Lecture 5 - Vector Data Flashcards
What is vector representation?
- complex rep based on primitive objects
- represents spatial location of features explicitly
What are the different ways that vector data can be put together into a vector data model?
- Spaghetti model
- Vertex dictionary
- Dual independent map encoding (DIME)
- Arc/node topological model
What is a TIN model?
A triangular irregular network is a vector model used to represent continuous surfaces
Explain the spaghetti model
- simple (one point and polygon)
- easy to manage & understand
- no topology
- lots of duplication (large storage space needed
- data model is map expressed as a list of cartesian (x, y) coordinates)
- no inherent structure (messy like spaghetti)
Explain what a point, line, and polygon would look like in spaghetti data model
Point: identifier and single x,y coordinate pair
Line: identifier and list of x,y coordinate pairs
Polygon: identifier and list of x,y coordinate pair identical to first pair (closed loop)
What are the weaknesses of the spaghetti model?
- redundancy: common boundaries recorded twice
- no topological info (connectivity): no automatic error checking; spatial relationships must be derived by computation
- nested polygons: polygon order important for graphic display; area measures may be incorrect
- slivers/gaps: logical errors occur if boundaries are not listed the same
Explain the vertex dictionary/ whole polygon model
- polygons described by listing coordinates of points in order as you “walk around” outside boundary of polygon
- vertices stored in one file while lines/polygons stored in another
- no duplication but model still does not use topology
What are the weaknesses of the vertex dictionary model?
- coordinates/borders for adjacent polygons stored twice (can result in slivers/gaps)
- no topological info about polygons
Explain the DIME model
- nodes (line intersections) are identified with codes
- assigns a direction code (a “from node” and a “to node”)
- street addresses and UTM coordinates explicitly defined for each link
- polygons described by listing ID numbers of points in order as you “walk around” outside boundary
- second file lists all points and their coordinates
- solves duplicate coordinate/border problem
- still no topological info
Explain the Arc/Node topological model
- uses topology
- overcomes major weaknesses of spaghetti model (allowing for GIS analysis, like overlaying, network, etc.)
- requires all lines to be connected, polygons closed, and loose ends removed
What is topology?
- relationships in spatial terms b/w connected or adjacent geographical objects
- connectivity: point-line relationship; line-line connection
- area definition: line-polygon relationship
- contiguity: polygon-polygon adjacency
- mathematical model used to define spatial relationships
Why is topology important?
- connections/relationships b/w objects that are independent of their coordinates (geometry changes under transformations)
Explain point, arc, vertices, node, and polygon in terms of vector primitives/features
- point: defined by single x,y pair
- arc: line defined by ordered set of x,y pairs (can be straight of curved)
- vertices: points on an arc that are not nodes
- node: end points of an arc, or intersection of two arcs
- polygon: area defined by arcs making up its boundary
What are the advantages of the topological model?
- spatial analysis: performed without using coordinate data but using topological data alone (more efficient)
- data validation: ensures that associated data form a consistent and clean topological fabric (reduces data set size and additional items can be added to attribute tables)
What are the disadvantages of the topological model?
- when data later is inputted or edited, the topology must be updated (can be time-consuming, but ArcGIS does for you)
- originally did not deal well with continuous surfaces (TIN model helps with)
