Objects And Fields Flashcards
What are Discrete objects
Objects + spatial footprints = features
Points lines and areas (or volumes) having known properties
Characteristics of objects
Objects can be manipulated and edited
Objects can be found in the real world
Objects may overlap
Objects can be counted
Characteristics of fields
Things it’s worth measuring at almost every location on the planet
Fields contain variables that have one value everywhere
The value of the variable is a function of time and space
Field is a way of conceiving geography as a set of variables each having one value at every location on the planet
What does a geo-atom look like
What features represent variables?
Polygons Grids TIN Sample points Contours
What are the characteristics of representations of variables?
Entities is property of the representation, not the phenomenon
Cannot be manipulated
Cannot overlap
How does the conceptual model influence the data model
Given I conceptualise the lake as an object I use a vector model
Given I conceptualise the lake as a field I use a raster data model
What does the best representation depend on?
Application area (e.g. orientation) Supported operations (e.g. area) Costs (centroid vs polygons) Scale (cities as points)
What is the difference between models and rendering?
Conceptual models decide what representation we want, in this case objects
The data model is the type of representation, the this is a line
Representation renders at a certain width if matches certain characteristics
What is a cell?
A cell is the smallest distinguishable unit.
The better resolution the more information can be stored
How does topology help with constraint checking
Topology defined in the domain (housing can’t go into someone else’s land) therefore would be a digitised error
How do you store topology?
Points - point ID
Lines - begin in node and end node
Polygons - lines constituting a polygon ID
What is a tesselation?
tesselations is an exhaustive regular it irregular partitioning of space with no gaps or overlaps in most cases square cells of raster data and also other alternative classifications of tester data
Limitations of raster data
Winner takes all
KO-criterion
Requires a decision may change the interpretation
How to convert raster to run-length codes
9 9 6 6 6 6 6 7 6 6 6 6 6 6 6 6 9 9 6 6 6 6 7 7 9 8 9 6 6 7 7 5 Goes to 2:9, 5:6, 1:7 8:6 2:9, 4:6, 2:7 1:9, 1:8, 1:9, 2:6,2:7, 1:5
What are the issues with data compression
Data compression can dramatically reduce the amount of storage required to save data but at the same time increases processing costs
What is quadree compression?
Only store information at higher resolution that is necessary
In raster that is the smaller cells, in the data model the node is split further
What are the two main algorithms that convert vector to raster
Any cell rule - continuous but wider line
Near cell rule - thin line but may be discintinuous
How to convert from raster to vector
Raster to cell Center points to smoothed lines
- post processing required e.g to smooth line
Object data model
Owner has a relation to the parcel contained in a county
Which is the best data model?
Vector models produce small data sets
Topological relations are handled using vectors
Analysis is simpler on raster data in many cases
Overlays are simple in the raster model
TINs provide flexible facet size
Object models include relations and Hierarchies