![]() Map scale = 1 : ground resolution * screen dpi / 0.0254 meters/inch It can be calculated from the ground resolution as follows, given the screen resolution in dots per inch, typically 96 dpi: Like the ground resolution, the map scale varies with the level of detail and the latitude of measurement. For instance, at a map scale of 1 : 100,000, each inch on the map represents a ground distance of 100,000 inches. The map scale indicates the ratio between map distance and ground distance, when measured in the same units. Ground resolution = cos(latitude * pi/180) * earth circumference / map width Using an earth radius of 6378137 meters, the ground resolution (in meters per pixel) can be calculated as: ![]() The ground resolution varies depending on the level of detail and the latitude at which it’s measured. For example, at a ground resolution of 10 meters/pixel, each pixel represents a ground distance of 10 meters. The ground resolution indicates the distance on the ground that’s represented by a single pixel in the map. Map width = map height = 256 * 2 level pixels In general, the width and height of the map (in pixels) can be calculated as: At each successive level of detail, the map width and height grow by a factor of 2: Level 2 is 1024 x 1024 pixels, Level 3 is 2048 x 2048 pixels, Level 4 is 4096 x 4096 pixels, and so on. At the lowest level of detail (Level 1), the map is 512 x 512 pixels. ![]() In addition to the projection, the ground resolution or map scale must be specified in order to render a map. The spherical projection causes approximately 0.33% scale distortion in the Y direction, which is not visually noticeable. Since the projection is used only for map display, and not for displaying numeric coordinates, we don’t need the extra precision of an ellipsoidal projection. To simplify the calculations, we use the spherical form of this projection, not the ellipsoidal form. Using a square aspect ratio for the map, the maximum latitude shown is approximately 85.05 degrees. Since the Mercator projection goes to infinity at the poles, it doesn’t actually show the entire world. It’s a cylindrical projection, which means that north and south are always straight up and down, and west and east are always straight left and right. Square buildings should appear square, not rectangular. This is especially important when showing aerial imagery, because we want to avoid distorting the shape of buildings. It’s a conformal projection, which means that it preserves the shape of relatively small objects. We chose to use the Mercator projection, which looks like this:Īlthough the Mercator projection significantly distorts scale and area (particularly near the poles), it has two important properties that outweigh the scale distortion: To make the map seamless, and to ensure that aerial images from different sources line up properly, we have to use a single projection for the entire world. This document describes the projection, coordinate systems, and addressing scheme of the map tiles, which collectively are called the Bing Maps Tile System. To make this interaction as fast and responsive as possible, we chose to pre-render the map at many different levels of detail, and to cut each map into tiles for quick retrieval and display. Bing Maps provides a world map that users can directly manipulate to pan and zoom.
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