lecture 4 Flashcards
(25 cards)
what does geographic data refer to?
locations on earths surface or locations relative to the earths surface
what does spatial data refer to?
any locational frame or reference
what are the two types of geospatial features?
spatially continuous or spatially discreet
what are fields?
continuous variables; a feature that occurs everywhere eg. height, topography
what are some characteristics of spatially continuous features?
occurs everywhere, sometimes called fields, measurement anywhere in the domain.
what are some characteristics of spatially discreet features?
distinct spatial extent (boundaries/framework for feature), attributes only within the extent, sometimes called entities or objects (buildings, roads, rivers)
What are the ways discreet data can be represented in GIS?
grid, polygon, point, polyline
describe point features
single point, used to define single, identifiable locations on the earths surface (could be a point in a tree, in grass, on a building); single X, Y (cartesian coordinate)
describe polyline features
joined points, think things like roads, rivers, boundaries; two cartesian coordinates, start and end with nodes points
describe polygon features
looped/bounded by line area; think things like lakes, forests; closed loop of cartesian coordinates; start and end with node points, mostly simple in GIS so cannot overlap
describe grid features
represented as cells/pixels, in rows and columns, covering a spatial extent; think things like elevation, land cover, temperature. also images. rastewr format allows or
what is important to remember about line segments in vector data models?
even thought they may look smooth at lower scales, all line segments are straight
what is the benefit of using vector data models to store geospatial locations?
efficient as represents only the necessary points and connections.
what does topology refer to in spatial analysis?
spatial relationships between features; adjacency, touching, dis-jointness.
benefit of understanding topology?
allows for spatial queries/searches, which helps to identify specific things (connectivity of features like roads etc.)
why should polygons in vector data not overlap?
violates the Euclidean geometry principles; can lead to errors in spatial analysis. GIS normally has checks to prevent.
what are vector data models better at representating?
discreet data, so suitable for spatial analysis tasks
raster data models are suitable for what?
gridded data, satellite images/national land cover datasets
what can raster data represent?
continuous variables (elevation eg.) and categorical variables (land cover types eg.)
when is raster data memory intensive?
when very high resolution
how do nominal raster data sets work?
of categorical data, values represent different categories, but values do not have numerical meaning/order.
how do ordinal raster data sets work?
of categorical data, represent categories that can be ordered. the differences between medium, eg, and small may not be the same as between medium and large though.
how are different categories represented in discreet data encoding?
using specific values/codes
is each pixel specified in a raster grid?
yes; each pixel is associated with one discreet value which will correspond to a distinct feature/characteristic.
