Vector Data Model

1
GEOG 202-Introduction to GIS
Md. Khalequzzaman, Ph.D. Dept. of Geology
Physics Lock Haven University Lock Haven, Pa 17745
2
Components of GIS

• Hardware (PC-based or workstations)
• Software (e.g. ESRIs ArcView, ArcEditor, ArcInfo
  w/in ArcGIS each of which contains ArcMap,
  ArcCatalog, and ArcTool and other software)
• Brainware (people) knowledge, training,
  development, research, implementation of goals
• Infrastructure data, organizations, institutes
  to support GIS (70-80 cost)

3
Disciplines Partnering with GIS

• Cartography (basis of GIS)
• Remote Sensing
• Geosciences/Natural Sciences
• Math/Statistics
• Computer

4
Steps in Building Data

• Geographic spatial features (real-world viewed by
  users)? Converted into geospatial relations
  (points, lines, areas)?Logical data structure
  (non/topology) created?Vector/Raster data model ?
  Computer software (GIS) used by users to portray
  the real-world features in simplified maps and
  relational databases

5
Vector vs. Raster Data Model

• Discrete features
• Points, lines, areas
• Can have topological (coverage) or
  non-topological (shape files)
• Can be simple or higher level (TIN)
• Can have more attribute data

• Continuous features
• Uses grid grid cells to model spatial
  variations of a feature
• Simple data Structure (row columns) with cell
  locations unique value
• Easy to analyze
• Minimum attribute data

6
Vector Data Model

• Data model is a logical means of organization of
  data for use in an info system (GIS)
• Vector data have X, Y, coordination of spatial
  features
• Have spatial relations recorded (i.e. beginning,
  ending, left/right, etc.)
• Data Types points, lines (series of open-ended
  points points/nodes), and areas (series of closed
  lines)
• We as users try to model spatial features in form
  of database that can be stored, manipulated,
  analyzed using GIS software

7
More on Vector Data Model

• Lines and areas are built from sequence of points
  in order
• Lines/areas have a direction to the ordering of
  points (arcs and nodes)
• Vector can store information about topology
• Vectors can represent features very accurately
• Vectors are far more efficient than raster data

8
Why Topology Matters

• Topology allows automated error detection and
  elimination
• Need to build/clean topology for
  digitized/scanned maps
• Unconnected nodes are snapped
• Slivers from digitizing are eliminated
• Topology makes map overlay feasible
• GIS operations are done w/o accessing the point
  files

9
Spatial Data Types
Ref Introduction to GIS by K. Chang
10
Topology
Ref Introduction to GIS by K. Chang
11
Non-Topology
Ref Introduction to GIS by K. Chang
12
Format Exchanging of GIS Data

• Can import different data formats or use utility
  programs (e.g Import 71) to convert data
• DLG and TIGER data are vector data available in
  SDTS formats
• SDTS facilitate data transfer in US since 1992
• GIS accept TIF, GIF, JPEG, AutoCAD data formats
  (needs right extensions) which are not
  georeferenced
• DEM DOQ are in raster format
• Vector to Raster conversion is easy, opposite is
  hard
• Data exchange by import/export involves errors in
  attributes geometry
• Efficient data transfer is important for the
  future of GIS

13
Vector Data Input

• Finding and/or inputting data is the most time
  consuming task
• Two Sources of GIS Data
• Existing Data (Internet)
• Data may need pre-processing (projection)
• Each State has a GIS data clearinghouse
  (www.pasda.psu.edu)
• Fed.Geog.Data.Comm (FGDC) USGS (DLG LULC),
  USEPA, PA DEP are other examples
• Private vendors (GeoComm.com) U of MD
  mapmart,com landinfo.com, etc.
• ESRI has free modules
• Metadata Very important to read/print

14
Metadata

• Data about data
• Projection info
• Data quality info
• Data currency organization
• Info about how to transfer data
• Contact info

15
Creating New Data

• Remotely sensed data (e.g. DOQ)
• Georef 1m x 1m resolution good as a background
  map for other themes

Ref Getting Started with GIS, by K. Clarke
16
Creating New Data (Contd)

• GPS differential GPS have cm-scale resolution
• Measures ranges (distances of receivers by 24
  NAVSTAR satellites using radio signals atomic
  clocks)
• Ref. or base stations can reduce noise
• Diff. GPS comes w/ software to download data
• Hand-held GPS data can be downloaded too

Ref Getting Started with GIS, by K. Clarke
17
Creating New Data (Contd)

• Field Survey Data
• defines parcels by azimuth distances
• Uses Total Station, Stadia, Laser Ranging, and
  COGO software

Ref Getting Started with GIS, by K. Clarke
18
Creating New Data (Contd)

• Street Addresses
• Point features, streeet segment length, even/odd
  house numbers with side descriptions (e.g. TIGER
  files) can be geocoded
• TEXT Files lat/long in dec. degrees cane
  converted to an event theme

Ref Getting Started with GIS, by K. Clarke
19
Creating New Data (Contd)

• Manuel Digitizing
• Digitizing tables used (tedious process)
• Electronic mesh senses cursor position
• Paper maps (e.g. Topo Maps) are sources of most
  GIS themes that we use
• Point or stream mode digitizing done
• Now-a-days mainly done in 3rd world
• Need to record ref. coor. ?converted to
  real-world coor. (GIS coverage)
• Digitizing polygons need labels (ID)
• Careful planning needed before you digitize

20
Manual Digitizing
Ref Getting Started with GIS, by K. Clarke
21
Creating New Data (Contd)

• Scanning
• Scan a map into binary grid file
• Can be converted back to vector thru tracing
  (i.e. line thinning ?extracting
  ?toporeconstruction or smoothing)
• Software available to clean scanned images
• Resolution is an issue (e.g. a line 5-6 pixels)
• DRG (like DLG) are scanned topo maps
• Digital maps are as good as the source map

22
Scanning
Ref Getting Started with GIS, by K. Clarke
23
Creating New Data (Contd)

• On-Screen (head-up) Digitizing
• Relatively easy to do w/ a background
• Good for updating/editing changes
• You can add points, lines, polygons
• You will work on an exercise/project
• You dont have to register points as in manual
  digitizing
• Geometric Transformation
• Digitized maps have same coor. As source map
• You may have to convert to real-world coor.
  projection using control points.