Most GIS projects require georeferencing some raster data. Georeferencing is
the process of assigning real-world coordinates to each pixel of the raster.
Many times these coordinates are obtained by doing field surveys - collecting
coordinates with a GPS device for few easily identifiable features in the image
or map. In some cases, where you are looking to digitize scanned maps, you can
obtain the coordinates from the markings on the map image itself. Using these
sample coordinates or GCPs ( Ground Control Points ), the image is warped and
made to fit within the chosen coordinate system. In this tutorial I
will discuss the concepts, strategies and tools within QGIS to achieve a high
accuracy georeferencing.
1.Georeferencing in QGIS is done via the ‘Georeferencer GDAL’ plugin. This is a
core plugin - meaning it is already part of your QGIS installation. You just
need to enable it. Go to Plugins ‣ Manage and Install
Plugins and enable the Georeferencer GDAL plugin in the
Installed tab. See Using Plugins for more details on how to
work with plugins.
The plugin is installed in the Raster menu. Click on Raster
‣ Georeferencer ‣ Georeferencer to open the plugin.
The plugin window is divided into 2 sections. The top section where the
raster will be displayed and the bottom section where a table showing your
GCPs will appear.
Now we will open our JPG image. Go to File ‣ Open Raster.
Browse to the downloaded image of the scanned map and click
Open.
In the next screen, you will asked to choose the raster’s coordinate
reference system (CRS). This is to specify the projection and datum of your
control points. If you have collected the ground control points using a GPS
device, you would have the WGS84 CRS. If you are geo-referencing a scanned
map like this, you can obtain the CRS information from the map itself.
Looking at our map image, the coordinates are in Lat/Long. There is no datum
information given, so we have to assume an appropriate one. Since it is
India and the map is quite old, we can bet the Everest 1830 datum would give
us good results.
You will see the image will be loaded on the top section.
You can use the zoom/pan controls in the toolbar to learn more about the map.
Now we need to assign coordinates to some points on this map. If you look
closely, you will see coordinate grid with markings. Using this grid, you
can determine the X and Y coordinates of the points where the grids
intersect. Click on Add Point in the toolbar.
In the pop-up window, enter the coordinates. Remember that X=longitude and
Y=latitude. Click OK.
You will notice the GCP table now has a row with details of your first GCP.
Similarly, add at least 4 GCPs covering the entire image. The more points
you have, the more accurate your image is registered to the target
coordinates.
Once you have enough points, go to Settings ->
Transformation settings.
In the Transformation settings dialog, choose the
Transformation type as Thin Plate Spline. Name your
output raster as 1870_southern_india_modified.tif. Choose
EPSG:4326 as the target SRS so the resulting image is in a
widely compatible datum. Make sure the Load in QGIS when done
option 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.
Once the process finishes, you will see the georeferenced layer loaded in
QGIS.
The georeferencing is now complete. But as always, it’s a good practice to
verify your work. How do we check if our georeferencing is accurate?
In this case, load the country boundaries shapefile from a trusted source
like the Natural Earth dataset and compare them. You will notice they match
up pretty nicely. There is some error and it can be further improved by
taking more control points, changing transformation parameters and trying a
different datum.
