Lecture 6 - Vector Analysis

Question 1

What is the objective of spatial analysis?

Answer 1

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

Question 2

What are the components of spatial analysis?

Answer 2

• possible input: tabular attribute data, spatial data/layers, combo of spatial and tabular
• possible output: maps/map features, charts, table

Question 3

What are the challenges of geographic analysis?

Answer 3

• 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

Question 4

What is a query?

Answer 4

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

Question 5

What are the 2 types of queries?

Answer 5

1. Attribute
2. Spatial
   - combo: powerful way of exploring data patterns

Question 6

Explain attribute queries

Answer 6

• 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)

Question 7

Explain spatial queries

Answer 7

• 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.)

Question 8

What is SQL?

Answer 8

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

Question 9

Give examples of some SQL commands

Answer 9

ex. used LIKE
Others:
- select, where (query)
- insert, delete, update (editing)
- create table (management)

Question 10

What are the limitations of attribute-based queries?

Answer 10

• cannot answer important geographical questions (must combine with spatial queries)
• ex. what land parcels are adjacent to the site with contaminated soil?

Question 11

Explain Boolean operators

Answer 11

• 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

Question 12

Explain measurements

Answer 12

Different types

• diff b/w 2 points
• area

Tedious and inaccurate by hand - use GIS tools
- measurements often made on horizontal projections of objects

Question 13

What is buffering?

Answer 13

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

Question 14

What is overlay?

Answer 14

• 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

Question 15

What are the 2 aspects of vector overlay?

Answer 15

1. Geometric (shapes of everything)

2. Attribute Management (data merged; resulting table is combo of attributes)

Question 16

What is a point-on-polygon overlay?

Answer 16

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

Question 17

What is a line-on-polygon overlay?

Answer 17

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?

Question 18

What is a polygon-on-polygon overlay?

Answer 18

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

Question 19

What are the issues with overlay?

Answer 19

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

Question 20

What is a network?

Answer 20

Interconnected arcs that comprise a set of features through which resources flow

Question 21

What is a network analysis?

Answer 21

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

Question 22

What interconnecting logical components are network models based on?

Answer 22

• nodes: defines start, end, and intersections
• chains: line features joining nodes
• links: join together points to make up a chain

Question 23

What are some examples of network analysis applications?

Answer 23

• routing (buses)
• flow tracing (pollutant dispersion
• allocation functions (facility location
• facilities management (phone, computer network, etc.)

Question 24

What are 3 types of network analyses?

Answer 24

1. Network routing
2. Route tracing
3. Network location-allocation

Question 25

Explain network routing

Answer 25

- shortest path b/w 2 points (direction instructions, bus routing, deliveries, etc.)

Question 26

Explain route tracing

Answer 26

- trace flows of goods, people, info through network (especially uni-directional flows) - ex. model downstream pollutant

Question 27

Explain network location-allocation

Answer 27

- 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

Question 28

How can we represent field-based (continuous) phenomena/surfaces within vector data model?

Answer 28

1. Sample real world surface | 2. Rep field-based surface using vector objects

Question 29

Explain sampling for continuous surface rep.

Answer 29

- 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

Question 30

Explain how IDW is used for sampling continuous surfaces

Answer 30

- 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.

Question 31

Explain representing field-based surfaces in vector GIS

Answer 31

- 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

Question 32

What factors are useful in modelling surfaces?

Answer 32

- slope: steepness - aspect: compass direction - visibility: line of sight b/w 2 points

Question 33

Explain the TIN model

Answer 33

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

Question 34

How are TINs generated?

Answer 34

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)

Question 35

What are the advantages of the TIN model?

Answer 35

- few data points required - records more data for areas of complex relief - captures discontinuities well (ex. ridges) - slope and aspect easily recorded

Question 36

What are the disadvantages of the TIN model?

Answer 36

- requires significant processing power to generate | - difficult to combine with other map layers via overlay (lots of polygons involved)