Goals:

Introduction to “geographical information systems” (GIS) and some of the basics techniques. The focus will be mainly on how one can digitize an image of a map—the process of georeferencing—and extract useful geographical information from it.

Software:

• QGIS, a Free and Open Source Geographic Information System (install it from https://qgis.org/en/site/)

Class:

• Basics of QGIS
• Basics of Georeferencing

Projection Issues

See, https://en.wikipedia.org/wiki/List_of_map_projections.

A Digital Map: Layers of Goodness

• Layers:
  • Analytical Layer
  • Our Data
  • Annotation/Legend
• Social Geography
  • Political Boundaries
  • Settlements, etc.
• Physical Geography
  • Types of surface
  • Continents / Coastal Line
  • Elevation profile
  • Rivers, Lakes, etc.
• Base Layer: Graticule

Main Types of Data: Points, Lines, Polygons

• Analytical Layer
  • Our Data
    • Points:
      • item, x[1], point(lat, lon)[2]
    • Lines:
      • item, x[1], from(lat, lon)[2], to(lat, lon)[2]
    • Polygons:
      • item, x, area(lat, lon; lat, lon; … lat, lon)[2]
  • Annotation/Legend

[1] where x is a categorical parameter; [2] lat/lon: decimal coordinates (not DMS)

Georeferencing

QGIS must be installed and running. Note: Interfaces might differ from system to system; the same applies to different versions of QGIS. Before we proceed, you may want to add Google Layers. To Add Google maps layer: (on the left pane, Browser) XYZ > New connection > paste one of the following links (Source: https://geogeek.xyz/how-to-add-google-maps-layers-in-qgis-3.html):

1. Google Maps: https://mt1.google.com/vt/lyrs=r&x={x}&y={y}&z={z}
2. Google Satellite: http://www.google.cn/maps/vt?lyrs=s@189&gl=cn&x={x}&y={y}&z={z}
3. Google Satellite Hybrid: https://mt1.google.com/vt/lyrs=y&x={x}&y={y}&z={z}
4. Google Terrain: https://mt1.google.com/vt/lyrs=t&x={x}&y={y}&z={z}
5. Google Roads: https://mt1.google.com/vt/lyrs=h&x={x}&y={y}&z={z}

NB: Images of maps to work with below

Step-by-step instructions

• In your XYZ Tiles, double click on any Google layer that you added (or on OpenStreetMap)
  • A layer should be added to the main map (see, Layer pane)
• Open Georeferencer: Raster > Georeferencer
  • if you do not see it there, it must be activated first:
    • Plugins > Manage and Install Plugins... > [search for georeferencer], then tick a box against Georeferencer GDAL
• In Georeferencer: File > Open Raster (also the first button on the main panel)
  • Choose a file. Lets start with US_eastStates_03.jpg (this one has a grid, so it will be easier to process)
  • In Coordinate Reference System Selector, choose WGS 84 / Pseudo-Mercator, then clock OK
• Open Settings > Transformation settings (or click the yellow gears button)
  • Choose settings like on the screenshot below (you can also change Compression setting, which will generate a smaller map-image):
• Now, adding georeference points:
  • If you have a grid, it is very easy: just type in coordinates
  • If you don’t, it is tricky: you need to find the same features on the image and on the map (for this, click on From Map Canvas)
• [!!!] Ideally, if you know projection of the map, you only need a few points; this almost never happens, so, the more points you create, the more precisely your image will be georeferenced. This is easy when you have a grid.
• When you are done with collecting points, click on PLAY button (green triangle)
  • The image of your georeferenced map should appear in the main window.
• [!!!] Keep in mind that you add layers to your map and a layer on top may cover the layer below!

In general, this procedure can be extremely helpful if:

• you need to collect geographical information from historical maps
• you want to use some historical map as the base layer for mapping your data

Collecting Point Data (Very Simple Way)

• Create a CSV file; copy/paste the content from below (three lines of text!)

city,state,lon,lat,lon1,lat1
New York,New York,-8234229.297,4970380.761,-8234229.297,4970380.761
Philadelphia,Pennsylvania,-8368867.370,4858182.367,-8368867.370,4858182.367

• In QGIS, load this file by:
  • Layer > Add Layer > Add Delimited Text Layer
  • Settings should look like on the screenshor below
    • CSV (comma separated format)
    • Point coodinates (X field: lon; Y field: lat)
    • Geometry CRS: EPSG:4325 - WGS 84 / Pseudo Mercator
    • (you will have only two lines of data, instead of five)
    • Make sure to click Add when you are done!
• The layer is there, but we need to add labels to see the cities
  • Right-click on the layer > Properties
  • On the tab Labels change the settings like on the screenshot below
  • Click Apply, then OK.
• You should see two cities now on the map:
  • If nothing appears, try: Right-click on the layer > Set CRS > Set Layer CRS: Choose WGS 84 / Pseudo-Mercator
• Now, we can use Coordinate Capture plugin to collect data from our georeferenced map:
  • If activated, coordinarte capture should be visible in lower right corner
    • To activate coordinate capture: Plugins > Manage and Install Plugins... > [search for coordinate capture], then tick a box against Coordinate Capture
• Collecting coordinates:
  • Keep the initial CSV open in some editor
  • In Coordinate Capture, click Start capture
    • Click on any point on the map > coordinates of that location will appear in the plugin
    • Click Copy to Clipboard
  • Go back to the open CSV:
    • Add a new line:
      • Type the name of the city
      • Add comma and type the name of the state (alternatively, just add another comma)
      • Now, paste what you copied into Clipboard (this will add two sets of coordinates)
    • Save the file
  • Back in QGIS:
    • Switch the layer off and on: after that the newly-added city must appear on the map.
  • Repeat, until all required data is collected.

Images of Maps

Reference Materials:

• Clifford, Jim, Josh MacFadyen, and Daniel Macfarlane. 2013. “Installing QGIS 2.0 and Adding Layers.” Programming Historian, December. https://programminghistorian.org/lessons/qgis-layers.

• Clifford, Jim, Josh MacFadyen, and Daniel Macfarlane. 2013. “Georeferencing in QGIS 2.0.” Programming Historian, December. https://programminghistorian.org/lessons/georeferencing-qgis.

Homework:

1. Georeference two of three offered maps of the US during the Civil War;
2. Collect coordinates for 20 place names from each map (into a CSV format: use the same structure as given above).
3. Prepare a blogpost on your website with screenshots of both maps and paste the content of your CSV files below each map.
4. Codecademy’s Learn Python, Unit 3-4.
5. Github: publish the confirmation screenshot as a post on your new site.