12: Vector II Flashcards
SPATIAL INTERPOLATION
Attempt to make a reasonable estimate of the values of a field at places where the field had not actually been measured
Process of intelligent guesswork
THIESSEN POLYGONS
A user may want to allocate catchment areas to a point dataset, and this can be done with Thiessen polygons (also Voronoi polygons)
Each polygon is created whereby any location inside the polygon is closer to that point than to any other sample point.
Simple algorithm, but can be computationally complex
polygons are smaller, more dense where there are more points
at the edge of data polygons extent out to infinity, would clip data to avoid this.
NETWORK ANALYSIS
‘Classic’ networking problems
Use for:
- optimal route (least cost, shortest path, etc)
- finding closest facility
- resource allocation (service areas - look at existing services and demand to locate a new something)
LOCATION-ALLOCATION
Topic in operations research and spatial analysis
Simultaneously determine the location of facilities and the allocation of demand of facilities
Supply and demand values can also be used to determine the maximum catchment area of a particular supply centre based on the demand located along adjacent links in the network
Inverse Distance Weighting (IDW)
Estimates unknown measurements from the weighted average of measurements at known points
Creates a smooth surface by gives the greatest weight to the nearest point (Tobler’s first law-near things are more related to each other)
What is a network?
A system of connected linear features through which resources flow
Examples: rivers, roads, pipelines, utilities
What is distance through a network?
The real length of nodes and links that make up the roads instead of the distance between two places as the crow flies
Topologically structured networks
Spaghetti data - the GIS doesn’t know anything about the structure, analysis isn’t possible
Structured data - points and lines (nodes and links) has hierarchy and tells the GIS how the data is structured
Nodes and verticies
- every segment has a starting and ending node
- nodes contain attribute information that helps define the level of network connectivity
- connectivity = one line segment shares a node with an adjacent line segment
Attributes of a road network
Origin + destination coordinates Street name/highway number -direction -type -proper name -type suffix (avenue) -direction of flow -ramps Addresses Directionality -one way Classification -freeway, arterial, collector, residential Speed limit, congestion (impedance), travel time Traffic volume Length Scenic value CONNECTIVITY -TURN TABLES
Turn tables
Because a network has many turns with different conditions (options at each node), a turn table is used to assign the turn impedance values (defining what turns are possible at each node)
Large impedance indicates turn not possible or illegal
Network optimisation techniques
- shortest path analysis
- closest facility
- travelling salesman problem
- allocation
- location-allocation
Network example applications
- routing emergency response vehicles over a street network
- optimising school bus routing over a street network
- tracing pollution upstream to a source over a stream network
- new retail location strategies
Shortest path analysis
shortest path finds the path with the minimum cumulative impedance between nodes on a network
the path may connect just two nodes or have specific stops between
the shortest path in distance may not be the same as the fastest path
Closest facility
a type of shortest path analysis to find the closest facility such as a hospital, fire station or ATM, to any location on a network
the algorithm first computes the shortest path from the selected location to all candidate facilities and then chooses the closest facility among the candidates
travelling salesman problem
more complex form of shortest path analysis because of two constraints:
- the salesman must visit each of the selected stops only once
- the salesman may start from any stop but must return to the original stop
the objective is determine which route or tour the salesman can take to minimise the total impedance value (distance travelled)
allocation
the study of the spatial distribution of resources through a network
resources - often public facilities such as schools, libraries or fire stations
distribution of resources defines service zones
the main objective of spatial allocation analysis is to measure the efficiency of facilities
Define the location-allocation problem
set of techniques which determine the optimum or near optimum locations for providing goods and services to a spatially dispersed population, according to some socioeconomic criteria
Requires inputs of supply, demand and distance measures
SUPPLY - consists of facilities or centres at point locations
DEMAND - may consist of points, lines or polygons (depending on data source) often using census data
DISTANCE - between supply and demand (distance matrix) - road network
Two common models:
- Minimum distance model
- Maximum coverage model
Maximising coverage algorithm
Everyone needs to be served by the solution, how do we locate the facility so that we cover everyone well and minimises the distance/makes the most efficient solution
Maximises the demand covered within a specified time or distance
Has been applied for:
- emergency services
- convenient oriented retail facilities (banks, fast food, etc)
Minimum distance model
Also called p-median location model
Minimises the total distance travelled from all demand points to their nearest supply centres
Has been applied for:
- food distribution
- public libraries
- health facilities
