RASTER DATA & VECTOR DATA IN REMOTE SENSING & GIS
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GIS Data Types: There are two types of data are used in GIS platform; that is, spatial data and non-spatial data.
Spatial data are those that have coordinates: latitudinal and longitudinal that shows position of feature. It represents the location of geographical entities as well as spatial dimension that are represented with the help of point, line and polygon/area. The spatial data further divided into two types; Raster data and Vector data.
While on the other hand non-spatial data are those representing a set of information that is systematically organized and computing against spatial data. These types are also known as attribute data.
For instance, if the spatial data contain a polygon representing a state, than in attribute data it has information about its administrative division; like area, population etc. The non-spatial data can be two types; Statistical which have numerical value and Descriptive that are stored in the form of word or text.
Raster Data & Vector data
Raster Data: A raster consists of a matrix of cells (or pixels) organized into rows and columns (or a grid) where each cell contains a value representing information, such as temperature.
The raster data are the data that have individual pixels where each pixel has its spatial location in referenced to real earth. When the data is geo-referenced the data give each and every pixel its locational information. Thus the attribute is represented as a single value of each pixel or cell that is called as DN (Digital Number Value).
Raster Data Structure:
Raster data comes in the form of individual pixels. It is represented by matrix or grid of pixel. Each pixel preserves locational information. This type of spatial data usually bulky and required large storage capacity.
Example of raster data as; satellite imagery, digital elevation model (DEM), aerial photography, Scanned maps etc.
All pixels in a raster data must be the same size, determining the resolution. The pixel can be any size, but they should be small enough to perform maximum detail analysis. A pixel or cell represent a square kilometer, a square meter or even a square centimeter.
Pixel or cell are arranged in rows and columns that construct a certain matrix format. The rows of the matrix are parallel to X – axis and the columns are parallel to the Y-axis.
Advantages to Raster Data:
- It is very simple data structure.
- Compatible with remotely sensed imagery
- Continuous features are best represented using raster.
- Overlay analysis is easy to perform with raster model.
Disadvantages to Raster Data:
- The use of large cells to reduce data volumes means that phenomenologically recognizable structures can be lost and there can be a serious loss of information
- The raster maps are considerably less beautiful than line maps
- Network linkages are difficult to establish
- Projection transformations are time consuming and difficult.
Vector Data: A representation of the world using points, lines, and polygons. Vector models are useful for storing data that has discrete boundaries, such as country borders, land parcels, and streets.
In the vector data, the spatial information are recorded as x, y coordinates.
The point features are recorded as single x and y pair of coordinates.
The line features as well as polygons features are recorded as a series of x and y coordinates. Thus the vector attributes recorded against feature ID numbers are assigned by system itself.
Vector Data Structure:
Vector Data are in the form of point, line and polygon that are recorded in Spatial information as x and y coordination.
There are 3 ways of representation:
- It has 0 dimension (cannot represent neither length nor width)
- Represented by single x y coordinate pair
- It has area size zero.
- Mostly used for denotes a single particular feature. (Town, Power pole)
Point Vector Data Type: Simple XY Coordinates
- It has 1 dimension (can represent the length)
- Represented by connecting two or more pairs of x, y coordinates
- It has its length value
- Commonly used to demarcate roads, rivers, stream and so on.
Vector Data Type Line: Connect the dots and it becomes a line feature
- It has 2 dimension (can represent the length as well as the width)
- Represented by connecting four or more pairs of x, y coordinates
- The starting point should be the ending point.
- Preserve an area.
- Commonly used to demarcate features having closed boundary (National Parks, Agricultural field)
Vector Data Type Polygon: Connect the dots and enclose. It becomes a polygon feature
Advantages of Vector Data:
- Good representation of phenomenology
- Easily makes connection between topology and network.
- Accurate graphics
- Easily for making projection and coordinates transformation.
- Retrieval, updating and generalization of graphics and attributes possible.
Disadvantages of Vector Data:
- Complex data structure
- Simulation may be difficult.
- Display and plotting can be expensive, particularly for high quality color.
- Spatial analysis and filtering within polygons are impossible.
Raster and Vector File Format:
Raster File Format
Vector File Format
Cartesian coordinate system
Keyhole Markup Language KML
MapInfo TAB format
Spatial Data File