Lecture 4 Flashcards
What is GIS?
- “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)
What is Spatial data analysis?
- 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
Measurement
•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
Measurement of length
•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
Precision vs accuracy
Raster less precise
Inaccuracy through lack of precision (+error)
Vector always more precise
Inaccuracy through error
What are Buffers?
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
What is Euclidean Distance (Raster)?
- Raster output
* Each cell gives a distance to the closest source cell (or feature)
What are Thiessen Polygons?
- 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)
What is reclassification
•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
Reclassify attribute values
–To simplify data
–To analyse data
–Done differently in vector and raster
•Convert Townsend Deprivation Index to Affluent/Deprived
Queries
•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
Vector overlay
•Uses disciplines of:
–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)
Clip
- 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
Intersect
- Similar to clip, cuts out features which overlap
- Unlike clip will also copy the attributes all features
- Boolean operator = AND
Union
- Combines two features and their attributes
* Boolean operator
Raster calculations
- 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
Raster Calculations Examples
•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. > < ≥ ≤
No data indicates…
Why?
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
Masks
Use multiplication (AND Boolean operator) to restrict data to cells overlapping with mask
Raster Overlay
- 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
Terminology: Cartographic Model
•“… 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)
Generally, real-world problems are complex
–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
Flowcharting
•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?
