Geography Reference
In-Depth Information
Labels —A point carrying textual information about what the point represents or the polygon that the
point resides in.
Junctions —Points in geometric networks where ends of lines (edges) are joined. Discussed when we cover
geometric networks in Chapter 9.
Lattice points —Set of points in a raster, usually defined to be at the centers of cells.
One-Dimensional Entities in a Two-Dimensional Field: Lines
You saw examples of lines representing streams and sewers in Chapter 1. Lines were also used there
to delineate boundaries of polygons. A line may be used to represent a linear feature that is too narrow
to have a meaningful area. As with zero-dimensional entities, vertices on a line may have a “z” (e.g.,
altitude) value.
In GIS Terms, a line is a simple geometric entity that consists of a sequence (that is, an ordered set) of
vertices, which are simply coordinate pairs or triples. Between each adjacent pair of vertices there is a
segment. A segment is frequently simply a straight line, but in vector-based geodatabase feature classes,
it can also be a part of a circle or ellipse, or it may be a spline (called a Bézier 7 curve). A line that consists
of multiple straight-line segments connected at vertices can approximate a curve. Lines can therefore be
used to represent curvilinear features, such as roads and streams. The segments of a line are not allowed
to intersect each other.
Paths —A path is a line as described previously, composed of a sequence of connected segments (or a
single segment). The term “path” is used in vector-based geodatabase feature classes.
Polyline —A polyline is made up of one or more paths. If there are multiple paths, the paths may be
connected or disjoint. Even if a polyline representing a feature consists of multiple paths, it has only one row
in the attribute table. If polylines are used in shapefiles, the segments of the path must be straight lines.
Rings —In a geodatabase vector feature class, when a path encloses an area (polygon), the path is called
a ring. A ring starts and ends at the same place. A ring is a sequence of nonintersecting segments that
form a closed loop. Its primary purpose is to enclose areas. If the segments are directed straight lines
(vectors), then the area enclosed is a polygon, in both the mathematical and GIS sense. If the segments are
curvilinear elements (arcs of circles or ellipses, or splines), then the enclosed area is a GIS polygon, but
not a geometric one. A ring has an unambiguous inside and outside. The length of a ring is automatically
stored in the associated attribute table.
Arcs —Used in coverages which are not discussed here. (The arc concept is the basis for the designation
“Arc” of ArcInfo. An Esri arc is basically a vector. “Info” was the name of the original brand of database
that the system used.)
Routes —Routes are subsets or supersets of polylines. They allow the user to define collections of linear
features (e.g., the parts of a road system that constitute bus route #99), or measured distances along a
linear feature (where a stream changes from clear to turbid). Routes make use of an m (measure) number
that specifies the distance along a feature to a location at which something changes.
7 Developed by Pierre Bézier in the late 1960s for computer-aided design (CAD) and computer-aided manufacturing
(CAM) operations for the Renault automobile company. It involves an anchor point at each end of a segment.
 
 
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