GIS: Raster & Vector Data Models by J. Wolfe, IS 645, Summer 2000 - Prof. James A. Wolfe, Study notes of Information Technology

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[email protected], WV - 25755Marshall UniversityCITEJamie Wolfe

StructuresData and Data Raster and Vector

Lecture 07, June 06, 2000to GIS IS 645 Introduction

IS 645: Geographic Information Systems, Summer 2000, J. Wolfe

Today’s class topics

◗ Raster and Vector Models ◗ Raster Models

File Formats

Data Models

Data Structures ◗ Vector Models

Data Models

Topological Models

Representation of surfaces ◗ Issues with data exchange

IS 645: Geographic Information Systems, Summer 2000, J. Wolfe

data Representing Feature and Attribute

◗ A feature on a map may have many attributes ◗ For example:

A road may have many attributes:

Length

Surface type

Maintained by

Each attribute has values:

Length = 500

Surface type = Cement

Maintained by = County

IS 645: Geographic Information Systems, Summer 2000, J. Wolfe

Raster and Vector

◗ can be presented in raster form or vector formGraphic representations and geographical space ◗ stored to represent the graphicunit can represent only one value. All units area combination of individual “units”, where eachIn the raster format, the graphic is represented as

composed by the combination of individual pixelsExample: bitmaps of images, where the image is ◗ using the relationshipare stored. Intermediate points are determinedor function. Only the points and the relationshipby a set of points, joined by a certain relationshipIn the vector format, the graphic is represented

Example: A CAD drawing (engineering drawing)

IS 645: Geographic Information Systems, Summer 2000, J. Wolfe

Vector overlay over raster file

IS 645: Geographic Information Systems, Summer 2000, J. Wolfe

Raster and Vector Data models

IS 645: Geographic Information Systems, Summer 2000, J. Wolfe

Common Raster data file formats

◗ Raster File formats:

BMP (bitmaps) - no compression

DIB (Device Independent Bitmaps)

GIF (Compuserve’s Graphical Interchange Format)

RLE - Run Length Encoding

for Telephone and Telegraph ) - RLE, Huffman codingCCITT Group 3, 4, (International Consultative Committee

JPEG (Joint Photographic Experts Group)

TIFF (Tagged Image File Format)

DEM (Digital Elevation Model)

IS 645: Geographic Information Systems, Summer 2000, J. Wolfe

Data Compression Techniques

◗ in exchange for a greatly increased compressionLossy: Where there is a certain loss of accuracy ◗ compress/expand cycleexact input stream will be generated after theLossless: Where there is a guarantee that the ◗ Data compression = Modeling + Coding ◗ LZW, etc.,length Encoding, LZ (uses adaptive dictionary),Lossless coding techniques: Huffman, Run- Stream Input Model Encoder StreamOutput Symbols Probabilities Codes

IS 645: Geographic Information Systems, Summer 2000, J. Wolfe

BMP

◗ Used by MS Windows and OS/2 systems ◗ is number of pixels multiplied by bits/pixelpixel is coded as index of the color table. File sizebits) to store color depth (based on palette). EachAn 8-bit bitmap uses a color table of size 256 ( ◗ based on default paletteColor may vary from computer to computer ◗ provide a depth of 16 million colorsA 24-bit bitmap uses 3 bytes for each pixel to

IS 645: Geographic Information Systems, Summer 2000, J. Wolfe

DIB: Device Independent Bitmap

◗ display device.Windows to display the bitmap on any type ofDevice-independent bitmap (DIB) format allows ◗ represent color.independent of the method used by a display tobitmap specifies pixel color in a formThe term "device independent" means that the

IS 645: Geographic Information Systems, Summer 2000, J. Wolfe

RLE: Run Length Encoding

◗ Earliest and simplest method of data compression ◗ the character itselfbytes: the number of times character appears andA repeated string of characters is replaced by two ◗ with Huffman codingBasis for many CCITT group standards. Combined ◗ lineusing 1D scheme, next K lines coded using firstTwo dimensional coding schemes: First line coded ◗ Size of K varies over applications

IS 645: Geographic Information Systems, Summer 2000, J. Wolfe

RLE Example: Vector Map

IS 645: Geographic Information Systems, Summer 2000, J. Wolfe

RLE Example: Raster map with codes

IS 645: Geographic Information Systems, Summer 2000, J. Wolfe

RLE Example: RLE file based on codes