Lecture 4

Question 1

What is GIS?

Answer 1

• “a system for capturing, storing, checking, integrating, manipulating, analysing and displaying data which are spatially referenced to the Earth” (DoE, 1987, p.132)
• “a set of tools for collecting, storing, retrieving at will, transforming, and displaying spatial data from the real world for a particular set of purposes” (Burrough and McDonnell, 1998, p.11)

Question 2

What is Spatial data analysis?

Answer 2

• Key strength of GIS
• Adds value to the data
• Turns data into usable information
• Need to consider the nature of the data and choose an appropriate analysis to get the most from the data

Question 3

Measurement

Answer 3

•Distance
•Area
•Perimeter
•Calculated using geometry in vector
•Calculated by counting cells and multiplying by cell size in raster
–Accuracy problems caused by cell size, origin and orientation

Question 4

Measurement of length

Answer 4

•Pythagorean:
–straight-line (Euclidean) distance between two points on a flat plane
•Great Circle:
–gives the shortest distance between two points on a spherical globe, given their latitudes and longitudes
•Manhattan:
–Linked to raster data. The sum of horizontal and vertical components when measuring a diagonal

Question 5

Precision vs accuracy

Answer 5

Raster less precise
Inaccuracy through lack of precision (+error)
Vector always more precise
Inaccuracy through error

Question 6

What are Buffers?

Answer 6

Polygons surrounding input features or layers at a specified distance or distances
–Fixed distance
–Variable distance
–Individual features
–All features (individual buffers dissolved)
•Buffers in raster data can be achieved by calculating Euclidean distance

Question 7

What is Euclidean Distance (Raster)?

Answer 7

• Raster output

* Each cell gives a distance to the closest source cell (or feature)

Question 8

What are Thiessen Polygons?

Answer 8

• Polygons defining the area closest to each point from a set of points
• Defined mathematically using Euclidean geometry
• Named after American meteorologist Alfred H. Thiessen (1872-1931)

Question 9

What is reclassification

Answer 9

•Creation of new output layer based on values in input layer
–one layer in, one layer out
•Input: may be categorical or continuous
•Output: generally categorical
•Reclassifying data into intervals or categories
•Simplifies data
•May be needed for further calculations
•For viewing purposes, the same effect can be achieved by changing the symbology without changing the data

Question 10

Reclassify attribute values

Answer 10

–To simplify data
–To analyse data
–Done differently in vector and raster
•Convert Townsend Deprivation Index to Affluent/Deprived

Question 11

Queries

Answer 11

•Attribute Query
–What is..?
–ArcMap: Select by Attribute

•Spatial Query
–Where is..?
–ArcMap: Select by Location
•Useful for finding information from the data
•Useful for checking data quality

Question 12

Vector overlay

•Uses disciplines of:

Answer 12

–Geometry
–Topology
•Allows comparison of layers which occupy the same geographic space
•Can be used to select features (select by location)
•Can be used to create new data (geoprocessing tools)

Question 13

Clip

Answer 13

• Extract input features that overlay the clip features
• Clip features act as a ‘biscuit cutter’
• Does not transfer attributes from the clip feature
• Useful for creating a new feature of a particular area of interest

Question 14

Intersect

Answer 14

• Similar to clip, cuts out features which overlap
• Unlike clip will also copy the attributes all features
• Boolean operator = AND

Question 15

Union

Answer 15

• Combines two features and their attributes

* Boolean operator

Question 16

Raster calculations

Answer 16

• Calculations performed on raster data
• Can be between a raster dataset and a number
• Can be between a raster dataset and another raster dataset
• Each calculation produces a new raster dataset

Question 17

Raster Calculations Examples

Answer 17

•Arithmetic
–E.g. + - x ÷
•Mathematic
–E.g. Power, square root, log
•Boolean
–AND, OR, NOT
•Trigonometric
–E.g. Sin, Cos, Tan, Asin etc.
•Can be conditional
–E.g. > < ≥ ≤

Question 18

No data indicates…

Why?

Answer 18

a cell without a value
0 is a value
Exclude it from calculations
Missing data – E.g. from radar shadow
Masks – can be used as a raster equivalent of a clip feature

Question 19

Masks

Answer 19

Use multiplication (AND Boolean operator) to restrict data to cells overlapping with mask

Question 20

Raster Overlay

Answer 20

• Combining data from different raster datasets
• Similar results to vector overlay processes, e.g. intersect, union, clip
• Only one set of value attributes can be stored in a raster dataset
• Combining different layers and understanding the results requires careful consideration

Question 21

Terminology: Cartographic Model

Answer 21

•“… the term coined … to designate the process of using combinations of commands to answer questions about spatial phenomena” Demers (1997, p353)
•
•“a set of interacting, ordered map operations that act on raw data, as well as derived and intermediate map data, to simulate a spatial decision making process” Tomlin (1990)

Question 22

Generally, real-world problems are complex

Answer 22

–E.g. How many children live within 5km of the runway?
•Cannot answer with a single GIS operation
–Multiple inputs and operations needed
•Logical, specific order, hierarchical structure

Question 23

Flowcharting

Answer 23

•Integral to cartographic modelling
–Use to PLAN approach and COMMUNICATE methodology
•Generic procedure
–Start with question
–Work backwards: data, operations
•What data do you need?
•How will the operations be implemented?