2.1 Geospatial Data Models (Vector, Raster, Tabular, and TIN)
Key Takeaways
- Vector data models represent discrete features using coordinate pairs (points, lines, polygons) and include 3D multipatches for volume.
- Raster data models use a grid of cells (pixels) for continuous surfaces (elevation, temperature) or discrete categorical data (land cover).
- Triangulated Irregular Networks (TINs) provide an efficient vector-based representation of continuous 3D surfaces using non-overlapping triangles.
- LAS point clouds are industry-standard formats for managing massive datasets of 3D lidar points.
- Standalone tables contain tabular data (attributes) without spatial geometry but can be joined to spatial data.
Understanding geospatial data models is paramount for any GIS professional and is heavily tested on the ArcGIS Pro Associate exam. A data model defines how real-world geographic features and phenomena are digitally represented and stored. In ArcGIS Pro, the primary models include vector, raster, TIN, point cloud, and tabular data.
The Vector Data Model
The vector data model represents geographic features using discrete geometry consisting of x,y (and sometimes z,m) coordinate pairs. It is ideal for features with distinct boundaries such as parcels, roads, and utility poles.
Vector Geometry Types
- Points: Defined by a single x,y coordinate pair. Points represent features that have no area or length at the given scale (e.g., a fire hydrant, a city location).
- Lines (Polylines): Defined by an ordered sequence of vertices (x,y coordinates) connected by straight or curved line segments. They represent linear features like roads, rivers, and utility lines.
- Polygons: Defined by a closed sequence of x,y coordinates enclosing an area. Polygons represent features with boundaries and area, such as lakes, parcels, and political boundaries.
- Multipatches: A 3D geometry type that represents the outer surface, or shell, of features that occupy a discrete volume in 3D space. Multipatches consist of planar rings and triangles and are used for 3D buildings, geologic bodies, and atmospheric volumes.
[!NOTE] In ArcGIS Pro, vector features can store z-values (elevation) and m-values (linear referencing measures) at each vertex. Enabling z- and m-values is done during the creation of a feature class.
The Raster Data Model
The raster data model represents geographic phenomena using a continuous grid of cells (or pixels), where each cell holds a value representing information. Rasters are ideal for data that changes continuously across a landscape.
Continuous vs. Discrete Rasters
- Continuous Rasters: Represent phenomena that vary smoothly over space, such as elevation (DEMs), temperature, or precipitation. Cell values are typically floating-point numbers.
- Discrete (Thematic) Rasters: Represent categorical data with distinct boundaries, such as land cover, soil types, or zoning. Cell values are integers, and these rasters often have an associated attribute table.
Band Configuration
- Single-band Rasters: Contain one matrix of cell values. Examples include a single grayscale image or a Digital Elevation Model (DEM).
- Multi-band Rasters: Contain multiple spatially coincident matrices (bands) of cell values. A common example is a natural color aerial photograph (Red, Green, Blue bands) or a multispectral satellite image (e.g., Landsat) which may have 7, 8, or more bands.
Advanced 3D and Surface Models
Triangulated Irregular Networks (TINs)
A TIN is a vector-based representation of a continuous surface. It is composed of contiguous, non-overlapping triangles formed by connecting irregularly spaced sample points containing x, y, and z coordinates. TINs are highly efficient because they store more points in areas of complex, highly variable terrain and fewer points in flat areas, unlike rasters which have a uniform grid resolution. TINs are primarily used for elevation modeling, volumetric calculations, and line-of-sight analysis.
LAS Point Clouds
LAS is an industry-standard binary format for storing airborne lidar data. In ArcGIS Pro, LAS datasets allow you to quickly examine, display, and analyze lidar point clouds natively without converting them to another format. LAS points can be classified (e.g., ground, vegetation, buildings) and used to generate derived products like DEMs, DSMs (Digital Surface Models), or TINs.
Standalone Tables
Not all GIS data contains explicit spatial geometry. Standalone tables (e.g., CSV files, Excel spreadsheets, or dBase tables) store tabular information consisting of rows (records) and columns (fields/attributes). While they lack geometry, standalone tables are critical in GIS. They can be dynamically linked to spatial layers using Joins or Relates, provided there is a common key field. Furthermore, tables containing coordinate information (like latitude/longitude or x/y) can be converted into point feature classes using the XY Table To Point tool.
Comparing Vector and Raster
| Feature | Vector Model | Raster Model |
|---|---|---|
| Data Structure | Complex (coordinates and topology) | Simple (grid array) |
| Storage Size | Generally smaller | Generally larger (especially at high resolutions) |
| Spatial Precision | High (exact coordinates) | Depends on cell size (resolution) |
| Best Used For | Discrete boundaries (parcels, roads) | Continuous surfaces (elevation, imagery) |
| Analysis Strengths | Network analysis, proximity, overlay | Map algebra, surface analysis, spatial interpolation |
In practice, GIS workflows frequently combine these models. For example, you might use a vector polygon layer of watersheds to extract summary statistics from a continuous raster elevation model.
Which geospatial data model is most appropriate for representing a continuous surface such as temperature or elevation across a region?
What is the primary advantage of a Triangulated Irregular Network (TIN) compared to a raster Digital Elevation Model (DEM)?
Which 3D vector geometry type in ArcGIS Pro is specifically designed to represent the outer shell or surface of a discrete volume, such as a building?