Vector-Based GIS Data Processing

Chapter 6

Vector Data Model

Feature Classespoints

lines

polygons

Layerslimited to one class of data

Figure p. 186

Vector Data Model

ShapefilesArcView

non-topological

most methods discussed in book will work in ArcView (the program compensates)

Characteristics of Vector Processing

compareslayersfeatures within layerslayers with features in other layersfeatures with features in other layersattributes

topological relationshipsconnectivityadjacencycontainmentnote: must maintain topology when altering data

Data record

ground coordinates in double-precision

accuracy varies with data collection methods

errors may propagate

Data Processing Functions

Figure p. 188 (top)

input

analysis: figure p. 188 (bottom)

output

Vector Data Input: Phases

acquisitiondigitizing• maps or remote-sensing data

• table top or scan & vectorize

import existing data

field collection

editing

formatting/translating

linking

Digitizing Existing Maps

Digitizersparts: table, cursor, controller

specifications• table size

• resolution

• accuracy: stability, repeatability, skew

processfigure p. 190

Digitizing Existing Maps

preparationcheck source quality

obtain control points (tic points)

prepare specifications: fig p. 191 (top)

Digitizing Existing Maps

digitize templatefig p. 191 (bottom)

features common to all layer• neat lines

• tic points

• common elements

create only once• save time

• ensure accurate overlay

source• base map

• input coordinates

edit & build topology

Digitizing Existing MapsDigitize map

register mapone layer at a timedigitize all lines (spaghetti model)• stream mode vs point mode• mark lines as digitized• only lines appropriate for layer

post-digitizing data processing• assign IDs• editing: map must remain on digitizer until this step is

complete

Scanning & Vectorization

“heads-up” digitizing

scanning: raster (or other) image

raster-vector conversion

processing

Scanning

scanner specificationsresolution

accuracy

scan size

output format

software quality

processquick input

no layers (entire map is scanned)

Raster-Vector Conversion

automaticmust be started

must be watched

problems: broken lines, thin lines, contrast

manual (“heads-up”)time-consuming

least editing (potentially)

Raster-Vector Conversion

text - automatic

symbol - manual

graphical editing

Importing Vector Data

outside sourcemetadata

translator

field survey (imports points)direct

via table

CAD

Importing Vector Data

transformationground coordinates

projection

need metadata

need to understand what was used (and will be used) and why

Graphical Data Editing

errors (fig p. 195)intersection errors

node errors

polygons are closed

polygon labeling

topology

processset environment

build topology

edit & correct

join adjacent layers

Graphical Data Editing

set editing environmentfig p. 196

edit tolerance

weed tolerance

grain tolerance

Graphical Data Editing

topology buildingbuild structure & relationships• fig p. 197

identify errors & automatically correct• fuzzy & dangling tolerance [fig p. 198 (top)]

rebuild topology (& repeat)

data editing & error correctionfig p. 198 (bottom)

back up file before & during editing

Graphical Data Editing

join adjacent layers (if necessary)fig p. 199

edge match

join maps

dissolve redundant polygon boundaries

rebuild topology

Attribute Data Conversion

vector data, no attributes

steps to creating attribute table(s)define structure of data file

populate data file• manual entry

• add ID column to existing data table

• check for errors

link to associated layers

Attribute Data Conversion

“event” themes (ArcView)XY• uses XY coordinates in a table

• creates a point theme

point, linear, continuous, address• uses existing shape file as reference

• creates point or line theme

Vector-Based Data Analysis

attribute database queryanalysis of attribute table data

selected items - fig p. 202• retrieve associated graphical elements

• may build new table

operations - fig p. 203• select

• project

• join

ArcView: Query Builder

Vector-Based Data Analysis

statistical computationmean, max, min, std. dev., quantile

ArcView• display within a theme

• can summarize attributes of one theme based on polygons in another theme

Vector-Based Data Analysisaddress geocoding - fig p. 204

provides street addresses with XY coordinatesuses reference theme• projected line file of street network with address ranges• U.S. Census TIGER files

event table• contains street addresses

geocoding editor• both input files must be compatible with editor• parses event table addresses• compares addresses to reference data• creates new point file and table with XY coordinates

Vector-Based Data Analysiscalculation of areas, perimeters & distancesfeature-based overlay analysis

point-in-polygon - fig p. 207 (upper left)• drops line downward from point

buffering (feature-based) - fig p. 207 (upper right)

point, line, polygon edge (inside or outside)buffer zone is a polygon - fig p. 208neighborhood analysisfixed or variable distance - fig p. 208

Vector-Based Data Analysis

reclassification (layer-based)fig p. 209

simplification

select attribute data

assign new, common value

merge appropriate adjacent polygons

Vector-Based Data Analysis

layer-based overlay analysisfeatures of two layers are intersected• point on polygon

• line on polygon

• polygon on polygon

creates new layer and attribute table - fig p. 210

Vector-Based Data Analysis

layer-based overlay analysis (con’t)operators - fig p. 211• union

• intersect

• identity

• clip (cookie cutter)

• erase cov

• split

Vector-Based Data Analysis

layer-based overlay analysis (con’t)define objectives & data analysis criteriaapply topological overlay operators - fig p. 212 & 213postoverlay processing & interpretation of resultsremoval of sliver polygons - fig p. 213• use template• use tolerance or sliver criteria

Vector-Based Data Analysis

network analysisinterconnected, linear features

network layer - fig p. 214• line segment/edge (has many attributes)

• junction/intersection (point or node)

• turn (attribute of junction)

Vector-Based Data Analysis

network analysis (con’t)functions• pathfinding - fig p. 215

• allocation

• tracing

• spatial interaction (uses gravity modeling)

• distance matrix

• location-allocation modeling

Vector-Based Output Functions

presentation methods - fig p. 217

map composition - Chapter 7

vector-raster conversionlose attributes

Vector Application Programming

“tool-box” approachextensions

scripts

advantagesstandardize processes

automate frequent functions

enhance capabilities

Vector Application Programming

internet map serverfig p. 223