HIST 326 Digital History: Getting Started with QGIS

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Last Updated: Oct 13, 2017 9:18 AM

Georectifying in QGIS

  • Find a map to overlay
    The Rochester Image Database has a nice selection of high-resolution maps from the Rochester area. For today's demo, you can download the Town of Henrietta Plat Map here
  • Install the following QGIS plugins
    Plugins → Manage and Install Plugins
    • OpenLayers Plugin
    • Georeferencer GDAL
  • Load OpenStreetMap map as your base layer
    Web → OpenLayers plugin → OpenStreetMap → OpenStreetMap
  • Set your Project CRS to WGS 84 / Pseudo Mercator
    Project → Project Properties → CRS
  • Zoom to your target location
    Hint: RIT's coordinates under the specified CRS are -8646787,5324730; Set scale to 1:25,000
  • Load map image to overlay (raster file)
    Raster → Georeferencer → Georeferencer
    File → Open Raster
  • Add 4-5 points via Ctrl + A
    Hint: Ideally you want to find a point to the northwest, northeast, southwest, and southeast, along with one central point
    You do not need to know the coordinates – you can use the "From map canvas" option to select the point on the base map
  • Adjust your transformation settings
    Settings → Transformation Settings
    Recommended settings:
    • Transformation type: Thin Plate Spline
    • Resampling method: Nearest neighbour
    • Target SRS: Selected CRS (EPSG:3857, WGS 84 / Pseudo Mercator)
    • Output raster: where-you-want-to-save-your-file
    • Compression: None
    • Mark: Use 0 for transparency when needed
    • Mark: Load in QGIS when done
  • Start Georeferencing via Ctrl + G

Terms to Know

Coordinate Reference System (CRS): System using two or three coordinates (x,y) to explain the location of a point on a two-dimensional map relative to real locations on the earth

Geospatial Information System (GIS): System designed to deal with spatial information with a geographical aspect (e.g. ArcGIS, QGIS)

Georectify: Similar to georeferencing; Often used to describe transforming and warping the image to remove distortion

Georeference: Assign a CRS to an image (e.g. scanned map or satellite image) by specifying appropriate coordinates relative to a known CRS (e.g. Google Maps)

Plat Map Map drawn to scale to indicate divisions of the land; Often used for urban planning and related activities such as zoning, utilities maintenance, and public space designation

Raster Data: Data made up of cells (or pixels) organized into columns and rows (e.g. scanned maps or satellite images)

Vector Data: Geometric data made up of interconnected vertices or points to create points, lines, and polygons (e.g. marker or road drawn on a map)

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