Disentangle Things by Ivan Hanigan

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Ripping data from georeferenced images with QGIS

Introduction

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.

Methods

Follow instructions on this website http://www.qgistutorials.com/en/docs/advanced_georeferencing.html
Try out the method on a month that is already available as spatial data, to assess precision.
Then start mapping earlier data with no correlates.

Data

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

drt-nswdpi-raw

Results

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.