Raster Formats (Continued)
Types of Information in a Digital MapRaster
Geographic Information
The geographic information in a digital map provides the position and shape of each map feature. For example, a road map’s geographic information is the location of each road on the map.
In a vector map, a feature’s position is normally expressed as sets of X,Y pairs or X,Y,Z triples, using the coordinate system defined for the map (see the discussion of coordinate systems, below). Most vector geographic information systems support three fundamental geometric objects:
- Point: A single pair of coordinates.
- Line: Two or more points in a specific sequence.
- Polygon: An area enclosed by a line.
Some systems also support more complex entities, such as regions, circles, ellipses, arcs, and curves.
Attribute Information
Attribute data describes specific map features but is not inherently graphic. For example, an attribute associated with a road might be its name or the date it was last paved. Attributes are often stored in database files kept separately from the graphic portion of the map. Attributes pertain only to vector maps; they are seldom associated with raster images.
GIS software packages maintain internal links tying each graphical map entity to its attribute information. The nature of these links varies widely across systems. In some the link is implicit, and the user has no control over it. Other systems have explicit links that the user can modify. Links in these systems take the form of database keys. Each map feature has a key value stored with it; the key identifies the specific database record that contains the feature’s attribute information.
Should problems arise, it is important for you to know how your software establishes and maintains attribute links.
Display Information
The display information in a digital-map data set describes how the map is to be displayed or plotted. Common display information includes feature colors, line widths and line types (solid, dashed, dotted, single, or double); how the names of roads and other features are shown on the map; and whether or not lakes, parks, or other area features are color coded.
However, many users do not consider the quality of display information when they evaluate a data set. Yet map display strongly affects the information you and your audience can obtain from the map -- no matter how simple or complex the project. A technically flawless, but unattractive or hard-to-read map will not achieve the goal of conveying information easily to the user.
Oddly enough, many common data sets contain no display information. For example, USGS Digital Line Graph files provide no display information at all. Each feature contains an attribute that describes the entity but does not indicate display features. Users, and their GIS software, must interpret those attributes and decide how each will look on the final display.