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Georeferencing maps in QGIS

A practical demonstration of how to georeference historical 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.

Baghdad map without grid 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…”

Georeferencer screenshot 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 screenshot 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.

GCPs on map screenshot 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.
Final map screenshot The map has been added to QGIS.

Maps with grids

If your map has a grid then we can use it to accurately georeference the map, but we need to understand the grid properly first!

Degrees or metres?

Map grids usually use on one of two different units – degrees or metres. Maps that show latitude and longitude use a geographic coordinate system. This maps the Earth as a globe in degrees north or south of the equator and east or west of the prime meridian which, for historical reasons, runs north-south through Greenwich in London.
Globe with lat-lon grid 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).
Maps that use metres employ one of several different mathematical models to convert from the earth’s surface which is curved to an assumed flat area on which measurements in metres are meaningful. These are called projected coordinate systems and there are a number of different schemes in use. These are less accurate (because the Earth isn’t flat!) but can be more useful as distances are measured using units (meters, kilometres) that we are comfortable with. Also, because of the curvature of the earth’s surface, you can only use a single projection over a relatively small area – not, for example, an entire continent. This means that different maps will use different projections depending on the area covered and the date at which the map was created.
British grid and lat-lon grid 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.
We cannot cover coordinate systems properly here. However, all you need to know is whether your map uses degrees or metres and, if it uses metres, the projection it uses.

Understanding the map grid

The first step to georeferencing your map is to understand what grid it uses so you can setup the georeferencer properly.
Have a look at the numbers around the edges of your grid.
A geographic coordinate system grid using latitude and longitude will have values followed by a degrees symbol (e.g. 30°) and sometimes one or two extra numbers (e.g. 30° 45’ or e.g. 51° 30’ 30”). These are minutes and seconds – they divide degrees (and minutes) into 60 equal parts. Less often you will also see decimal degrees (e.g. 30.75°). They will also usually be followed by a letter: N or S for latitude to differentiate between the northern and southern hemisphere, and E or W for longitude to differentiate between the eastern and western hemisphere. Sometimes no letter is given and a minus is used for the southern and western hemispheres instead.
A projected coordinate system will have very long easting and northings followed by “m” for metres (e.g. 575500 m). What can be confusing is that as these numbers are so long, mapmakers often do not include them all every time, so you need to look carefully at the whole grid to see if some numbers have been excluded from the start or end of the coordinates.
Often a map will have both geographic and projection coordinate system grids on one map, one only as tickmarks around the edge of the map. In this case you can choose which to use for georeferencing.
Map grid example 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

Now you know which grid to use, you can setup 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.
If you are using a latitude and longitude grid to georeference your map, you can use “EPSG:4326 – WGS84”, if you are using a metres grid then you will have to find the details of the projection on the map itself and then find and select it in QGIS using the “Select CRS” button. QGIS already ‘knows’ most projections and can convert between them automatically.
Projection information map example The purple text is an example of the projection information you will find on a map…
Projection in QGIS screenshot … 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

We can now finally start adding GCPs using the grid!
  • 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!
If you are using metres make sure you include the full coordinate (not the shortened version). For example, from the blue-gridded map above, “1541000” not “41”. If you are using latitude and longitude, make sure you include any minutes or seconds given on the grid. For this you do not need the symbols, just leave a space between the numbers – so “31° 30’ 45”” should be typed as “31 30 45”.
GCP corner screenshot 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.
If you have no data added yet, type “world” into the coordinate box at the bottom of the window to show a simple world map to help orient yourself!
Four GCP screenshot 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!
The right number depends on the accuracy, scale and size of the map. Eight GCPs evenly distributed around the edge of the map is a useful minimum to aim for. If possible, add a few towards the middle of the map too.
  • 7) When you have finished adding GCPs you can run the georeferencer by navigating to “File” > “Start georeferencing”.
Your finished map will be added to QGIS when the process finishes.
Finished map screenshot 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!

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