9. gis data collection. overview introduction primary data capture secondary data capture data...
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9. GIS Data Collection
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Overview
IntroductionPrimary data captureSecondary data captureData transferCapturing attribute dataManaging a data capture project
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Data Collection
One of most expensive GIS activitiesMany diverse sourcesTwo broad types of collection
Data capture (direct collection)Data transfer
Two broad capture methodsPrimary (direct measurement)Secondary (indirect derivation)
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Data Collection Techniques Raster Vector
Primary Digital remote sensing images
GPS measurements
Digital aerial photographs
Survey measurements
Secondary Scanned maps Topographic surveys
DEMs from maps
Toponymy data sets from atlases
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Stages in Data Collection Projects
Planning
Preparation
Digitizing / TransferEditing / Improvement
Evaluation
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Primary Data Capture
Capture specifically for GIS useRaster – remote sensing
e.g. SPOT and IKONOS satellites and aerial photographyPassive and active sensors
Resolution is key considerationSpatialSpectralTemporal
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Spatial and temporal characteristics of commonly used Earth observation remote-sensing systems and their sensors
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Typical Reflectance Signatures
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Vector Primary Data Capture
SurveyingLocations of objects determined by angle and distance measurements from known locationsUses expensive field equipment and crewsMost accurate method for large scale, small areas
GPSCollection of satellites used to fix locations on Earth’s surfaceDifferential GPS used to improve accuracy
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Total Station
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Secondary Geographic Data Capture
Data collected for other purposes can be converted for use in GISRaster conversion
Scanning of maps, aerial photographs, documents, etcImportant scanning parameters are spatial resolution and bit depth
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Vector Secondary Data Capture
Collection of vector objects from maps, photographs, plans, etc.Digitizing
Manual (table) Heads-up and vectorization
Photogrammetry – the science and technology of making measurements from photographs, etc.COGO – Coordinate Geometry
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Scanner
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Types of scanner
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Vector Over Raster
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Digitizer
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Adjacent raster cells with the same attribute values are aggregated. Class boundaries are then created at the intersection between adjacent classes in the form of vector lines
Batch vectorization of a scanned map
(A) original raster file
(B) vectorized polygons
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Typology of human errors in digitizing: (A) undershoots and overshoots; (B) invalid polygons; and (C) sliver polygons
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Examples of spatial error in vector data
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Radius Topology Feature Snapping
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Error induced by data cleaning
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Mismatches of adjacent spatial data sources that require rubber-sheeting
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Edge matching
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Rubber sheeting
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Photogrammetry
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Data Transfer
Buy vs. build is an important questionMany widely distributed sources of GIKey catalogs include
US NSDI Clearinghouse networkGeography Network
Access technologiesTranslationDirect read
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Comparison of data access by translation and direct read
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SummaryData collection is very expensive, time-consuming, tedious and error proneGood procedures required for large scale collection projectsMain techniques
Primary• Raster – e.g. remote sensing• Vector – e.g. field survey
Secondary• Raster – e.g. scanning• Vector – e.g. table digitizing