Raster Mapping Overview


Raster Mapping Overview

Most moving map packages use only a vector database, in which each database element (e.g., a highway intersection) is represented as a specific data record. Such designs result in a piecewise-style map display that is not easily configurable, although the database is typically compact. RASTRAC also uses a raster database, made up of image files that represent the image as a very large rectangularly-oriented dot matrix, like your television or computer monitor. A raster image, in general, contains one dot element for every dot displayed on your screen. For example, a 20x20 dot image contains 400 dot elements. Of course, each dot has a color, which may be black, white, or any other color. A data element of one bit can be used to encode only 2 colors. A data element of one byte (8 bits) can encode up to 256 colors , and a data element of 24 bits can encode up to 16,777,216 colors. In these examples, the 1, 8, or 24 values are referred to as the number of bits per pixel (pixel is a shortening of picture element, sometimes called a pel). Some file formats provide for image compression, in which redundant data is optimally encoded to reduce file size. Some compression formats like LZW (named after its inventors: Limpel, Ziv & Welch) and RLE (Run-Length Encoding) result in NO loss of information. Others result in information loss, while yielding greater compression ratios (like JPEG).

You can zoom a map by holding down the left mouse button on a map and dragging an outline of the area of interest. When you release the left mouse button the map will zoom to the area outlined by the ‘zoom-box’. To move the ‘zoom-box’ for a raster or layered map, hold the shift key while moving the mouse to place the ‘zoom-box’ where you want it (this will not work for a PW_Street map). If you want to cancel the zoom, hold down the control (Ctrl) key when you release the mouse button.