1. GEOG 60 – Introduction to Geographic Information Systems Professor: Dr. Jean-Paul Rodrigue Topic 3 – Geographical Data Structures A – Geographic Data Models B – Inputting Spatial Information

2. Conditions of Usage ■For personal and classroom use only Excludes any other forms of communication such as conference presentations, published reports and papers. ■No modification and redistribution permitted Cannot be published, in whole or in part, in any form (printed or electronic) and on any media without consent. ■Citation Dr. Jean-Paul Rodrigue, Dept. of Economics & Geography, Hofstra University.

3. Geographic Data Models ■1. Raster Representation ■2. Vector Representation ■3. The ArcView Shapefile Model A A

4. Spatial Objects ■Definition Delimited geographic areas. Ex. park, golf course, wildlife reserve. Associated attributes. Attributes are qualities or characteristics of spatial objects. Ex. census data, types of vegetation, heights or depths. ■Types of geographical delimitation Three conventional delimitations. Points, lines and polygons. 1 1 Grass, 1 feet Shrubs, 6 feet Forest, 25 feet

5. Raster Representation ■Issue A model for a computer to represent real geographical elements as graphical elements (on screen or on paper). ■Two models of representation possible: Raster (grid-based) Vector (line-based) 1 1 Representation

6. 1 1 Raster Representation ■Cellular organization Divides space in a series of units. Each unit is generally similar in size to another. Grid cells is the most common raster representation. Features are divided into cellular arrays. Coordinate (X,Y) is assigned to each cell, as well as a value. Allows for registration with geographic reference system. JPEG, GIF, BMP and TIF are raster formats. ■Tessellation Geometric shapes that can completely cover an area. Squares / Rectangles. Triangles. Hexagons.

7. Raster Representation Real world Grid Raster Point Line Area Value =0 =1 =2 =3 Row Column 1 1 Triangles Hexagons

8. Raster Representation ■Advantages Easy to conceptualize. Overlay operations are easy. A two-dimensional array forms a coverage. ■The problem of resolution For a small grid: Coarse resolution but limited storage space. For a large grid: Fine resolution but large storage space. 1 1 Size = 7x7x4 = 196 Cell = 10 m x 10 m = 100 m 2 Size = 10x10x4 = 400 Cell = 7 m x 7 m = 49 m 2 700 meters

9. The Mixed Pixel Problem 1 1

10. (X,Y) (X2,Y2) (X3,Y3) (X4,Y4) (X5,Y5) Line Vector Representation ■Concept Assumes that space is continuous, rather than discrete. Infinite (in theory) set of coordinates. ■Points Spatial objects with no area but can have attached attributes. A single set of coordinates (X and Y) in a coordinate space. ■Lines Spatial object made up of connected points (nodes). Have no width. ■Polygon Closed areas that can be made up of a circuit of line segments. Line segments that make up a portion of a polygon. 2 2 Point (X5,Y5) (X,Y) (X2,Y2) (X4,Y4) (X3,Y3) Polygon (X,Y)

11. Vector Representation 2 2 Node 1Node 2Node 3 Node 4 Node 5 Node 6 Node 7 Node 8 NodeXY 1234567812345678 12 22 53 24 5 36 17 38 4 16 42 17 9 43 21 44 1 23 4 5 6 7 8 ArcFromTo ABCDEFGHIJABCDEFGHIJ 64125473876412547387 41232558764123255876 Arc A Arc B Arc CArc D Arc E Arc F Arc G Arc H Arc I Arc J

12. A B C Vector Representation 2 2 5 1 23 4 6 7 8 A B CD E F G H I J Poly# of arcs Arc list ABCABC 445445 B,C,E,F A,F,G,J E,D,H,I,G

13. Vector Representation Pt. ID XY 1 2 3 4 5... 24.5 24.8 27.8 30.1 14.2... 27.4 24.1 22.5 29.9 30.1... Point Table Ln. IDPt. 1Pt. 2 35 36 37 38 39... 1 4 6 2 8... 3 2 8 10 11... Line Table Pol. IDLn. ID 74 75... 38 35 29 28 42... Poly. Table Pol. IDAttrib. 74 75 76 77 78... 104.2 100.1 105.7 102.7 106.1... Attrib. Table Relational Links 2 2

14. GIS Vector Models – 3 Major Models 2 2 Coordinate and Topological files Attribute tables Relational database Unique identifier Hybrid (Features) - Relational join – (Attributes) Relational database Integrated Element – Class – Attribute Object store Object-Oriented

15. The ArcView Shapefile Model ■Format Hybrid data model. Store spatial information in a vector format. Sequential list of features. Less processing intensive (faster drawing time). Two-dimensional (x,y) and three-dimensional (x,y,z) features supported. Each shapefile represent one shape type. Point, Polyline, and Polygon are the most common. Three major components: Main file (*.shp): Store the list of geographical features. Database file (*.dbf): Store the attributes in a table. Index file (*.shx): Links the database and geographical features. 3 3

16. a b c d The ArcView Shapefile Model Main file (*.shp) Index file (*.shx) dBase table (*.dbf) a b c d 3 3

17. The ArcView Shapefile Model id geomshp_len 101 102 103 104 105... 4507.4 3491.1 2321.8 682.9 1279.1... type 2 1 3 5 4... Predefined fields surface asphalt concrete asphalt gravel asphalt... width 85.3 45.1 75.9 35.2 60.3... lanes 4 2 4 2 4... name abc def ghi jkl mno... object identifier (optional) geometry custom fields geometry tracking field (optional) 3 3

18. Inputting Spatial Information ■1. The Input Subsystem ■2. Choosing What to Input ■3. Editing Vector Objects C C

19. Input Subsystem ■Digitizing Most difficult and time consuming task in mapping and GIS. Takes about 75% of the time in a mapping project. About 75% of the costs of operating a GIS system. ■Digitizing and errors Since digitizing is very time consuming, you must get it right on the first time. Error correction is excessively long and costly. The larger the file, the bigger the number of potential errors. Each model, raster or vector, requires special digitizing equipment. 1 1

20. Input Subsystem ■Mouse The mouse is the most basic input system. A rolling ball with two sensors, one of X, one for Y. It continuously sends a set of X,Y coordinates to the CPU. Buttons are sending interrupts to the CPU. By itself, it cannot be used to encode mapping information, but it is suitable for tracing. 1 1 X,Y Pointer

21. Input Subsystem ■Tracing and digitizing Tracing is a form of digitizing. Mainly imply using a scanned image. Easier for less experienced users. Precision is limited to the resolution of the scanned image. 1 1

22. 1 1 Input Subsystem ■Digitizer Table containing a matrix of very small cables. A mouse-like device, often called a puck (cursor), moves over the table. Creates an electromagnetic field disruption on the grid, the center of which is the X,Y coordinate. One of the most precise digitizing technique. Varies according to: Stability: Tendency of coordinates to change with temperature. Repeatability: With the same location, are the X,Ys exact? Linearity: Keeping up with movements of the cursor. Resolution: The smallest unit or measure it can handle. Skew: Differences between variation of Xs and Ys.

23. 1 1 Input Subsystem ■Scanner An horizontal light makes a pass and each line is read by a photoelectric cell (like a photocopy machine). Resolution: Number of pixels per units of surface the cell can read. 1,200 DPI (Dots per inch) is common. Color depth: Number of different colors the cell can read. 1-bit only supports B&W, 4-bit 16 colors (or shades of gray), 8-bit 256 colors and 12-bit 16.7 million colors. Two types: Drum scanners (rotating drum), Flat-bed scanners.

24. Choosing What to Input ■Some Rules Find what are your goals. Digitize the information you really need: Most base maps contain a lot of information. Try to choose a conventional source of spatial data. Use the level of accuracy corresponding to your task: High levels of accuracy equal high levels of diminishing returns. Input data as separate themes: Each theme should be specific. Each theme has a specific geographic feature (point, line or polygon). 2 2

25. Choosing What to Input 2 2 PointsLinesPolygons Store Locations Streets Highways Parks Type of Feature Information Themes

26. Choosing What to Input ■How Much to Input Choosing the right amount of information to encode is difficult. Depends on the level of accuracy. 2 2 Not enough Too ManyGood Solution

27. Base Map (Air Photograph) 2 2

28. Digitizing Major Roads and the Hydrography 2 2

29. Digitizing Lots 2 2

30. Final Product 2 2

31. Editing Vector Objects ■Points Simply changing the coordinate. Dragging and dropping the most common. Lines Changing the coordinate of one or more points. Splitting a line in two. Merging lines. ■Polygons Changing the coordinate of one or more points (the last point is also the first point). Splitting a polygon in two. Using a boundary to draw another polygon. Merging polygons. Creating an island in a polygon. Creating an intersection. 3 3

32. Editing Vector Objects 3 3 Moving a point (vertex) Splitting a line Intersection Merging lines Line A Line B

33. Editing Vector Objects 3 3 Moving a point (vertex)Splitting a polygon Polygon A Polygon B

34. Editing Vector Objects Using a boundary 3 3 Merging polygons

35. Editing Vector Objects Creating an island 3 3 Creating an intersection

36. Editing Vector Objects ■Snapping Make a vertex take the coordinates of a reference. Spanning tolerance defines the “search space”. Avoid overshoots and undershoots for lines. Avoid gaps and overlaps for polygons. ■Snap to Vertex Snaps the next vertex to the nearest vertex in an existing line or polygon. ■Snap to Boundary Snaps the next vertex to the nearest line segment in an existing line or polygon boundary. 3 3

37. Editing Vector Objects ■Snap to Intersection Snaps the next vertex to the nearest node common to two or more lines or polygons. ■Snap to Endpoint Snaps the next vertex to the nearest endpoint of an existing line. For lines only. 3 3