1. GIS Data Structure: an Introduction

2. Some Key GIS Elements: Layer A category of geographic features
Also called:
“Data Plane”
Coverage
Shapefile
Geodatabase
Retail
Streets
Parcels
Elevation
Land use
Real world

3. More key GIS elements: Entities vs. Attributes
Where is it?
Attributes
What’s there?
A GIS must be able to manage both

4. Organization? Data Model: Links entities and attributes, and
Allows analysis of multiple data layers
Two Types:
Raster
Vector

5. Raster Model
Space is divided into a series of units, regular or irregular

6. Raster Model One attribute per grid cell
Many grouped attributes = entity
Water
Residential
Commercial

7. Raster Model – data organization2 1 3
Water
Residential
Commercial 1 2 3

8. Some familiar raster data …
Multispectral scanner data
(1 band) 37 41 45 42 39 36 33 38 40 34 30 32 29 24 28 27 22 25 19 20 26 18 16 23 21 15 14 12 11
DEM
(Digital Elevation Model)

9. Raster Model: Strengths
Good for geographic features..
With no distinct shape or size
That occur over the whole surface of the earth, or
Change in a gradual manner over the earth’s surface
Good for:
Topography (elevation)
temperature
precipitation
Environmental phenom.

10. Raster Model: Good for …
Environmental phenomena
Topography (elevation)
Temperature
Precipitation
Pollution patterns
Data gathered as pixels
Remotely sensed data
Scanned “pictures”:
Orthophotos
Scans of topo maps
Good for:
Topography (elevation)
temperature
precipitation
Environmental phenom.

11. Cool Raster Data 1: USGS Digital Orthophoto Quads (DOQ)
Collaged & rectified aerial photos
Image and detail of a photo, with scale properties of a map
Black-and-white and Color Infrared
Scale: 1:40,000
1-meter resolution
Ask: Raster or vector ??

12. USGS Digital Orthophoto Quads (DOQ)
Uses:
A base layer for a GIS:
Evaluate other data layers’ accuracy
Update other data layers
Source of new data layers
Ask: Raster or vector ??

13. Cool raster data 2: USGS Digital Raster Graphic (DRG)
No attributes
Geo-referenced
Uses:
Combined with GPS for determining locations
Combined with DEM for 3-D view of area
Uses:
Combined with DEM for 3-D view of area
Combined with GPS for determining locations (fieldwork, “Onstar”)

14. USGS: DRG + DEM

15. Raster Model: Disadvantages
Earlier, data storage needed lots of space…
Complex calculations of large raster data pushes limits

16. Raster Model: Disadvantages
Precise locations are lost
Exact measurements are difficult
Output (maps) tend to look coarse
Point
Line
Area

17. Vector Model
Geographic entities are considered to be either points, lines or areas.
Points, lines, and areas are defined by one or more coordinate pairs.
Point
(x,y)
(x,y)
Line
(x,y)
Area (or polygon)

18. Vector Model: Data Organization
In a vector model, how many attributes can you store for each entity in a layer ?
Entity table 2
Attribute table
Water
Residential
Commercial
Cell tower
Road

19. Vector Considerations
More points = closer to real feature
Still an abstraction of real space!
(x,y)
Line

20. Vector Model: Its strengths
Data this model is good for:
Entities that are clearly linear
Entities that are considered points
Entities that cover areas
There is little internal variation within the areas
Broad categories are of more interest than variations
Ask for examples!

21. Vector Model: Its strengths
Other :
Efficient data storage
Makes nice, sharp-looking maps!
Ask for examples!

22. Summary: Raster and Vector
Ways of organizing entity and attribute data in a GIS:
Raster models for:
Continuously-varying phenomena
Executing analytical functions
Vector models for:
Precisely-located phenomena
Cartographic quality
Efficient analysis

23. Choosing a data model … If… How to analyze topics like …
Raster data is good for phenomena that exists everywhere, and changes gradually,
& Vector data is good for phenomena that “make sense” as points, lines, and polygons,
How to analyze topics like …
Locating a road, based on topography?
Air pollution in relation to land use?

24. Converting Between Raster & Vector
Vector > Raster
Grid with the appropriate cell size overlays the vectors
Each cell is assigned the attribute code of the vector to which it belongs

25. Converting Between Raster & Vector
each raster cell is assigned an attribute value
vector is created by storing x and y coordinates for midpoints of cells

26. Conclusion about Converting Between Raster and Vector
It’s possible … but be aware of limitations and use caution in interpreting results