UNDERSTANDING OF SPATIAL DATA MODELS AND THEIR ASSOCIATED PLANAR GEOMETRIES
OVERVIEW
A solid grasp of spatial data models and their underlying geometries is fundamental to effective GIS analysis and data management. This sub-section explores how real-world geographic phenomena are abstracted into digital representations—primarily through vector and raster data models. Candidates will examine the geometric foundations of points, lines, and polygons (planar geometries) as well as the implications of topological versus non-topological models. Understanding how spatial entities are structured, stored, and related within different data models enables GIS professionals to select appropriate methods for analysis, editing, and visualization. Mastery of these concepts is essential for building accurate, efficient, and interoperable geospatial systems.
KEY CONCEPTS AND TERMINOLOGY
- Spatial data model - Basic properties and process for a set of spatial features
a. According to Bolstad:
• Cartographic Models – temporally static, combined spatial datasets, operations, and functions for problem-solving.
• Spatio-temporal models – dynamics in space and time, time-driven processes
• Network models - modeling of resources (flow, accumulation) as limited to networks.
b. According to Goodchild:
• Data models - entities and fields as conceptual models
• Static modeling - taking inputs to transform them into outputs using sets of tools and functions.
• Dynamic modeling - iterative, sets of initial conditions, apply transformations to obtain a series of predictions at time intervals.
c. According to DeMers:
• Based on purpose descriptive - passive, description of the study area prescriptive - active, imposing best solution
• Based on methodology stochastic - based on statistical probabilities deterministic - based on known functional linkages and interactions
• Based on logic inductive - general models based on individual data deductive - from general to specific using known factors and relationships
B. Vector - coordinate based data model that represents points, lines, and polygons.
a. Points – discrete locations on the ground
• Represented by a coordinate pair.
b. Lines – linear features, such as rivers, roads, and transmission cables
• Composed of vertices
• Begin and end at vertices.
• Represented by an ordered list of vertices.
c. Polygons – form bounded areas, such as islands, land masses, and water features.
• Composed of nodes and vertices
• The start node is the same as the end node.
d. Attributes associated with each feature.
C. Raster - composed of rectangular arrays of regularly spaced square grid cells and each cell has a value (attribute)
• Examples include soil pH, elevation, and salinity of a water body.
• Single or multiple bands
• Each cell typically has 1 attribute value, except for multi-dimensional raster data.
• Multidimensional raster data represents data captured at multiple times, depths, or heights. It is commonly used in atmospheric, oceanographic, and earth sciences.
Sources and Formats:
o Satellite observations: Data collected at specific time intervals.
o Numerical models: Data generated by aggregating, interpolating, or simulating from other data sources. Common storage formats include:
o netCDF: Often used for oceanographic data.
o GRIB: Commonly used for weather data.
o HDF: NASA frequently uses this format for scientific data storage.
o Esri Cloud Raster Format (CRF): Also supports multidimensional raster data storage.
• Raster coordinates are stored by ordering the matrix.
D. Pixel - smallest resolvable piece of scanned image - pixel is always a cell but a cell is not always a pixel.
E. Geodatabase - object oriented spatial model (feature classes, feature datasets, non-spatial tables, topology, relationship classes, geometric networks)
• Basic components include feature classes, feature datasets, non-spatial tables.
• Complex components include topology, relationship classes, geometric networks.
• Relationship classes – model real-world relationships that exist between objects such as parcels and buildings.
F. GRID - A grid is a structured arrangement of data points or values in equally spaced rows and columns, also known as raster data. It’s commonly used to organize and analyze data, especially in fields like geography, meteorology, and computer graphics. It is often used to represent features on the Earth’s surface, such as elevation, land cover, temperature, precipitation, and more. Geospatial data is typically organized into grids where each cell corresponds to a specific location.
G. TIN - Triangulated Irregular Network - portions vector data into contiguous, non-overlapping triangles
• Create Delaunay triangles.
• Advantages of TIN - small areas with high precision elevation data. More efficient storage than DEM or contour lines
• Disadvantage of TIN - it requires very accurate data sources and costs are expensive;TIN production and use are very computer intensive)
H. Topological - features need to be connected using specific rules.
I. Hierarchical - database that stores related information in a tree-like structure.
• Records can be traced to parent records to a root record.
J. Network - collection of topologically connected network elements (edges, junctions, turns)
• Each element is associated with a collection of network attributes.
K. Object Oriented - data management structure stores data as objects (classes) instead of rows and tables as a relational database
• Examples include SQL Server, Oracle, PostgreSQL