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9. GIS Data Collection

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

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9. GIS Data Collection

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  1. 9. GIS Data Collection

  2. Overview • Introduction • Primary data capture • Secondary data capture • Data transfer • Capturing attribute data • Managing a data capture project

  3. Data Collection • One of most expensive GIS activities • Many diverse sources • Two broad types of collection • Data capture (direct collection) • Data transfer • Two broad capture methods • Primary (direct measurement) • Secondary (indirect derivation)

  4. Data Collection Techniques

  5. Preparation Evaluation Planning Digitizing / Transfer Editing / Improvement Stages in Data Collection Projects

  6. Primary Data Capture • Capture specifically for GIS use • Raster – remote sensing • e.g. SPOT and IKONOS satellites and aerial photography • Passive and active sensors • Resolution is key consideration • Spatial • Spectral • Temporal

  7. Spatial and temporal characteristics of commonly used Earth observation remote-sensing systems and their sensors

  8. Typical Reflectance Signatures

  9. Vector Primary Data Capture • Surveying • Locations of objects determined by angle and distance measurements from known locations • Uses expensive field equipment and crews • Most accurate method for large scale, small areas • GPS • Collection of satellites used to fix locations on Earth’s surface • Differential GPS used to improve accuracy

  10. Total Station

  11. Secondary Geographic Data Capture • Data collected for other purposes can be converted for use in GIS • Raster conversion • Scanning of maps, aerial photographs, documents, etc • Important scanning parameters are spatial resolution and bit depth

  12. 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

  13. Scanner

  14. Types of scanner

  15. Vector Over Raster

  16. Digitizer

  17. Batch vectorization of a scanned map (A) original raster file (B) vectorized polygons 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

  18. Typology of human errors in digitizing: (A) undershoots and overshoots; (B) invalid polygons; and (C) sliver polygons

  19. Examples of spatial error in vector data

  20. Radius Topology Feature Snapping

  21. Error induced by data cleaning

  22. Mismatches of adjacent spatial data sources that require rubber-sheeting

  23. Edge matching

  24. Rubber sheeting

  25. Photogrammetry

  26. Data Transfer • Buy vs. build is an important question • Many widely distributed sources of GI • Key catalogs include • US NSDI Clearinghouse network • Geography Network • Access technologies • Translation • Direct read

  27. Comparison of data access by translation and direct read

  28. Summary • Data collection is very expensive, time-consuming, tedious and error prone • Good procedures required for large scale collection projects • Main techniques • Primary • Raster – e.g. remote sensing • Vector – e.g. field survey • Secondary • Raster – e.g. scanning • Vector – e.g. table digitizing

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