Lecture 3 Flashcards
Data types
Nominal
- Establishes identity
- Not used for mathematical calculation
- E.g. postcodes, soil type/ other land cover types
Data types
Ordinal
- Establishes a comparative order or ranking, still don’t have a numeric to give an exact quantity but gives an indication of what is ‘better’ or ‘worse’
- A is greater/better than B
- No accurate relative information
- How much greater is A than B?
- E.g. Best counties to live in
Data types
Interval
–Allows calculation of differences •A is 10 points greater than B –No origin value •Cannot say A is twice as great as B – not relative to an origin value –E.g. Temperature
Data types
Ratio
–Have absolute or real zero, example is age (everyone starts from same point_
–Differences are significant and calculations predictable, put them into models, Jane is twice as tall as Sam etc.
–E.g. age, distance
Raster Data (Pixel) Types in ArcGIS
Integer – Whole Numbers
–Best used to represent discrete data (thematic or categorical).
–E.g. land cover
Raster Data (Pixel) Types in ArcGIS
Floating Point
–Best to represent continuous data (surface data)
–E.g. elevation, air pollution- can have a range of values
Raster Data (Pixel) Types in ArcGIS
For both data types:
- Integer – Whole Numbers
- Floating Point
–Unsigned means all numbers are positive
–Signed can be negative
–No Data values – the absence of a value (not a value of zero!)
Symbology
atch with real world for comprehension
By symbol e.g plane for airport
By colour e.g blue for river
Point data can use arbitrary symbols or icons
Ordinal data should have logical progression
Bigger road- thicker line
Different types of symbology:
Based on data type
Map purpose
Clarity
Raster symbology
Interval and radio data
Colours / shading show sensible progression
High to low
Reinforced with legend
Map types and data classifications
Two main map types
•Topographic
–Reference maps containing a diverse set of information
–Can be a composite of different information
Some information can be left out, information useful for purpose will be left in…
•Thematic
–Relates to a particular theme or topic
–E.g. population, crime, income, land use
Proportional symbol
E.g Earthquake intensity
Quantitative point data
Choice symbol size range- you can pick the range size
Dot density
•Used to show spatial distribution
•Dot usually indicates a count of the variable
More dots in an area the higher the average income
Different colours can represent ethnicity, also.
–Aggregate location
•Can map different categories
Choropleth
•Main type of map for quantitative area data
–Choice of intervals (number and size)
–Choice of shading
–Choice of spatial unit
Equal-Interval
•Data divided into a chosen number of equal intervals or classes •Simple for dividing up the data Clear distinction in bands •Can give unequal distributions •Can give empty classes
Quantile
- Data divided so an equal number of values fall in each class
- Quartile = 4 classes (Quintile = 5 classes)
- Can give irregular intervals
- No indication of frequency distribution of data
Natural Breaks (Jenks)
- Divides into classes according to natural breaks between groupings of data
- ‘Common sense’ method
- Reliant on data providing clear natural breaks
Standard Deviation
- Divides data according to distance from the mean (standard deviation)
- Intervals determined from the data
- Can give unusual class boundaries
Why are paper maps still useful?
But…
–Transportable
–Reliable
–Easy to use
•But…
–Fixed scale/extent
–Static view
–Flat
Why is GIS good?
•GIS more flexible –‘seamless’ –Can be dynamic (animated) –3-D visualisation –Interactive (layers added/removed)
Map design
•Purpose or agenda –What will be mapped –Target audience •Representing reality –Projection- means of getting 3D data sets onto a map –Spatial referencing –Features and data types –Scale –Generalisation –Symbology –Annotation
Scale
Large scale
Covers small areas
Large amount of detail
1:1000
1:25,000
Scale
Small scale
•Covers large areas
•Small amount of detail
–1:250,000
–1:1,000,000
Ratio
Units do not matter as long as they are the same.
Map composition
- Map body
- Inset/Overview map
- Title
- Legend
- Scale
- Direction (North Arrow)
- Map metadata (projection info, date, author)
- Copyright
- Reference grid
Map Presentation
•Clear, descriptive title with key information:
–Location
–Content
–Date
•Map body should be largest element on the map – Zoom map content to fit.
•If using a map template, do not leave unused placeholders, e.g. ‘Click to enter map title’!
•Think about who will see it and how they will use the map.
Legend
- All items in the legend should be visible on the map
- Plain English layer names in legend – not filenames or field names
- Number of decimal places on legend labels
What is best used to represent discrete data (thematic or categorical)?
–E.g. land cover
Integer – Whole Numbers
What is best used to represent continuous data (surface data)
–E.g. elevation, air pollution- can have a range of values
Floating Point
