QGIS Georeferencing [Tutorial]

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Georeferencing

Georeferencing refers to the assigning of ground ( real ) coordinates to an object. In spatial representations, objects as buildings, trees, roads, heights, temperature are represented using a given set of coordinates (x,y,z).  Georeferencing helps to make sure that such objects are located in their right space.

Suppose you need to use an old map in a spatial analysis project in QGIS and this map is only available in paper format. You need to convert it from analog to digital form  either  through scanning  or taking a picture of that map using your smart phone.

Scanned map of central Uganda

However, when you import this image in a GIS environment, it will be lacking spatial location. Today we will look straight on how to attach spatial info on that image. In this tutorial we will use a map of Uganda. click here to download the map CRS - UTM ARC1960 36N

Step 1: Import the image in QGIS. Study the image to identify the coordinate system , gid location and other attribute information necessary for Georeferencing

Step 2: Open the plugin tool and activate the GDAL Georeferencing tool.

Step 3: Open the Georeferencing tab( Raster-> georeferencing). Import the image in this window

Step 4: Open transformation settings and set the parameters you would like to use in the transformation, coordinate system and also specify where to save your new image after geo referencing 

Step 5: Zoom to a particular area and add well distributed control points ( in this tutorial we shall use only four). after adding all the points, click start georeferencing( the green play button)

coordinates used in Georeferencing

Step 6. Close the Georeferencing tool and import the saved image in QGIS. Set the project coordinate system as coordinate system of the map.

Finally add a base map to check whether your image lies in the
correct geographical location as shown below.

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