In Geographic Information Systems (GIS), a polyline is a fundamental type of vector data used to represent linear features on the Earth's surface. According to [symbology, Esri software], a polyline is defined as "A shape defined by one or more paths, in which a path is a series of connected segments".
Understanding Polylines: Paths and Segments
Breaking down the definition:
- Path: A path is a sequence of connected line segments. It represents a continuous linear feature or a part of one. A single polyline feature can consist of one or multiple paths, even if they are disconnected.
- Segment: A segment is the basic building block of a path. It is a straight line drawn between two points, known as vertices. A path is formed by joining these segments end-to-end.
Think of it like drawing a line on a map. You start at one point, draw to another (creating a segment), then draw from that point to the next (creating another segment), and so on. This series of connected segments forms a path. If you lift your pen and start drawing another distinct line somewhere else to represent the same overall feature (like islands in a river), that would be another path within the same polyline feature.
Characteristics and Significance
Polylines are essential for representing geographical objects that have length but no significant width or area at the chosen map scale. Unlike polygons, polylines do not enclose an area. They are characterized by:
- Sequence of Vertices: Defined by an ordered list of coordinate pairs (vertices).
- Connectivity: Segments within a path are connected.
- Linear Representation: Ideal for features like roads, rivers, and boundaries that follow a line.
- Multiple Paths: A single polyline object can represent features that are logically one entity but spatially disconnected (e.g., a single river feature might include separate paths for braided channels).
Common Examples of Polylines in GIS Data
Many real-world features are represented as polylines in GIS datasets:
- Roads and highways
- Rivers and streams
- Railway lines
- Utility lines (water pipes, power cables)
- Contour lines (representing elevation)
- Property boundaries (often represented line by line before forming polygons)
- Walking trails
Why Polylines are Important in GIS
Polylines play a crucial role in various GIS applications:
- Mapping and Visualization: Displaying linear features on maps.
- Measurement: Calculating the length of features like roads or rivers.
- Network Analysis: Modeling transportation networks (roads, railways) or utility networks (water, power) for routing, flow analysis, and service area determination.
- Spatial Analysis: Identifying intersections, calculating distances along lines, or buffering linear features.
In essence, polylines provide the digital structure necessary to store, analyze, and display geographic information that exists as lines or networks.
