How to Georeference a Map in QGIS
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. The coordinates can be obtained from the scanned map itself, if latitude and longitude values are printed on map as corner tick marks or grid lines.
The process of georeferencing a map using QGIS is explained in the following steps:
- A. Open QGIS Desktop
- B. Go to the Raster menu – Georeferencer and click on Georefrencer.. option. A new window titled Georefrencer will open. option.(If Georeferencer option is not available in Raster menu then install Georefrence Plugin from Manage and Install Plugins.. option in Plugins menu)
- A. Inside Georefrencer window click on Open Raster option in File menu. A file selection dialog will open.
- B. From the file selection dialog window navigate to the folder containing raster file (scanned map) and select the raster file (scanned map) and press open button. Selected raster file will display in the Georefrencer window.
- Now select Add Point option from menu bar or from the Edit menu
- A. Now click on the left top corner of the map (raster image) at the intersection of latitude and longitude.
- B. A pop-up window (Enter map coordinates) will open
- C. Enter the longitude value of clicked point in the box labeled X/East. Here the value can be entered in DMS format (e.g 75 15 0) or Degree decimal format (e.g. 75.25)
- D. Enter the value of latitude of clicked point in the box labeled Y/North
- E. Press OK button
- After adding all the four corners of the map, select Save GCP Points As.. from File menu and save the entered values (shown in GCP table at the bottom of screen) as disk file for further us/edit.
- Now select Transformation Settings from settings option in main menu option. A popup window (Transformation settings) will open
- Here select the Transformation type as Polynomial 1 and Resampling method as Nearest neighbor.
- Select the output location and file name for georefrenced raster file in output raster option.
- Specify the Target SRS (spatial reference System) as EPSG:4326 and press OK button.
- Now go to the File menu and click Start Georeferencing option or Press Ctrl+G.
- Georefrncing is complete. Now open the georefrenced raster map in QGIS for digitization.