©2005 Austin Troy. All rights reserved Lecture 3: Introduction to GIS Understanding Spatial Data Structures ( Raster and Vector ) by Austin Troy, Leslie Morrissey, & Ernie Buford, University of Vermont

©2005 Austin Troy. All rights reserved Representation Entities Fields Bona Fide Fiat Introduction to GIS

©2005 Austin Troy. All rights reserved Representation Entities Fields Vector Raster Introduction to GIS

4 Pendekatan / Model Raster pada Representasi Data TGA Semua obyek geografis dalam bentuk TGA dinyatakan dengan sel atau piksel yang merupakan titik yang mempunyai koordinat dan atribut. Merupakan pendekatan yang sesuai dengan data inderaja berupa citra dijital yang merupakan salah satu data masukan sistem informasi geografis. Keuntungan dan keterbatasannya: –Membutuhkan tempat penyimpanan data yang besar –Penyajian kurang baik / kurang halus tergantung resolusi –Representasi yang sangat kompatibel dengan proses komposit lapis data SIG –Merupakan data baku pembentuk citra dijital pada sistem inderaja

5 Pendekatan / Model Vektor pada Representasi Data TGA Merupakan representasi yang cocok untuk penyajian dalam format peta (konvensional). Obyek geografis disajikan dalam titik atau segmen garis. Keuntungan dan keterbatasannya: –Membutuhkan tempat penyimpanan data yang kompak –Penyajian garis yang sangat halus –Proses overlay dan perhitungan luas area memerlukan algoritma yang lebih kompleks –Merupakan data baku pembentuk data spasial untuk keperluan SIG/peta

6 Representasi Data TGA dengan Pendekatan Raster dan Vektor (Source: Purwadhi, 1997)

7 Representasi Data Spasial Secara eksplisit berbentuk struktur data raster: Atribut obyek dinyatakan dengan simbol / warna / tingkat keabuan yang merupakan nilai sel atau piksel Secara implisit berbentuk struktur data vektor dengan bentuk topologi titik/garis/area(poligon): Atribut obyek dinyatakan dengan himpunan vektor yang menyatakan keterhubungan

©2005 Austin Troy. All rights reserved Points (no dimensions) Lines, or “arcs” (1 dimension) or Areas, or “polygons” (2 or 3 dimensions) Vector Introduction to GIS

Vector Introduction to GIS Zoom in

Vector Data Locational data tell where features are Attribute data tell what features are (lots of info!) Topological data tell where features are in relation to one another Introduction to GIS Topology defines relationships between features: e.g., parcel #16 is adjacent to parcel #234 Main St. is connected to First St. First St. is a one way street (direction) …. Basis for powerful spatial analyses

Vector Problems Introduction to GIS Vector wetland polygons over a CIR photograph (graphic courtesy Leslie Morrissey)

©2005 Austin Troy. All rights reserved Raster Grids, or pixels Cell size is constant Area of each cell defines the resolution Raster files store only one attribute, in the form of a “z” value, or grid code. Introduction to GIS

Raster Introduction to GIS

Raster cell size & resolution Introduction to GIS (graphic courtesy Leslie Morrissey)

Representing a terrain surface – raster vs. vector Introduction to GIS

©2005 Austin Troy. All rights reserved Raster and Vector representations of the same land use: Introduction to GIS

©2005 Austin Troy. All rights reserved Vector vs. Raster: bounding & lines Raster: poor representation (stairstep) Vector: better precision (discrete) Introduction to GIS

©2005 Austin Troy. All rights reserved Vector vs. Raster: Sample points Cancer rates across space Introduction to GIS

Raster: Location & Distance Error Introduction to GIS (graphic courtesy Leslie Morrissey)

Raster: Area (In)Accuracy Introduction to GIS (graphic courtesy Leslie Morrissey)

Raster: Encoding Problems Introduction to GIS (graphic courtesy Leslie Morrissey)

©2005 Austin Troy. All rights reserved WHEN TO USE RASTER OR VECTOR??? Introduction to GIS

©2005 Austin Troy. All rights reserved Raster and Vector Analytic advantages and disadvantages Technical advantages and disadvantages Specific Usages Tossups Introduction to GIS

Raster Advantages Introduction to GIS Represents high spatial variability Best for mapping/modeling continuous phenomena Represents gradients best Better for fuzzy (transitional) boundaries Space filling – grid cell for all real estate Combining layers with map algebra A B C A + B = C

Raster Disadvantages Introduction to GIS Locational accuracy and ability to represent features accurately depends on cell size Data layer represents only one theme or “attribute” Poor representation of linear features and boundaries No topology Raster coding problems Big files! (storage/transfer issue)

Vector Advantages Introduction to GIS Great for discrete locations and boundaries not continuous data Maps linear features well High locational accuracy (depends on scale) Based on topology Lots of attribute data (multiple attributes) Small files (downloadable from Internet)

Vector Disadvantages Introduction to GIS Polygons are assumed to be homogeneous, so may miss inclusions, mixes Lines and polygon boundaries define discrete divisions (whether or not they exist on the ground) Poorly represent continuous surfaces, gradients Scale (of source) determines mapping accuracy large scale = more detail small scale = less detail

©2005 Austin Troy. All rights reserved Moving between vector and raster Introduction to GIS

Converting vector to raster Introduction to GIS (graphic courtesy Leslie Morrissey)

Converting raster to vector Introduction to GIS (graphic courtesy Leslie Morrissey)

Conversion problems Introduction to GIS

TIN – Triangulated Irregular Network Introduction to GIS (graphic courtesy Leslie Morrissey) (DEM -- Digital Elevation Model)