands for North American Datum of 1983).
The TerraService provides a set of methods to convert between "Geographic" projection and UTM. That is, convert between longitude and latitude points and UTM NAD 83 points and vice-versa. TerraServer image tiles carry meta-data fields of the longitude and latitude of the corner points and center point of the tile. The TileId mate-data fields can be used to compute the UTM coordinates of any point in a tile. The Scale, Scene, X, and Y fields can be used to compute the UTM NAD 83 coordinates for the lower left hand pixel in the TerraServer tile (XOffset=0 and YOffset=200). The following formulas are required to compute the UTM NAD 83 for the lower left hand pixel:
Int32 UtmZone = Scene;
Int32 metersPerPixel = (1 << ((Int32) Scale - 10)); Int32 UtmEasting = X * 200 * metersPerPixel; Int32 UtmNorthing = Y * 200 * metersPerPixel;
