GIS Data Models

1. GIS Data Models

2. Geographic information • Characteristics of Geographic Information • Location! • volume • Dimensionality • Point • Line • Area • Continuity • Feature • field

3. Building complex features • Simple geographic features can be used to build more complex ones. • Areas are made up of lines which are made up of points represented by their coordinates. • Areas = {Lines} = {Points}

4. Properties of Features • size • distribution • pattern • contiguity • neighborhood • shape • scale • orientation.

5. Basic properties of geographic features

6. GIS Analysis • Much of GIS analysis and description consists of investigating the properties of geographic features and determining the relationships between them.

7. GIS Capability • A GIS package should be able to move between • map projections, • coordinate systems, • datums, and • ellipsoids.

8. Draw a map of your favorite place in Texas.

10. GIS Data Models

11. Maps as Numbers • GIS requires that both data and maps be represented as numbers. • The GIS places data into the computer’s memory in a physical data structure (i.e. files and directories). • Files can be written in binary or as ASCII text. • Binary is faster to read and smaller, ASCII can be read by humans and edited but uses more space.

12. The Data Model • A logical data model is how data are organized for use by the GIS. • GISs have traditionally used either raster or vector for maps.

13. Two approaches to handling spatial data with GIS: • Raster model • Vector model • Points, • lines, • polygons

14. Features and Maps • A GIS map is a scaled-down digital representation of point, line, area, and volume features. • While most GIS systems can handle raster and vector, only one is used for the internal organization of spatial data.

15. Rasters and vectors can be flat files … if they are simple Flat File Vector-based line 4753456 623412 4753436 623424 4753462 623478 4753432 623482 4753405 623429 4753401 623508 4753462 623555 4753398 623634 Raster-based line Flat File 0000000000000000 0001100000100000 1010100001010000 1100100001010000 0000100010001000 0000100010000100 0001000100000010 0010000100000001 0111001000000001 0000111000000000 0000000000000000

16. A raster data model uses a grid. • One grid cell is one unit or holds one attribute. • Every cell has a value, even if it is “missing.” • A cell can hold a number or an index value standing for an attribute. • A cell has a resolution, given as the cell size in ground units.

17. Raster GIS • Raster Data Model • Rows and Columns of Cells (Array) • Area of Cell equals Spatial Resolution • Value for each cell records type of object or condition • Cells do not correspond to spatial entities in real world • A road is a group of cells, not a single entity • Cells are considered Homogeneous Units

18. Two approaches to handling spatial data with GIS: • Raster model • Vector model • Points, • lines, • polygons

19. Generic structure for a grid Grid extent Grid cell s w o R Resolution Columns Figure 3.1 Generic structure for a grid.

21. Definitions • Raster - A format for storing, processing, and displaying graphic data in which graphic images are stored as values for uniform grid cells or pixels. • Pixels - Abbreviation for picture element, the smallest indivisible element that makes up an image. In raster processing, data is represented spatially on a matrix of grid cells, called pixels, which are assigned values for image characteristics or attributes.

23. More Definitions • Resolution - A measure of the accuracy or detail of a graphic display, expressed as dots per inch, pixels per line, lines per millimeter, etc. • Spatial Resolution - The accuracy associated with the capture of ground information as reproduced in a digital format or graphic display. For example, 10-foot pixels vs. 100-foot pixels.

25. Definitions • Minimum Mapping Unit - The smallest element we can uniquely represent in our data.

26. Sources of Raster Data • Satellite data • LANDSAT • SPOT • Scanned aerial photography • Digital Orthophotography • Scanned maps and documents

27. From where do we get Raster Data? • SCANNED Aerial photographs • photographs are NOT raster images but SCANNED images ARE • SCANNED maps • Satellite images

30. From where are the data in a raster cell taken?

31. Why does it matter where the cell data come from? • It’s hard to tell just by looking at the image!

32. The mixed pixel problem

33. Grids and missing data Figure 3.8 GIS data layer as a grid with a large section of “missing data,” in this case, the zeros in the ocean off of New York and New Jersey.

34. Why use Raster? • Overlay Analysis/Overlay Operations • Arithmetic Operations • Addition • Subtraction • Division • Multiplication • Logical (Boolean) Operations • Where conditions occur or do not occur together • AND, OR, NOT, GT, LT, etc.

36. Raster GIS Applications • Integrate images to georeferenced data • i.e., parcel deed image linked to parcel centroid • Document Imaging • Natural Resource applications where: • Positional accuracy relaxed • Imagery-oriented

37. Raster Applications • Utility Corridor Siting • Environmental Mapping • Natural Communities Mapping • Forest resource planning • Spatial data variability decisions • Forest inventory • Wildlife habitat analysis

38. More Raster Applications • Wetlands Vegetation Inventory & Analysis • Agricultural analysis • Planetary analysis (including lunar) • Vector Updating • Digital Terrain Modeling • Flood Control & Emergency Preparedness • Communication System Engineering

39. Any Technology has Pro’s & Con’s

40. Raster Limitations • Aesthetics • Data storage requirements • Overlay operations performed on every cell • Sparse data sets require as much processing as dense ones

41. RASTER -- summary • A grid or raster maps directly onto a programming computer memory structure called an array. • Grids are poor at representing points, lines and areas, but good at surfaces. • Grids are good only at very localized topology, and weak otherwise. • Grids are a natural for scanned or remotely sensed data. • Grids suffer from the mixed pixel problem. • Grids must often include redundant or missing data. • Grid compression techniques used in GIS are run-length encoding and quad trees.

42. GIS Data Models

43. Rasters are faster, but... • Points and lines in raster format have to move to a cell center. • Lines can become fat. Areas may need separately coded edges. • Each cell can be owned by only one feature. • As data, all cells must be able to hold any cell value. • It is very difficult to precisely position features in space.

44. Vector GIS Data Model • Precisely position features in space • Points, Nodes, vertex, single X,Y coordinate pair • Lines, Arcs, series of X,Y coordinate pairs • Area, Polygons, area as a closed loop of X,Y coordinate pairs

45. Areas are lines are points are coordinates

46. The Vector Model • A vector data model uses points stored by their real (earth) coordinates and so requires a precise coordinate system. • Geographic Coordinate System • Latitude/Longitude • Cartesian Coordinate Systems • X,Y Coordinate system • State Plane • UTM (Universal Transverse Mercator) • Lines and areas are built from sequences of points in order. • Lines have a direction to the ordering of the points. • Polygons can be built from points or lines. • Vectors can store information about topology.

47. Raster/Vector Comparison

48. VECTOR • At first, GISs used vector data and cartographic spaghetti structures. • Collection of coordinate strings with no structure • Cartesian coordinates stored in data structure • No spatial relationships stored • Inefficient data storage technique • Vector data evolved the arc/node model in the 1960s. • In the arc/node model, an area consist of lines and a line consists of points. • Points, lines, and areas can each be stored in their own files, with links between them.

49. Arc/node map data structure with files 13 1 x y 11 e 2 x y l i 12 3 x y F 10 2 s 4 x y t 7 n 5 x y i 5 o POLYGON “A” 6 x y P 9 7 x y 4 8 x y 6 1 9 x y 2 10 x y 3 11 x y 8 12 x y 13 x y 1 File of Arcs by Polygon 1,2,3,4,5,6,7 1 A , Area, Attributes : 1,2 1,8,9,10,11,12,13,7 2 Arcs File Figure 3.4 Arc/Node Map Data Structure with Files.

50. The topological vector model uses the line (arc) as a basic unit. Areas (polygons) are built up from arcs. • The endpoint of a line (arc) is called a node. Arc junctions are only at nodes. • Stored with the arc is the topology (i.e. the connecting arcs and left and right polygons).