GIS Data Structure: an Introduction

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

More key GIS elements: Entities vs. Attributes 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

Raster Model – data organization 2 1 3 Water Residential Commercial 1 2 3

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)

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.

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.

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 ??

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 ??

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”)

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) Line (x,y) Area (or polygon)

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

Vector Considerations More points = closer to real feature Still an abstraction of real space! (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 Ask for examples!

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

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… 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?

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

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

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