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