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Ripping data from georeferenced images with QGIS


Drought Declarations are made by government in Australia to determine when and where a drought is occuring. The Declarations are linked to agricultural and social support policies. We are working on a climatic drought index that should correlate with these Drought Declarations. The New South Wales Government has provided us with spatial data from 1986 but they also have a graphical visualisation available for earlier times, especially noteworthy is the extreme 1982-1983 drought. This post is a document of the process I will try to derive spatial data from the image.



Download the drought maps. The current maps are at: http://www.dpi.nsw.gov.au/content/agriculture/emergency/drought/planning-archive/climate/advance-retreat

But there is a higher resolution map archived at: http://pandora.nla.gov.au/pan/120345/20120529-0000/advance-retreat-drought-map-april-2011.pdf



Step 1 - load the reference layer

  • Load up the NSW boundaries by selecting "Add PostGIS Layers" > Connect > gislibrary
  • Then select a map layer of NSW

Step 2 - set up the raster data into the Georeferencer

  • Launch the Georeferencer from Raster > Georeferencer > Georeferencer.
  • If you do not see that menu item, you will need to enable the Georeferencer GDAL plugin from Plugins > Manage and install Plugins > Installed.
  • Zoom in on the month of interest
  • Now click on the Add Point button on the toolbar and select an easily identifiable location on the image. Corners, intersections, poles etc. make good control points.
  • Once you click on the image at a control point location, you will see a pop-up asking you to enter map coordinates. Click the button From map canvas.
  • Find the same location in your reference layer and click there. The coordinates are auto-populated from your click on the map canvas. Click Ok. Similarly, choose at least 4 points on the image and add their coordinates from the reference layer.
  • Now go to Settings > Transformation settings (yellow cog wheel).
  • Choose the settings as shown below.
  • Transformation type = Thin Plate Spline
  • output raster = someting
  • target EPS = 4283 (match what your reference layer is)
  • Make sure the Load in QGIS when done button is checked. Click OK.
  • Back in the Georeferencer window, go to File > Start georeferencing. This will start the process of warping the image using the GCPs and creating the target raster.

Step 3 - rip the data to a shapefile

  • create a new shp (Layer > New)
  • type = ploygon
  • Specify CRS = EPSG:4283
  • attribute = drought, text
  • save to a shp like 'drt198610rip' and OK
  • right-click, toggle editing, add new feature
  • save layer edits



This work


Compared to NSW DPI drought declarations data



The process produces data that is very prone to imprecision. We may be able to use some extra data from before 1986 though.

Posted in  drought awap grids data operation

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