Spatial data are computer representations of spatial features of the real world- Representation of Real World. A modeling language for a GIS database is a spatial data model. A spatial database holds a digital representation of the real world. Among spatial data models, we can distinguish two major types, field and object-based models.

  • Field-based models consider spatial phenomena to be of a continuous nature where in every point in space a value of the field can be determined. Examples of such phenomena are temperature, barometric pressure, or elevation.
  • Object-based models consider space to be populated by well distinguishable, discrete, bounded objects with the space between objects potentially being empty. Examples include parcels, buildings, and trees, among others

It is therefore common to see the geographical phenomenon of spatial features in terms of field, objects, and boundary- boundaries simply define the extent of interest. i.e Geographic phenomenon= Field, Object, Boundary

The primitives that are used to represent features in the object view comprise point, line, area/polygon, and volume items. It is often called the vector-based approach

In the field view, the part of space relevant to us is divided up completely into regular or irregular tessellations—usually squares, triangles, or cubes—that define a two- or three-dimensional raster or grid. The more common name for the field view approach is the raster-based approach

GIS Data Types

A GIS deals with spatial data (e.g., parcels, rivers, wells,), their attributes and characteristics (e.g., location, area, length, name, depth …), and the relationships between the objects (e.g., a parcel boundary follows a river, a well is located in a certain parcel …). The objects are stored in the database with geometric primitives (volumes, areas, lines, and points) and the relationships between them (topology). Spatial data have the following characteristics:

Table : Characteristics of spatial data

Source: Kainz, Wolfgang (2004)

It is common to equate GIS Data with three “triplex” characteristics- description, reference, and time). A description is an attribute, reference is the spatial reference/data, and time as the reference and description may change over time.

The basic data type in a GIS reflects traditional data found on a map and is currently digitally in GIS or spatial databases. Accordingly, GIS technology utilizes two basic types of data. These are: spatial data (location and geometry), attribute data (descriptions), and spatial relationships

Spatial data: describes the absolute and relative location of geographic features.

Attribute data: describes characteristics of the spatial features. These characteristics can be quantitative and/or qualitative in nature. Attribute data is often referred to as tabular data.

The coordinate location of a forestry stand would be spatial data, while the characteristics of that forestry stand, e.g. cover group, dominant species, crown closure, height, etc., would be attribute data.

Relationships (rules or behavior): topological relationships such as coincidence, adjacency, and connectivity.

Spatial Data Models (Formats)

Traditionally spatial data has been stored and presented in the form of a map. Three basic types of spatial data models have evolved for storing geographic data digitally. These are referred to as:

  • Vector
  • Raster
  • Image

The following diagram reflects the two primary spatial data encoding techniques. These are vector and raster.

  • Vector Model (Format) Vector model uses discrete points, lines, and/or areas corresponding to discrete objects with a name or code number of attributes.
  • Raster Model (Format) Raster model uses regularly spaced grid cells in a specific sequence. An element of the grid cell is called a pixel which contains a single value of attributes.
  • Image data utilizes techniques very similar to raster data, however, typically lacks the internal formats required for analysis and modeling of the data. Images reflect pictures or photographs of the landscape.

The data model represents a set of guidelines to convert the real world (called entity) to digitally and logically represented spatial objects consisting of attributes and geometry.