9. gis data collection
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9. GIS Data Collection. Overview. Introduction Primary data capture Secondary data capture Data transfer Capturing attribute data Managing a data capture project. Data Collection. One of most expensive GIS activities Many diverse sources Two broad types of collection - PowerPoint PPT PresentationTRANSCRIPT
9. GIS Data Collection
OverviewIntroductionPrimary data captureSecondary data captureData transferCapturing attribute dataManaging a data capture project
Data CollectionOne 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)
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
Stages in Data Collection Projects
Planning
Preparation
Digitizing / TransferEditing / Improvement
Evaluation
Primary Data CaptureCapture specifically for GIS useRaster – remote sensing
e.g. SPOT and IKONOS satellites and aerial photographyPassive and active sensors
Resolution is key considerationSpatialSpectralTemporal
Spatial and temporal characteristics of commonly used Earth observation remote-sensing systems and their sensors
Typical Reflectance Signatures
Vector Primary Data CaptureSurveying
Locations 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
Total Station
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
Vector Secondary Data CaptureCollection 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
Scanner
Types of scanner
Vector Over Raster
Digitizer
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
Typology of human errors in digitizing: (A) undershoots and overshoots; (B) invalid polygons; and (C) sliver polygons
Examples of spatial error in vector data
Radius Topology Feature Snapping
Error induced by data cleaning
Mismatches of adjacent spatial data sources that require rubber-sheeting
Edge matching
Rubber sheeting
Photogrammetry
Data TransferBuy vs. build is an important questionMany widely distributed sources of GIKey catalogs include
US NSDI Clearinghouse networkGeography Network
Access technologiesTranslationDirect read
Comparison of data access by translation and direct read
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