Introduction
ERDAS IMAGINE is designed to integrate two data types, raster and vector, into one system. While the previous chapter explored the characteristics of raster data, this chapter is focused on vector data.The vector data structure in ERDAS IMAGINE is based on the ArcInfo data model (developed by ESRI, Inc.). This chapter describes vector data, attribute information, and symbolization.
You do not need ArcInfo software or an ArcInfo license to use the vector capabilities in ERDAS IMAGINE. Since the ArcInfo data model is used in ERDAS IMAGINE, you can use ArcInfo coverages directly without importing them.
See "Geographic Information Systems" on page 173 for information on editing vector layers and using vector data in a GIS.
Vector data consist of:
• points
• lines
• polygons
Each is illustrated in Figure 16.
Figure 16: Vector Elements node
node vertices
polygons
label point line
points
Points
A point is represented by a single x, y coordinate pair. Points can represent the location of a geographic feature or a point that has no area, such as a mountain peak. Label points are also used to identify polygons (see Figure 17).Lines
A line (polyline) is a set of line segments and represents a linear geographic feature, such as a river, road, or utility line. Lines can also represent nongeographical boundaries, such as voting districts, school zones, contour lines, etc.Polygons
A polygon is a closed line or closed set of lines defining a homogeneous area, such as soil type, land use, or water body. Polygons can also be used to represent nongeographical features, such as wildlife habitats, state borders, commercial districts, etc. Polygons also contain label points that identify the polygon. The label point links the polygon to its attributes.Vertex
The points that define a line are vertices. A vertex is a point that defines an element, such as the endpoint of a line segment or a location in a polygon where the line segment defining the polygon changes direction.The ending points of a line are called nodes. Each line has two nodes:
a from-node and a to-node. The from-node is the first vertex in a line.
The to-node is the last vertex in a line. Lines join other lines only at nodes. A series of lines in which the from-node of the first line joins the to-node of the last line is a polygon.
Figure 17: Vertices
In Figure 17, the line and the polygon are each defined by three vertices.
Coordinates
Vector data are expressed by the coordinates of vertices. The vertices that define each element are referenced with x, y, or Cartesian, coordinates. In some instances, those coordinates may be inches [as in some computer-aided design (CAD) applications], but often the coordinates are map coordinates, such as State Plane, Universal Transverse Mercator (UTM), or Lambert Conformal Conic. Vector data digitized from an ungeoreferenced image are expressed in filecoordinates.
line polygon
vertices
label point
Tics
Vector layers are referenced to coordinates or a map projection system using tic files that contain geographic control points for the layer. Every vector layer must have a tic file. Tics are not topologically linked to other features in the layer and do not have descriptive data associated with them.
Vector Layers
Although it is possible to have points, lines, and polygons in a single layer, a layer typically consists of one type of feature. It is possible to have one vector layer for streams (lines) and another layer for parcels (polygons). A vector layer is defined as a set of features where each feature has a location (defined by coordinates and topological pointers to other features) and, possibly attributes (defined as a set of named items or variables) (ESRI 1989). Vector layers contain both the vector features (points, lines, polygons) and the attribute information.Usually, vector layers are also divided by the type of information they represent. This enables the user to isolate data into themes, similar to the themes used in raster layers. Political districts and soil types would probably be in separate layers, even though both are represented with polygons. If the project requires that the coincidence of features in two or more layers be studied, the user can overlay them or create a new layer.
See "Geographic Information Systems" on page 173 for more information about analyzing vector layers.
Topology
The spatial relationships between features in a vector layer are defined using topology. In topological vector data, a mathematical procedure is used to define connections between features, identify adjacentpolygons, and define a feature as a set of other features (e.g., a polygon is made of connecting lines) (Environmental Systems Research Institute, 1990).
Topology is not automatically created when a vector layer is created. It must be added later using specific functions. Topology must be updated after a layer is edited also.
Digitizing on page 49 describes how topology is created for a new or edited vector layer.
Vector Files
As mentioned above, the ERDAS IMAGINE vector structure is based on the ArcInfo data model used for ARC coverages. This georelational data model is actually a set of files using the computer’s operating system for file management and input/output. An ERDAS IMAGINE vector layer is stored in subdirectories on the disk. Vector data are represented by a set of logical tables of information, stored as files within the subdirectory. These files may serve the following purposes:• define features
• provide feature attributes
• cross-reference feature definition files
• provide descriptive information for the coverage as a whole A workspace is a location that contains one or more vector layers.
Workspaces provide a convenient means for organizing layers into related groups. They also provide a place for the storage of tabular data not directly tied to a particular layer. Each workspace is completely independent. It is possible to have an unlimited number of workspaces and an unlimited number of vector layers in a workspace. Table 1 summarizes the types of files that are used to make up vector layers.
Figure 18 illustrates how a typical vector workspace is set up (Environmental Systems Research Institute, 1992).
Table 1: Description of File Types
File Type File Description
Feature Definition Files
ARC Line coordinates and topology CNT Polygon centroid coordinates
LAB Label point coordinates and topology TIC Tic coordinates
Feature Attribute Files
AAT Line (arc) attribute table PAT Polygon or point attribute table Feature
Cross-Reference File
PAL Polygon/line/node cross-reference file
Layer Description Files
BND Coordinate extremes LOG Layer history file
PRJ Coordinate definition file TOL Layer tolerance file
Figure 18: Workspace Structure
Because vector layers are stored in directories rather than in simple files, you MUST use the utilities provided in ERDAS IMAGINE to copy and rename them. A utility is also provided to update path names that are no longer correct due to the use of regular system commands on vector layers.
See the ESRI documentation for more detailed information about the different vector files.