Lecture 6 - Vector Analysis Flashcards
What is the objective of spatial analysis?
To transform data into useful info to satisfy the requirements/objectives of decision-makers at all levels of detail
- helps us identify data trends, create new relationships from data, view complex relationships b/w data sets, make better decisions
What are the components of spatial analysis?
- possible input: tabular attribute data, spatial data/layers, combo of spatial and tabular
- possible output: maps/map features, charts, table
What are the challenges of geographic analysis?
- lots of data
- spatial relationships difficult to measure
- inherent uncertainty due to scale
- difficult to make data sources compatible
- multiple objectives
- quantity vs. quality
- GIS can address some but not all difficulties
What is a query?
The selective display and retrieval of info from a database
- ability to query and retrieve data based on some user-defined criteria necessity in GIS
What are the 2 types of queries?
- Attribute
- Spatial
- combo: powerful way of exploring data patterns
Explain attribute queries
- select features using attribute data (using SQL)
- results can be mapped or presented in conventional database form
- can be used to produce maps of subsets of data or choropleth maps
- based on unique structure of DBMS (relational is RDBMS)
- uses external databases (large amounts stored in non-GIS databases)
Explain spatial queries
- allow us to examine locational relationships b/w 2 data sets
- clicking on features on the map to find out their attribute values
- GIS can perform geometric queries (ex. by point, circle, rectangle, etc.)
What is SQL?
Structured Query Language
- programming language designed for retrieval and management of data in RDBMS
- provides tools needed to manage RDBMS (creating tables, adding data, queries/searches)
- provides a standard for GIS software
Give examples of some SQL commands
ex. used LIKE Others: - select, where (query) - insert, delete, update (editing) - create table (management)
What are the limitations of attribute-based queries?
- cannot answer important geographical questions (must combine with spatial queries)
- ex. what land parcels are adjacent to the site with contaminated soil?
Explain Boolean operators
- used to combine search terms to construct more complex searches in a database
- AND: intersection of two sets (just the overlap) - ex. snow and ice
- OR: yields union of two sets (all of both sets) - ex. snow or ice
- NOT: yields exclusion of one set (even overlap is excluded) snow not ice
Explain measurements
Different types
- diff b/w 2 points
- area
Tedious and inaccurate by hand - use GIS tools
- measurements often made on horizontal projections of objects
What is buffering?
A transformation
- creates new objects/attributes (polygon) based on simple rules (involves simple geometric, arithmetic, or logical rules; may create new fields from existing fields of discrete objects)
- summarizes distance or proximity by creating distance buffers around selected features (points, lines, or areas)
- possible in both raster and vector formats
What is overlay?
- series of operations performed on spatial data in different layers (new layers of data produced; most required GIS technique)
- joins two layers to create a new layer (output contains both the spatial and attribute data from both input layers)
- map features and associated attributes are integrated to produce new composite maps
What are the 2 aspects of vector overlay?
- Geometric (shapes of everything)
2. Attribute Management (data merged; resulting table is combo of attributes)
What is a point-on-polygon overlay?
Determines whether a point lies inside or outside polygon
- overlay point objects on areas (“is contained in” relationship)
- result in new attribute for each point
- ex. combine wells and planning districts
What is a line-on-polygon overlay?
Determines what line sections lie within specific polygons
- overlay line objects on area objects
- lines are broken at each area object boundary; containing area has new attribute of each output line
- ex. what rivers and streams portions lie in counties?
What is a polygon-on-polygon overlay?
Determines what areas in dataset A lie within specific polygons of dataset B
- overlays 2 layers of area objects
- boundaries broken at each intersection
- number of output areas likely greater than number of input areas
- ex. input watershed boundaries and county boundaries
What are the issues with overlay?
Spurious polygons
- during polygon overlay, many new and smaller polygons are created, some of which may not represent true spatial variations (results from overlaying diff versions of the dame line)
- able to set a tolerance value for deleting spurious polygons
- can be confusing and inaccurate
- ex. road may be part of county boundary but also boundary b/w two fields
What is a network?
Interconnected arcs that comprise a set of features through which resources flow
What is a network analysis?
Uses topological relationships to represent and analyze flows along a set of interconnected paths
- based on connectivity, impedance, and direction of flow on arc
- ex. road network
What interconnecting logical components are network models based on?
- nodes: defines start, end, and intersections
- chains: line features joining nodes
- links: join together points to make up a chain
What are some examples of network analysis applications?
- routing (buses)
- flow tracing (pollutant dispersion
- allocation functions (facility location
- facilities management (phone, computer network, etc.)
What are 3 types of network analyses?
- Network routing
- Route tracing
- Network location-allocation
Explain network routing
- shortest path b/w 2 points (direction instructions, bus routing, deliveries, etc.)
Explain route tracing
- trace flows of goods, people, info through network (especially uni-directional flows)
- ex. model downstream pollutant
Explain network location-allocation
- provision of service to satisfy spatially dispersed demand
- allocate arcs to nearest supply centre and draw boundaries to define service areas
Two problems: - location: where to put central facilities
- allocation: which subsets of demand should be served from each site
How can we represent field-based (continuous) phenomena/surfaces within vector data model?
- Sample real world surface
2. Rep field-based surface using vector objects
Explain sampling for continuous surface rep.
- sample field variables by recording value at limited number of locations
- interpolation: estimate new values from neighbouring point locations to create a continuous surface
- IDW
Explain how IDW is used for sampling continuous surfaces
- inverse distance weighing estimates interpolated cell values by averaging values of sample data points in neighbourhood of each processing cell
- more influence/weight by points closer to estimate cell
- issue: uneven distribution of sample points, boundaries, etc.
Explain representing field-based surfaces in vector GIS
- isolines are lines that connect points of equal value (contour for equal elevation, isotherms for equal temp)
- interpolate contour lines based on values of point sample
What factors are useful in modelling surfaces?
- slope: steepness
- aspect: compass direction
- visibility: line of sight b/w 2 points
Explain the TIN model
Triangulated Irregular Network
- hybrid vector model (construct elevation surface from set of irregularly spaced data points)
- created by linking neighbouring points to form an irregular triangular lattice
- each triangle stores: list of nodes, list of neighbouring triangles, slope & aspect, and able to interpolate elevation within triangle
How are TINs generated?
Via Delaunay triangles
- polygons generated from a point layer
- all sample points connected with 2 nearest neighbours to form triangles
- divides space between points as evenly as possible
- elevation, slope, and aspect of triangle calculated from heights of 3 corners
- DTs as near equiangular as possible, and longest side is as short as possible (minimizes interpolation distances)
What are the advantages of the TIN model?
- few data points required
- records more data for areas of complex relief
- captures discontinuities well (ex. ridges)
- slope and aspect easily recorded
What are the disadvantages of the TIN model?
- requires significant processing power to generate
- difficult to combine with other map layers via overlay (lots of polygons involved)
