SPATIAL DATA

                A GIS is a database that is specially designed to work with map data. This type of information is called SPATIAL DATA .  The geographic data in a project can consist of LOCATION, SHAPE and SIZE as spatial data and DESCRIPTIONS and ASSOCIATED DATA as nonspatial attributes.

Spatial data is classified as three main types:

-          Point – A point feature is a spot or location that has no physical or actual spatial dimensions, but does have specific location.

-          Line – A line is a one dimensional feature having only length, no width. It has a beginning and an end.

-          Polygon – A polygon is an enclosed area, a 2D feature with at least three sides and that has area and perimeter, such as parcels of land, agricultural field or political districts.

RASTER AND VECTOR DATA

Spatial data in GIS has two primary data formats.

RASTER

Raster uses a grid cell structure. Raster cell’s value or code represent all of the features with the grid, it does not maintain true size, shape, or location for individual features because of the square cells.

VECTOR

Vector is more like a drawn map. Vectors are data elements describing position and direction. In GIS, vector is the map-like drawing of features, without the generalizing effect of a raster grid. Where no data occurs.

RASTER CODING

Maps can be digitized or scanned at a selected cell size and each cell assigned a code or value. The cell size can  be adjusted according to the grid structure or by ground units(each cell = 30M), also termed resolution. There are three basic and one advanced scheme for assigning cell codes.

q    Presence/Absence      

q    Cell Center

q    Dominate Area

Presence/Absence

The most basic method is to record a feature if some of it occurs in the cell space. This is the only practical way of coding point and line features, because they do not take up much area of cell. 

Cell Center

    This involves  reading only the center of the cell and assigning the code accordingly. This is not a good scheme for point or lines, because they do not necessarily pass through the exact center.

Dominate Area

Assign the cell code to the feature with the largest(Dominant) share of the cell. This is suitable primarily for polygons, although line features could be  assigned according to which one has the most linear distance in a cell.   

Percent Coverage

A more advanced method is to separate each feature for coding into individual themes and then assign values that show its percent cover in each cell, can add more for the dominant feature and delete the area for those features not coded.

Raster Resolution

Increasing the number of cells on a data set increases spatial resolution, which help to increase spatial accuracy. This is usually done as part of the initial data entry stage for each map.

Raster Gridding

To see how the raster format can create spatial inaccuracies, note the actual shape of the river and then compare it to the two gridded versions. However, the purpose of a project usually determines data accuracy need and resolution. 

Vector Data

Vector features appear more realistic than raster features and have spatial accuracy. The three vector feature types are point, line, and polygon. Shape points are the ends and bends that define the feature’s outline. At the beginning and end of every line or polygon feature is a node. At each bend Vertex. A shape is recorded by using the coordinates of its shape points. Chains connect the shape points to draw the feature’s outline.

                Vector system data files store only the coordinates of each node and vertex.  

Raster and Vector data structure

Reasons to convert Raster to Vector

There are four basic reasons to convert from raster to vector;

  q  Despite the accuracy problems, the better visual appearance of vector features may be preferred.

  q   Some plotters work only on vector data

  q   Comparison with vector data is best when both data files have identical formats

  q   Modern GIS system accept both vector and raster data files, some old versions have either one 

        data type.

Raster Advantages

q  It is a relatively simple data structure: a grid with a single code in each cell

q   The simple grid structure makes analysis easier

q   Due to simplicity of raster formats, the computer can be low tech

q   Remote sensing imagery is typically obtained in raster format

q   Modeling is the creation of a generalized data file or a set of universal procedures to accomplish a certain GIS task

Raster Disadvantages

q  Spatial inaccuracies are common with raster systems

q   Because each cell tends to generalize a landscape, the result is relatively low resolution compared to the vector format

q   Each cell must have a code, even where nothing exists. That is, even No data must be coded, usually 0 value

Vector Advantages

q  The geographic data is more accurate and credible

q   Displays are more pleasing to the eye

q   Very high resolution

q   High spatial accuracy

q   It have storage advantages

q   The general public usually understands what is shown on vector maps

q   Vector data can be topological

Vector Disadvantages

q  Formats may be more difficult to manage than raster formats

q   Vector format require more powerful, high-tech machines

q   Due to better computers and increased management needs, vector formats very expensive

q   Learning the technical aspects of vector system is more difficult than understanding the raster format