Georeferencing maps in QGIS

Once we have downloaded a map that is of interest to us, we need to get it into QGIS! Just like the Corona imagery we will need to georeference it first. If the map has a grid then you can use exact coordinates taken from that grid to locate it, if it doesn’t we use a similar approach to the Corona imagery.

Maps without grids

First let’s cover maps without grids.

_{This map of Baghdad has no grid. Figure 79 in Naval Intelligence Division 1944, Iraq and the Persian Gulf, Oxford University Press: Oxford.}

Opening your map in the georeferencer

• 1) Start QGIS – you can either continue with a previous project or create a new one.
• 2) Add a modern satellite image of your area using “Layer” > “Add Layer” > “Add Raster Layer”.
• 3) Open the georeferencer by navigating to “Raster” > “Georeferencer…”.
• 4) Open your map in the georeferencer by navigating to “File” > “Open Raster…”

_{Adding a map to the georeferencer.}

Setup the georeferencer

• 1) Setup the georeferencer by navigating to “Settings” > “Transformation settings”.
• 2) Change “Transformation type” to “Polynomial 1”.
• 3) Make sure “Target SRS” is set to “EPSG:4326 – WGS84”.
• 4) Replace “modified” with “GR” in “Output raster”.
• 5) Change “Compression” to “LZW”.
• 6) Tick “Save GCP points”.
• 7) Tick “Load in QGIS when done”.
• 8) Click OK to return to the georeferencer.

_{Transformation settings for a map with no grid.}

Add ground control points (GCPs)

• 1) Add GCPs by finding common points between the map and the modern satellite image – depending on the size and accuracy of the map, five to ten high quality and well-spaced points should be sufficient.

_{Adding GCPs to a map without a grid.}

Run the georeferencer

• 1) Run the georeferencer by navigating to “File” > “Start Georeferencing”.

When it is complete your map will be added automatically to your QGIS project.

• 2) Zoom in and check you are happy with the results.

_{The accuracy of the georeferencing will depend on how accurately the map was drawn and on how accurate the GCPs were placed. You can follow the same steps to remove inaccurate GCPs described in Step 5.4.}

• 3) Once you are happy you can close the georeferencer.

_{The map has been added to QGIS.}

Maps with grids

Degrees or metres?

_{This 3D globe model with a one-degree grid shows how using latitude and longitude allows you to locate your position very accurately, but makes it much harder to gauge distance – notice how a single degree at the equator is a very different size to a single degree located near the poles. Courtesy of NASA (WorldWind).}

_{This is the British National Grid used to map the United Kingdom using metres, you can see the latitude and longitude grid behind changes shape even over this small area. You will also notice that this system has definite boundaries. For example, it doesn’t cover most of the Republic of Ireland, or extend far into Continental Europe. Data courtesy of NaturalEarth.}

Understanding the map grid

_{A typical grid system on a map – the blue grid uses metres (note the long and short metre easting and northing numbers), and the black numbers and tickmarks use latitude and longitude (degrees and minutes).}

Setting up the georeferencer

• 1) Start QGIS and the georeferencer and add your map to the georeferencer, as described above.
• 2) Navigate to “Settings” > “Transformation Settings”.
• 3) Change “Transformation type” to “Polynomial 1”.
• 4) Select the correct “Target SRS” for your map.

_{The purple text is an example of the projection information you will find on a map…}

_{… and this is the right projected coordinate system to select in QGIS! This was found by typing “Iraq Zone” into the “Filter” box. You will notice that the numbers in the bottom-left box match what is provided on the map.}

• 5) Replace “modified” with “GR” in “Output raster”.
• 6) Change “Compression” to “LZW”.
• 7) Tick “Save GCP points”.
• 8) Tick “Load in QGIS when done”.
• 9) Click OK to return to the georeferencer.

Adding GCPs

• 1) Click “Edit” > “Add Point”.
• 2) Zoom in and click on the top-left corner of the map grid.
• 3) Type the coordinates of the grid in the two boxes and then click the “OK” button – make sure you get them the right way round!

_{Adding the first GCP at the top-left of the map grid.}

• 4) Add three more GCPs to the other corners of the map.
• 5) Check the QGIS windows to make sure the GCPs are appearing in the right place.

_{Checking the map’s GCPs are located correctly in QGIS.}

• 6) Once you are happy that the GCPs are correctly located, you can add some more!

• 7) When you have finished adding GCPs you can run the georeferencer by navigating to “File” > “Start georeferencing”.

_{The georeferenced map has been added to QGIS.}

_{Will Corona imagery or historic maps be more useful in your area of interest?}

Congratulations! We have reached the end of our week learning about historical data and archaeology. In our final week we will be learning about how to put all the information we have learned together!