gis data structure: an introduction. real world land use elevation parcels streets retail some key...
TRANSCRIPT
GIS Data Structure: an Introduction
Real world
Land use
Elevation
Parcels
Streets
Retail
Some Key GIS Elements:
• Layer– A category of
geographic features
– Also called:• “Data Plane”• Coverage• Shapefile• Geodatabase
More key GIS elements:Entities vs. Attributes
• Entities– Where is it?
• Attributes– What’s there?
A GIS must be able to manage both
Organization?
• Data Model:– Links entities and attributes, and – Allows analysis of multiple data layers– Two Types:
• Raster• Vector
Raster Model • Space is divided into a series of units, regular or
irregular
Raster Model • One attribute per grid cell• Many grouped attributes = entity
Water
Residential
Commercial
1
2
3
Water
Residential
Commercial
2 2 2 2 2 2 2 2 2 2
2 2 2 2 2 2 1 2 2 2
2 2 2 2 1 1 1 1 2 2
2 2 2 1 1 1 1 1 2 2
2 2 1 1 1 1 2 2 2 2
2 2 2 2 2 1 2 2 2 2
2 2 2 2 2 2 2 2 2 2
2 2 2 2 2 2 2 2 2 2
2 2 2 3 2 2 2 2 2 2
Raster Model – data organization
Multispectral scanner data(1 band)
Some familiar raster data …
37 41 45 42 39 36
33 38 40 37 34 30
32 36 37 34 29 24
28 32 33 27 24 22
24 28 27 25 22 19
20 26 24 20 18 16
18 23 21 18 15 14
16 20 18 15 12 11
DEM(Digital Elevation Model)
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
• Environmental phenomena– Topography (elevation)
– Temperature
– Precipitation
– Pollution patterns
• Data gathered as pixels– Remotely sensed data
• Scanned “pictures”:– Orthophotos– Scans of topo maps
Raster Model: Good for …
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
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
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
USGS: DRG + DEM
Raster Model: Disadvantages
• Earlier, data storage needed lots of space…
• Complex calculations of large raster data pushes limits
Raster Model: Disadvantages
• Precise locations are lost
• Exact measurements are difficult
• Output (maps) tend to look coarse
Point
Line
Area
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)
(x,y)
(x,y)
(x,y)
(x,y)Area (or polygon)
(x,y)
(x,y)(x,y)
(x,y)
(x,y)
(x,y)
(x,y)Line
Vector Model: Data Organization
Water
Residential
Commercial
Cell tower
Attribute tableAttribute table
Entity tableEntity table
2
In a vector model, how many attributes can you store for each entity in a layer ?
Road
Vector Considerations
• More points = closer to real feature
• Still an abstraction of real space!
(x,y)
(x,y)
(x,y)
(x,y)
(x,y)Line
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
Vector Model: Its strengths
• Other :– Efficient data storage
– Makes nice, sharp-looking maps!
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
Choosing a data model …• If…
– 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?
Converting BetweenRaster & 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
Converting BetweenRaster & Vector
• Raster > Vector– each raster cell is assigned an attribute value
– vector is created by storing x and y coordinates for midpoints of cells
Conclusion about Converting Between Raster and Vector
• It’s possible … but be aware of limitations and use caution in interpreting results