EAT 233/3 GEOGRAPHIC INFORMATION SYSTEM (GIS) CO3: Ability to produce detail mapping using geographic information systems (GIS) BY : AYU WAZIRA AZHARI
SPATIAL DATA & THE MODELLING
Spatial Data in GIS Spatial data in GIS has two primary data formats (the arrangement of data for storage or display): i.raster Generalizes the scene O OO 2 1 1 1 O2 1 1 1 into a grid cells, each 1 OO2 1 1 with a code to indicate 1 11 1 O1 the feature being depicted ii.vector Has polygons that appear normal, much like map OO 2 O0 2 1 1 1 1 1 O2 1 1 1 OO2 1 1 1 11 1 1 O 1
Raster & Vector Data Models In both (raster & vector) models, the spatial information is represented using finite, discrete homogeneous units. In the raster model, the homogeneous units are grid cells (or pixels). In the vector model, the homogeneous units are points, lines and polygons.
Figure 1: Raster: Grid cells ; Vector: Points, lines, polygons
Raster Data Model Vector Data Model Raster data model uses an array of cells, or pixels, to represent real-world objects. Because the raster cell s value or code represents all of the features within the grid, it does not maintain true size, shape, or location for individual features. Even where nothing exists (no data), the cells must be coded. Most GIS themes depict only the necessary features in area; showing everything on the landscape would be confusing. By definition, 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. The lines are analog, which means they are not broken into cells or fragments, but continue from start to finish in a continuous manner. Therefore, shape is better retained, much like an actual map.
Figure 2: Real world scene of raster and vector models
Raster Coding In the raster world a range of different methods is used to encode a spatial entity for storage and representation in the computer. Figure 3 shows the most straightforward method of coding raster data.
Figure 3: Feature coding of cells in the raster world
Resolution The minimum linear dimension of the smallest unit of geographical space for which data are recorded. In the raster model, the smallest units are rectangular (for most systems). The smallest units are known as cells or pixels. The array of cells is known as lattice, grid or matrix. High resolution refers to raster with small cells.
Gridding A higher density of cells in a raster system usually implies more accurate measurements. The size of the raster cells is therefore important. Figure 4 shows the effects of grid size than can create spatial inaccuracies.
Figure 4
Vector Data Based on vectors The fundamental primitive is points. Objects are created by connecting points with straight lines (or arcs). Areas are defined by sets of lines.
Figure 5: Vector
Raster & Vector Structures Raster and vector structures have different methods of storing and displaying spatial data. Figure 6 shows the difference between raster and vector data structure
Figure 6: The difference between raster and vector data structure
Comparision between Raster & Vector RASTER VECTOR Advantages: Advantages: 1. It is a simple data structure. 2. The simple grid structure makes analysis easier. 3. Because of the relative simplicity of raster formats, the computer platform can be low tech and inexpensive. 4. Remote sensing imagery is typically obtained in raster format easily integrated into a raster format GIS because of the identical data formats 1. The geographic data is more accurate and credible than the raster format. 2. Vector data is very high resolution. 3. The high resolution supports high spatial accuracy. 4. Vector formats have storage advantages. 5. The general public usually understands what is shown on vector maps. 6. Vector data can be topological. Disadvantages: 1. Spatial inaccuracies are common with raster systems. 2. Because each cell tends to generalize a landscape, the result is relatively low resolution compared to the vector format. 3. Because of spatial inaccuracies caused by data generalization, a raster format cannot tell precisely what exists at a given location. 4. Each cell must have a code, even where nothing exists. That is, even NO DATA must be coded, usually 0 value. Disadvantages: 1. It is a more complex data structure than a simple raster. 2. Vector formats require more powerful, hightech machines. 3. The use of better computers, increased management needs, and other considerations often make the vector format more expensive. 4. Learning the technical aspects of vector systems is more difficult than understanding the raster format, particularly when topology is introduced.