Georeference Paper Maps and Images in QGIS

Introduction

Images such as paper maps and scanned drawings carry no location information, so they do not appear in the correct position in GIS. A process called “georeferencing” solves this problem.

This article introduces the basic concepts and steps of georeferencing, which gives coordinate information to a map image in QGIS.

What is georeferencing?

Georeferencing is the process of assigning geographic location information (coordinates) to an image or drawing. You use it when you want to place a paper map, a scanned image, or a drawing in PDF format at the correct position in GIS.

In georeferencing, you match specific points in the image to real geographic coordinates (such as latitude and longitude). These points are called GCPs (Ground Control Points). Once you set several of them, the image can be displayed on top of other map layers.

QGIS 3.26 and later can also georeference vector data that has no location information, such as CAD data.

Try georeferencing in QGIS

This article uses a map of central Sapporo from the Meiji era, taken from the image data published in the National Diet Library Digital Collections.

Open the Georeferencer

Georeferencing has several steps. Follow them in order and you can complete it without getting lost.

From the menu bar, choose Layer → Georeferencer.

The Georeferencer opens in a separate window.

Window overview

The Georeferencer window has the following tools.

1. Start Georeferencing: output the georeferenced image data
2. Transformation Settings: set the parameters for the image output
3. Add GCP Point: specify the locations you want to align (a GCP is an alignment point)
4. Delete GCP Point: delete GCPs you do not need
5. Pan: move around the displayed image

Add the image data

First, add the image you want to georeference. Click Open Raster, the leftmost button on the toolbar.

Select the image you want to use and load it.

The image you loaded appears.

Place GCPs

Next, add GCPs to give the image location information.

The details depend on the transformation type, but to improve accuracy, you can generally align more precisely by placing four or more GCPs in a well-balanced way. Spreading GCPs across the four corners of the image in particular reduces the overall distortion.

Add a GCP

Click Add GCP Point, then click the location you want to align in the Georeferencer window.

The Enter Map Coordinates dialog opens. Click From Map Canvas.

The QGIS map canvas appears. Using a basemap such as GSI Maps as a reference, click the same location you just specified in the Georeferencer.

The Enter Map Coordinates dialog reappears. Check that the coordinates are filled in, then click OK.

Repeat these steps to place several GCPs. This example places nine GCPs, mostly along the outer part of the image.

In the GCP table, you can check the accuracy by looking at the residual values. A “residual” is the gap between a position on the image and the matching position on the map, measured in pixels. The smaller the residual, the more accurate the GCP position. The larger it is, the bigger the gap, and the more the GCP needs correcting. In this example, the residuals fall between 2 and 10 pixels, so the georeferencing is accurate enough.

Run the georeferencing

When the GCPs are in place, run the georeferencing. Click Transformation Settings and configure the georeferencing options.

This example uses the following settings.

1. Transformation type: select Polynomial 1 (first-order polynomial).

   Note: The transformation type is the calculation method that converts the pixels of the map image into real geographic coordinates.

2. Target CRS: select EPSG:30172 - Tokyo / Japan Plane Rectangular CS XII.

   Note: Set the target CRS to match the coordinate system (geodetic datum) of the map you are using.

   Choosing the wrong CRS can shift the georeferencing result far from the correct position.

3. Output raster: click the ... button and specify the folder and file name to save to.

4. Resampling method: select Bilinear (2x2 Kernel).

   Note: Resampling an image means adjusting its number of pixels when you change its resolution or size. Image quality and processing time differ by method.

5. When all the settings are complete, click OK.

After the settings are complete, click Start Georeferencing on the toolbar to run the transformation.

QGIS adds the georeferenced image data.

To check that the image overlays the correct position, set the transparency of the georeferenced image. First, right-click the georeferenced image and click Properties.

When Layer Properties opens, configure the following.

1. Select Transparency from the menu on the left.
2. Global Opacity: adjust the slider to set the transparency (for example, 70%).
3. NoData: to make the black areas created by the tilt transparent, enter 0 for Additional NoData value. Set this as needed.
4. Click OK to apply.

Check key features such as roads, rivers, and buildings. If they roughly match the basemap, the georeferencing is done correctly.

If the position is far off, review the positions and number of GCPs and georeference again. Be careful in particular: accuracy drops when the GCPs are clustered on one side of the image or when their residuals are too large.

Conclusion

Georeferencing may look difficult at first, but anyone can do it once they know the steps.

QGIS offers powerful georeferencing for free, so try reusing the materials you have on hand as digital maps.