Georeferencing using QGIS ~ Bản tin TMV

Georeferencing using Quantum GIS (QGIS)

This process follows the basic principles of georeferencing

Input options: Most image files. QGIS on most platforms does not open JPEG2000 images, so you will need to convert these to TIFF. See GDAL conversion and transformation
Output options: A georeferenced image file - GeoTIFF
Strengths: It is free, open-source software, and intuitive to use. It offers a good range of coordinate systems, transformation methods, and image formats.
Weaknesses: Software is in development and can be regularly updated. This means it is always improving but can therefore be necessary to keep learning how to use it.

Software Download and Installation:
QGIS can be downloaded from the official Quantum GIS website. Once installed, you will also need to enable the Georeferencer plugin.

Open QGIS and from the menubar select Plugins > Manage Plugins, and select the GDAL Georeferencer plugin.

Workflow:

Open your map in your Image Editing software.
Crop the non-geographic parts of the map; ie. remove the margin areas outside the map boundaries. Some useful free software for editing and cropping images includes ImageMagick, Irfanview and GIMP.
Open the QGIS software.
To begin, you need to add a base map layer that you will use to georeference your map.
A base map layer is basically an already georeferenced map.
In QGIS we have the option of adding base mapping that is held in different file formats.
It is possible to add images or datasets, and these can be either locally or remotely stored.
Option 1 - Add a georeferenced image file - Raster Layer:
Click this link to view a list of georeferenced maps we have available for you to use.
From the top menu bar, click Layer, then select Add Raster Layer

Use the File Browser to locate the base map file you want to add.
Select it and click Open.
The map should now be displayed in your workspace.
All maps you add are displayed as layers in the Layers window found on the left hand side of the QGIS workspace.

If you right click on a layer in the Layers window a menu will be displayed. Two useful zoom options are available in this menu. Use Zoom to Layer Extent to zoom to the extents of that Map Layer. Use Zoom to best Scale to zoom to the maps actual size based on its resolution.
Note: Changing the Coordinate System of your Project or a Layer
When QGIS is started, it sets the coordinate system to either that of your last project or defaults to the WGS84 Coordinate System. It is advisable to specify the coordinate system your project will use. Under the Settings option in the top menu bar, select Project Properties. The Project Properties dialogue box will be displayed. This dialogue box has options to select from a list of available coordinate systems or to search for a specific coordinate system by it's EPSG number. You can also change any Layers' coordinate system at any time by right clicking it in the Layers window and selecting Properties from the menu.
Our georeferenced maps use either the British National Grid (EPSG:27700), or the Spherical Mercator coordinate systems. For further information, see our page on Coordinate Reference Systems.
Option 2. Add a Web Map Service (WMS) layer from a web server.
Click this link to view a list of WMS layers we have available for you to use.
From the top menu bar, click Layer, then select Add WMS layer.

The Add Layers From A Server window will open.

Click the New button to set up a new WMS connection
The Create a new WMS Connection dialogue box will open.

In the Name field type a name for the WMS connection (eg. Bartholomew Post Office plan, 1865).
In the URL field, type the URL of the WMS layer you would like to add.
If you are using one of our WMS layers, copy and paste it from our list of WMS layers
You can of course, add a WMS layer from your own or another server that you are able to access.
Once you have completed these fields click the OK button. This will return you to the Add Layer from a Server window.
Now click the Connect button to connect to the WMS Layer.
The WMS Layer will now be displayed as a list of layers.
These layers correspond to the zoom levels of the WMS layer.
Click on all the layers to select them.

Now click the Add button to finish adding the WMS layer to your workspace
The WMS layer should now be displayed in your workspace and you can use the zoom and pan tools to navigate and view it within the QGIS workspace. You will find these in the toolbars displayed above the workspace.

Note: Changing the Coordinate System of your WMS Layer
By default, QGIS uses the WGS84 coordinate system, and will add and project your layers accordingly. It is possible to select a different coordinate system for a WMS layer when adding it to your workspace. When following the workflow above, once you have connected to your WMS layer and it is displayed as a list in the Add Layer from a Server window, click the Change coordinate reference system button. A dialogue box will be displayed with options to select from a list of available coordinate systems or to search for a specific coordinate system by it's EPSG number. You can also change any Layers' coordinate system at any time by right clicking it in the Layers window found on the right hand side of your workspace and selecting Properties from the menu.
Our georeferenced maps use either the British National Grid (EPSG:27700), or the Spherical Mercator coordinate systems. For further information, see our page on Coordinate Reference Systems.
We are now ready to add the map we want to Georeference.
From the top menu bar click Plugins and select Georeferencer then Georeferencer.
This opens the Georeferencer window.
We now want to open the map we would like to Georeference.
In the Georeferencer window's top menu bar click File and select Open Raster.
Now use the File Browser to locate the base map file you want to add.
Select it and click Open.
Your map should now be displayed in the Georeferencer window.
To georeference your map you need to add sets of Control Points to move it in to the correct position.
These sets of Control Points are used to link known locations on your map, and the georeferenced base map you have added.
Use the zoom and pan tools found at the top of the Georeferencer window to move the map to a location that you want to set a Control Point.

Now go to the Workspace window that displays your base map.
Use the pan and zoom tools at the top of this window to move the base map to the same location.

Return to the Georeferencer window and select the Add Control Point Tool.
Now click on your map in the Georeferencer window where you want to set a Control Point.

This will set a point on your map and bring up the Enter Map Coordinates dialogue box.

Click the From map canvas button. The Workspace window will now be displayed.
Now click on the base map at the matching location where you want to link your Control Point.

You will again be returned to the Georeferencer window.
Click OK to link the points you have set and finish setting this Control Point.
The minimum number of Control Points you need to set is three. For a better fit you will need to add more control points.
The Control Points you set are displayed in the Georeferencer Control Point (GCP) table at the bottom of the Georeferencer window.

Note: Deleting Control Points
If you are not happy with the placement of a Control Point and would like to delete it you can do so by right clicking the entry in the Georeferencer Control Point Table and seleting delete. You can also delete Control Points using the Delete Control Point tool. Select this tool from the Georeferencing toolbar, then use it to click on the Control Point in either the Georeferencer window or the Workspace window.
After placing the required number of Control Points, the Georeferencer window should look like this:
To finish and save your Georeferenced map correctly you need specify what transformation method will be used to transform it in to its new georeferenced position.
From the top menu bar of the Georeferencer window click Settings and select Transformation Settings.

The Transformation Settings window is displayed.

From the drop down menus, choose the Transformation Type and Resampling Method that will be used to output the new Georeferenced version of your map.
Under Output Raster, browse to the location you would like to save your Georeferenced Map and give it a suitable file name.
You can also specify the coordinate system the Georeferenced version map will be projected according to.
For British National Grid, specify EPSG:27700 in the Target SRS field.
Click the Load in QGIS checkbox.
This will add the new Georeferenced map to QGIS once saved so that you can check the result.
Click OK
To finish and export your georeferenced map, click the Start Georeferencing button found in the Main Toolbar at the top of the QGIS Workspace.
Note: Further information for choosing Transformation Type and Resampling Method:
In QGIS there are several methods for transforming the image, these include the linear / affine transformation, the Helmert, the 1st, 2nd and 3rd order polynomials, and the thin plate spline. These different Transformation Methods interpret your Control Points in different ways, and control how the map you are georeferencing is fitted and warped to your georeferenced base map.
For best fit, the thin-plate spline or higher-number polynomial transformations are often best. The spline transformation, as a true rubber sheeting method, transforms the source Control Points exactly to the target Control Points, and optimises for local accuracy opposed to global accuracy.
The Helmert transformation is often used for converting between coordinate systems in three-dimensional space. The higher-order polynomial transformations involve different degrees of warping, both of the source Control Points, and the pixels in between them.
The Resampling method determines how the pixels are output for our new Georeferenced map, determining how they are converted from one pixel array to another. The Nearest Neighbour default ususally gives good results.