lecture 3: introduction to gis part 1. understanding spatial data structures part 2. an introduction...
TRANSCRIPT
Lecture 3:
Introduction to GIS
Part 1. Understanding Spatial Data Structures
Part 2. An introduction to the Vector data model
Lecture by Austin Troy, University of Vermont
Perception, Semantics, and Space• How do we deal with representing semantic
constructions of spatial objects, like “mountain,” “river,” “street,” “city,”
• How about representing more conceptual semantic constructions like “temperature,” “migration pattern,” “traditional homeland,” “habitat,” “geographic range,” etc?
• Answer: we have various data models which use different abstractions of reality
Introduction to GIS
Entities and Fields• There are two general approaches for
representing things in space:– Entities/ Objects: precise location and
dimensions and discrete boundaries (remember, points are abstractions).
– Fields, or phenomena: a Cartesian coordinate system where values vary continuously and smoothly; these values exist everywhere but change over space
Entities and Boundaries• There are two general types of boundaries, bona fide
and fiat (D. Mark, B. Smith, A. Varzi)
• Pure bona fide boundaries represent real discontinuities in the world, like roads, faults, coastlines, power lines, rivers, islands, etc.
• Pure Fiat boundaries are a human cognitive or legal construction, based on a categorization, such as administrative unit, nation state, hemisphere
• Some have elements of both, like soil type areas
Introduction to GIS
Two major data models
• Entity approach roughly corresponds with the vector model
• Field approach roughly corresponds with raster model
• Any geographic phenomenon can be represented with both, but one approach is usually better for a particular circumstance
Introduction to GIS
Raster
• Spatial features modeled with grids, or pixels• Cartesian grid whose cell size is constant• Grids identified by row and column number • Grid cells are usually square in shape • Area of each cell defines the resolution • Raster files store only one attribute, in the form of a
“z” value, or grid code. • Consider the contrary….
Introduction to GIS
• Vector layers either represent:– Points (no dimensions)– Lines, or “arcs” (1 dimension) or– Areas, or “polygons” (2 or 3 dimensions)
• Points are used to define lines and lines are used to scribe polygons
• Each point line or polygon is a “feature,” with its own record and its own attributes
Introduction to GIS
Vector
Raster and Vector representations of the same terrain
Introduction to GIS
Raster: great for surfaces Vector: limited with surfaces
Vector vs. Raster: bounding
Introduction to GIS
Raster: bad with bounding Vector: boundary precision
• In Arc View and Arc GIS, we can covert vector layers to grids, based on an attribute, or grids to vector layers
• The disadvantage of vector to raster is that boundaries can be imprecise because of cell shape• Each time you convert, you introduce more error too
Moving between vector and raster
Introduction to GIS
• where boundaries are not precise
• that occur everywhere within a frame and can be expressed as continuous numeric values
• where change is gradual across space
• where the attribute of a cell is a function of the attributes of surrounding cells
Raster data analysis is better for representing phenomena:
Introduction to GIS
• Simple file structure
• Simple overlay operations
• Small, uniform unit of analysis
Raster technical advantages :
Introduction to GIS
Raster technical disadvantages :
• Big file size, especially for fine-grained data
• Difficult and error-prone reprojections
• Square pixels are unrealistic
Vector analysis is better :• Where there are definable regions • Where the relative position of objects is important• Where precise boundary definition is needed• Where multiple attributes are being analyzed for a
given spatial object• For modeling of routes and networks• For modeling regions where multiple overlapping
attributes are involved• EG: units with man-made boundaries (cities, zip
codes, blocks), roads, rivers
Introduction to GIS
• Smaller file size (in general)
• More graphically interpretable
• Allows for topology (see further on)
Vector technical advantages :
Introduction to GIS
Vector technical disadvantages :
• Complicated file structure
• Minimum mapping units are inconsistent between overlapping layers
Specific Vector Usages
• All legal and administrative boundaries (zip codes, states, property lines, land ownership)
• Building footprints and 3-D models• Roads• Bedrock geology• Pipelines, power lines, sewer lines• Flight paths and transportation routes• Coastlines
Introduction to GIS
Specific Raster Usages• Terrain modeling where micro-locational variability is
present and matters• Groundwater modeling, where surface flow outside of
channels is important• Representation of slope and aspect• Representations of distance and proximity to features• Spatial representation of probabilities (logit)• Modeling phenomena in nature with continuous spatial
variability and numeric attributes, like soil moisture, depth to bedrock, percent canopy cover, vegetative greenness index, species richness index
Introduction to GIS
• In many cases, though, the choice between raster and vector may not be so clear.
• Often it depends on the application
• The following are some examples where you could go either way:
Tossups
Introduction to GIS
Soil
• Soil type: Vector – Soil types are meant to represent discrete and
homogeneous areas and are qualitative. There is no “slight gradation” between soil types like with pH
• Soil pH: raster– pH is numeric, not categorical, and that number may vary
slightly within a single soil type polygon
– If pH were turned into categories, like High, Medium and Low, vector might be better
Introduction to GIS
Rivers• Most people think of a river as a discretely bounded
entity, hence vector • What about where the river size fluctuates
seasonally, e.g. desert rivers?• Or where the location of the river bed changes
slowly and gradually over the years• Or where the river becomes delta, and the distinction
between “river” and “swamp” becomes fuzzy? • Or where the river has a certain probability of
flowing or being dry at any given location and time
Introduction to GIS
• Depends on the type of analysis being done• With vector can do network modeling of stream and
river system, but only in the arcs– Vector stream model can take advantage of topologically
enabled analysis tools
• With raster, can do surface flow modeling– More realistic, because when it rains water flows
everywhere, not just in channels, shows accumulation
– Think of every piece of land as mini stream channel
Rivers
Introduction to GIS
• Vector works well for modeling vegetation stand type where categories are broad, e.g. mixed conifer, deciduous hardwood
• Raster works better where there is micro-locational heterogeneity in species distribution
• Raster also works better for representing ecotones, or edges between two stands
• The more specific and variable the classification, the more likely the raster approach will be needed
Vegetation Mapping
Introduction to GIS
Intro to Vector• Recall that there are three basic “feature” or
“object” types in the vector data type:– Point – Arc – Polyons
• In general a given layer holds a given feature type (e.g. “roads” is a line layer, “counties” is a polygon layer, “weather stations” is point)
Introduction to GIS
Intro to Vector• A point layer only consists of a bunch of (x,y)
coordiantes
• In a line (arc) layer, points define lines
• In a polygon layer, lines define areas
• Hence each level of vector features builds on the last
Introduction to GIS
Intro to Vector• Each point has a unique location
• 2 points define a line segment
• One or several line segments define an arc
• The endpoints of an arc are “nodes
• The angle points are “vertices” (sing. Vertex)
• The feature is the arc, not the line
• Two arcs meet at the nodes
Introduction to GIS
Intro to Vector• Several arcs can scribe a polygon
• Polygons are closed regions whose boundaries are made up of line segments connected at many angles.
• Polygons generally define an area of homogenous phenomena (e.g. forest stand, building, zip code, lake)
• These phenomena can be described by one or more stored attributes
Introduction to GIS
Vector Representation:lines•Ring: this is a series of line segments (a string) that close upon each other
•It is NOT a polygon!!
•The computer does not know that the area inside “belongs” to that object
Introduction to GIS
Vector Representation:lines•A polygon is encoded differently, because the computer “knows” that the areas within those arcs “belongs” to that polygon, while it does not with a ring
Introduction to GIS
• Topology: spatial relationships between objects are encoded; the spatial location of each point, line and polygon is defined in relation to every other point, line and polygon
• Topology allows for behaviors of objects in relation to other objects to be defined
• Topology allows for powerful analysis tools and can significantly reduce error and increase quality
• Vector files in ARC INFO are topologically encoded. Arc GIS 8.3 geodatabases will be as well. Currently geodatabases are partially topological
Introduction to GIS
Vector: Topology
• One of the most important functions of topology is ensuring data quality and “logical consistency”
• When you bring in line and polygon data from external sources, you will often find errors such as lines (arcs) that dangle or overshoot, polygons that don’t close, adjacent polygons that show up as not sharing a border (we’ll return to this later in the semester)
Introduction to GIS
Vector Topology: purpose
• A topological structure helps ensure these problems don’t happen because it allows for enforcing of user-defined spatial rules
• ArcGIS 8.3 (coming soon) will include new tools for defining and validating topology rules
• Topology can also be used for defining spatial rules between layers to minimize errors and ensure logical consistency between them
Introduction to GIS
Vector Topology
• Say we have the following layers: property lots, sidewalk, building footprints, zoning map
• We can specify topological rules, like:– Lots must be enclosed polygons– Buildings must be entirely within a lot– Sidewalks must be outside a lot polygon– Lots must fall entirely within a single zone– Lots must either share a border with another lot or with city
land, including streets and sidewalks.– In a low-density zone, no more than 20 lots can be touching
• We can’t do this yet, but will be able to shortly
Introduction to GIS
Topology rules: Example
Vector Topology TableConsists of four elements
1. Polygon topology table• Lists arcs/links comprising polygon
2. Node topology table• Lists links/arcs that meet at each node
3. Arc, or “link” topology table• Lists the nodes on which each link/arc ends and
polygons to right and left of each link/arc, based on start and finish nodes
4. Table with real world coordinates for each point
Introduction to GIS
Vector Topology Table
Graphical display of arcs, nodes, vertices and lines
Topology table for the ARCs making up the polygons
A table of the polygon topology
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Spaghetti Data Model•Just because feature looks like a point, line or polygon does not mean it’s topological
Spaghetti Model is:•Non-topological data model that looks like vector•collections of line segments and points with no real connection or topology•Only stores features coordinates; there are no relative relationships encoded in this model •each feature has no knowledge of other features that it intersects, is adjacent to, contiguous with or near
Introduction to GIS
Spaghetti Data•Generally have loose ends, nodes not “snapped,” polygons don’t fully close, etc•Polygons defined by coordinates of circumscribing points, so common boundaries between two polygons are often registered twice.•Generally come from CAD files or digitizing•They often look fine to the user, but are useless from the standpoint of spatial analysis•This approach is memory inefficient
Can “clean” these data, using user-defined tolerances
Introduction to GIS