1. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam WFM 6202: Remote Sensing and GIS in Water Management Akm Saiful Islam [Part-B: Geographic Information System (GIS)] Lecture-2: Data Model and Structure December, 2006 Institute of Water and Flood Management (IWFM) Bangladesh University of Engineering and Technology (BUET) WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam

2. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam Data Model A set of guidelines to convert the real world (called entity) to the digitally and logically represented spatial objects consisting of the attributes and geometry. Types of geometric data model –Vector Model -Model uses discrete points, lines and/or areas corresponding to discrete objects with name or code number of attributes –Raster Model - Model uses regularly spaced grid cells in specific sequence. An element of grid cell is called a pixel (picture cell)

3. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam Vector and Raster Model Vector model Raster model 256 color more colors

4. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam Example of vector based model

5. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam Example of raster representation

6. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam Comparison of Raster and Vector Data Model Advantages Raster modelVector model 1. It is a simple data structure1. It provides a more compact data structure that the raster model 2. Overlay operations are easily and efficiently implemented 2. It provides efficiently encoding of topology and as result more efficiently implementation of operation such as network analysis 3. High Spatial variability is efficiently represented in raster format 3. The vector model is better suited to supporting graphics that closely approximate Hand-drawing maps 4. The raster format is more or less required for efficient manipulation and enhancement of digital images

7. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam Raster modelVector model 1. It is less compact therefore data compression techniques can often overcome this problem. 1. It is a mode complex data structure. 2. Topological relationships are more difficult to represent. 2. Overlay operations are more difficult to implement. 3. The output of graphics is less aesthetically pleasing because boundaries tend to have a blocky appearance rather than the smooth lines of hand-drawn maps. 3. The representation of high spatial variability is inefficient. 4. Manipulation and enhancement of digital images cannot be effectively done in vector domain. Comparison of Raster and Vector Data Model Disadvantages

8. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam Geometry and Topology Geometry –Spatial objects are classified into point object such as meteorological station, line object such as highway and area object such as agricultural land, –which are represented geometrically by point, line and area respectively Topology –refers to the relationships or connectivity between spatial objects

9. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam Geometry of Vector Data - is given by point, line and area objects  Node  an intersect of more than two lines or strings, or start and end point of string with node number  Chain  a line or a string with chain number, start and end node number, left and right neighbored polygons  Polygon  an area with polygon number, series of chains that form the area in clockwise order (minus sign is assigned in case of anti- clockwise order).

10. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam Topological of Vector data Relationship between nodes and chains, the following topology should be built.  Chain : Chain ID, Start Node ID, End Node ID, Attributes.  Node: Node ID, (x, y), adjacent chain IDs (positive for to node, negative for from node). For relationship between polygons the following additional topology need to be built  Chain geometry : Chain ID, Start Coordinates, Point Coordinates, End Coordinates.  Chain topology : Chain ID, Start Node ID, End Node ID, Left Polygon ID, Right Polygon ID, (Attributes).  Polygon topology : Polygon ID, Series of Chain ID, in clockwise order (Attributes).

11. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam

12. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam Building Topology: (See also Figure 3.9)

13. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam Topological Relationships between Spatial Objects

14. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam Geometry of Raster Data - is given by point, line and area objects  Point objects A point is given by point ID, coordinates (i, j) and the attributes  Line object A line is given by line ID, series of coordinates forming the line, and the attributes  Area objects An area segment is given by area ID, a group of coordinates forming the area and the attributes.

15. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam - One of the weak points in raster model is the difficulty in network and spatial analysis as compared with vector model.  Boundary  Boundary is defined as 2 x 2 pixel window that has two different classes  Node  A node in polygon model can be defined as a 2 x 2 window that has more than three different classes Topological features of Raster Data

16. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam Example of Raster boundary

17. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam Attributes –Attributes are often termed "thematic data" or "non-spatial data", that are linked with spatial data or geometric data. –An attribute has a defined characteristic of entity in the real world. –Attribute can be categorized as normal, ordinal, numerical, conditional and other characteristics. –Attribute values are often listed in attribute tables which will establish relationships between the attributes and spatial data such as point, line and area objects, and also among the attributes

18. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam Layers Spatial objects in digital representation can be grouped into layers. For example, a map can be divided into a set of map layers consisting of contours, boundaries, roads, rivers, houses, forests etc. Map Layers

19. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam Overlay of Spatial data layers Two different object layers can be overlaid which can result another layers

20. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam Data Structure for Continuous Surface Model In GIS, continuous surface such as terrain surface, meteorological observation (rain fall, temperature, pressure etc.) population density and so on should be modeled Grid at regular intervals –Bi-linear surface with four points or bi-cubic surface with sixteen points is commonly used Random points –Triangulated irregular network (TIN) is commonly used. Interpolation by wieghted polynomails is also used. Contour lines –Interpolation based on proportional distance between adjacent contours is used. TIN is also used. Profile –Profiles are observed perpendicular to an alignment or a curve such as high ways. In case the alignment is a straight line, grid points will be interpolated. In case the alignment is a curve, TIN will be generated.

21. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam Different Types of DEM

22. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam A Continuous field example

23. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam Delaunay Triangulation for TIN

24. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam Example of Contour map & 3D view

25. WFM 6202: Remote Sensing and GIS in Water Management © Dr. Akm Saiful IslamDr. Akm Saiful Islam ESRI models Shapefiles – as non-topological data format. Shape file treats points are pair of x, y coordinates, a line as a series of points and a polygon as a series of lines. –Can be displayed more rapidly on monitors. –Interoperable among other software. Coverage – as topological based vector data format. A coverage can be point coverage, line coverage or polygon coverage.