Friday, September 18, 2015

Data types used in GIS

Data types: GIS data is of two types.
i) Attribute data and
ii)Spatial data
Attribute data refers to the properties of spatial entities. It is also called aspatial data or non-spatial data or tabular data. Attribute data is defined as data that describe the characteristics of spatial features.
 Spatial data refers to location and is an element of reality. Spatial data is defined as data that describe the geometry of spatial features.

Data structure: The basic data structures for GIS are:
i) Vector
ii) Raster
iii) Triangulated Irregular Network (TIN) and
iv) Tabular information (attribute table)

Vector data structure consists of points, lines and polygons.
Points are a pair of (x,y) coordinates to specify a single location
Lines are defined by a set of coordinate pairs.
Polygons are sets of coordinate pairs that define the boundary of an enclosed area.
In vector data layers, the feature layer is linked to an attribute table. An individual feature is linked to a row (record) in the attribute table.

In a Raster data structure, the world is represented by an array of gridded cells. In this type of data structure, a point is described by one cell, a line or a polygon by a zone of cells. A raster grid can store values that represent categories. A grid attribute table has a value and a count field. The value field has a number representing information regarding the grid cell. The count field shows how many grid cells have the same value.
Grid cells can also store continuous values like elevation.
The main source of raster data is digital image photo or satellite imagery.
An example of raster analysis is neighbour cell analysis.

Comparison of Vector and Raster data structures
Vector data structures are used for:
i. Features with discrete shapes and boundaries
ii. Database management, Database query and reporting
iii. Network analysis and
iv. Generating high quality maps
Raster data structures are used for :
i. Continuous surfaces that change gradually over space (Eg: Soil, Land cover, Vegetation, Pollution)
ii. Spatial analysis and modeling (Eg: Agricultural suitability)

Triangulated Irregular Network (TIN) It is a three dimensional data structure for representing surfaces.
Examples of vector elevation data are:
i. Contour lines and
ii. Spot height points
Vector elevation data is good for visualization but cannot be used for analysis unless it is converted into a TIN.
TIN data structure is used for modeling small areas with high precision elevation data. It can use multiple data inputs. A TIN data structure provides more efficient storage than DEM or contour lines. It can be used to model:
i. Roads and road-cuts
ii. Dam construction and
iii. Urban flood modeling
A TIN is very effective in case of availability of high precision data.

Attribute Table is a flat file with columns and rows.
A row consists of ALL geographic features pertaining to a certain entity
A column consists of an item of information about a feature
The common attribute field types are:
i. Numeric (integers or decimals)
ii. Text (strings)
iii. Date and
iv. Binary Large OBject (BLOB)