spatial data model

• Spatial data models


  Store location along with a set of attributes, typically stored as coordinates in reference to a known location/origin
• Spatial data models


  • Store location information
  • Store descriptive attributes about the geographic features
• Vector vs Raster


  Two core data models used to represent spatial data
• TIN


  A unique type of spatial data model
• Cartographic data model/symbology


  Determines how spatial data is visually represented (e.g. point symbol, line style, polygon fill)
• Pyramiding


  Creating generalized versions of a detailed raster to improve display performance
• Topological relationships


  Spatial relationships between geographic features (e.g. adjacency, connectivity)
• Layers


  Collections of like geographic features, often limited to points, lines, polygons, and 3D models
• Projects


  Collections of related layers and other spatial data, stored together
• Spatial data examples


  • Monitoring wells with time series data
  • Facility locations and addresses
  • Continuous data like occupancy
• Data model


  The structure for describing spatial data, including the relational database tables
• Cartesian coordinate system


  Represents location using X and Y axes, can also include Z for 3D
• Latitude/Longitude


  A global coordinate system that divides the world into degrees, minutes, and seconds
• Decimal degrees


  A more precise way to represent latitude/longitude using decimal values
• The purpose of GIS is to support decision making
• Decimal degrees provide more precision for mapping a specific location compared to degrees, minutes, seconds
• Latitude is the positive/negative value representing north/south position, longitude is the positive/negative value representing east/west position
• Coordinate pairs are typically expressed as x,y with x being longitude and y being latitude, which is the reverse of how they are typically discussed
• Elevation is measured in real-world units like feet or meters, not degrees
• Measuring distances on a map is more accurate along the equator than near the poles due to map distortion
• Map projections are used to more accurately represent the curved earth on a flat map, but all projections involve some compromise
• Coordinate systems can also be based on measuring distance from the earth's center in 3D space
• Spatial data has both location information and associated attributes or descriptive information
• Quantitative attributes


  Numerical measurements or ratios that can be statistically analysed
• Spatial data typically links location information with associated attributes in a database
• Vector data model


  • Represents real-world features as simplified points, lines, areas, surfaces, volumes defined by coordinates
  • Implies precise mathematical location, limited only by number of bits used to store coordinates
  • Allows for precise calculations of length, area, volume
• Vector data may not accurately represent features with fuzzy or imprecise boundaries
• Point objects in vector data

  • Zero-dimensional, single coordinate pair or triplet
  • May represent a specific feature or just a vertex along a line
• Multi-point is a special type of point object that stores multiple geographic locations in a single record
• Point object


  Can represent a single location of vertex along a line, or a specific thing as a point in and of itself with descriptive attributes, or a node or point designating the start or end of a line
• Point object


  • May have attribution
  • A multi-point is a single record with a collection of many geographic locations
• Line object


  A 1D object composed of two or more point objects, can be straight or curved, can represent a connection or a directional link
• Directed link or chain linked directional objects


  • Transportation networks
  • Stream networks
• Area object


  A 2D object, a combination of strings that close to create a boundary and a totally closed loop, can be simple or complex with islands, rings or holes
• Raster model


  A gridded representation of the landscape where each cell or pixel is given a value, can represent continuous data or discrete data
• Raster model


  • Space filling, location is implied by the grid origin and cell size, can have multiple attributes per cell
• Continuous raster


  Has an unlimited number of floating point values, each cell is unique
• Discrete raster


  Has a limited number of integer values, cells with the same value represent the same class
• Raster data is typically larger in file size but faster for spatial analysis compared to vector data
• Vector data


  • Better for defined boundaries, required for network analysis and topology, produces crisper cartographic output