Hi! I’m Natalia Erazo, currently working on the Ecuador project aimed at examining biogeochemical processes in mangrove forest. In this tutorial, we’ll learn the basics of (free) QGIS, how to import vector data, and make a map using data obtained from our recent field trip to the Ecological Reserve Cayapas Mataje in Ecuador! We’ll also learn standard map elements and QGIS function: Print Composer to generate a map.
I. Install QGIS
II. Learn how to upload raster data using the Plugin OpenLayers and QuickMap services.
III. Learn how to import vector data: import latitude, longitude data and additional data. Learn how to select attributes from the data e.g., salinity values and plot them.
IV. Make a map using Print Composer in QGIS.
I. QGIS- Installation
QGIS is a very powerful tool and user friendly open source geographical system that runs on linux, unix, mac, and windows. QGIS can be downloaded here . You should follow the instructions and install gdal complete.pkg, numpy.pkg, matplotlib.pkg, and qgis.pkg.
II.Install QGIS Plug-in and Upload a base map.
- Install QGIS Plug-in
Go to Plugins and select Manage and Install plugins. This will open the plugins dialogue box and type OpenLayers Plugin and click on Install plugin.
This plugin will give you access to Google Maps, openStreet map layers and others, and it is very useful to make quick maps from Google satellite, physical, and street layers. However, the OpenLayers plugin could generate zoom errors in your maps. There is another plug in: Quick Map Service which uses tile servers and not the direct api for getting Google layers and others. This is a very useful plugin which offers more options for base maps and less zoom errors. To install it you should follow the same steps as you did for the OpenLayers plugin except this time you’ll type QuickMap Service and install the plugin.
Also, If you want to experiment with QuickMap services you can expand the plugin: Go to Web->Quick Map Services->Settings->More services and click on get contributed pack. This will generate more options for mapping.
2. Add the base layer Map:
I recommend playing with the various options in either OpenLayers like the Google satellite, physical, and other maps layers, or QuickMap Service.
For this map, we will use ESRI library from QuickMap services. Go to–> Web- ->QuickMapServices–> Esri–> ESRI Satellite
You can click on the zoom in icon to adjust the zoom, as shown in the map below where I zoom in the Galapagos Islands. You’ll also notice that on the left side you have a Layers panel box, this box shows all the layers you add to your map. Layers can be raster data or vector data, in this case we see the layer: ESRI Satellite. At the far left you’ll see a list of icons that are used to import your layers. It is important to know what kind of data you are importing to QGIS to use the correct function.
We will now add our data file which contains latitude and longitude of all the sites we collected samples, in addition to values for salinity, temperature, and turbidity. You can do this with your own data by creating a file in excel and have a column with longitude and latitude values and columns with other variables and save it as a csv file. To input data you’ll go to the icons on the far left and click on “Add Delimited Text Layer”. Or you can click on Layer-> Add Layer-> Add Delimited Text Layer.
You’ll browse to the file with your data. Make sure that csv is selected for File format. Additionally, make sure that X field represents the column for your longitude points and Y field for latitude. QGIS is smart enough to recognize longitude and latitude columns but double check! You can also see an overview of the data with columns for latitude, longitude, Barometer mmHg, conductivity, Salinity psu and other variables. You can leave everything else as default and click ok.
You’ll be prompt to select the coordinate reference system selector, and this is very important because if you do not select the right one you’ll get your points in the wrong location. For GPS coordinates, as the data we are using here, you need to select WGS 84 ESPG 43126.
As we saw earlier, the data contains environmental measurements such as: salinity, turbidity, temperature and others. We can style the layer with our sampling points based on the variables of our data. In this example we will create a layer representing salinity values.
You’ll right click on the layer with our data in the Layer Panel, in this case our layer: 2017_ecuador_ysi_dat.. and select properties.
The are many styles you can choose for the layer and the styling options are located in the Style tab of the Properties dialogue. Clicking on the drop-down bottom in the Style dialogue, you’ll see there are five options available: Single Symbol, Categorized, Graduated, Rule Based and Point displacement. We’ll use Graduated which allows you to break down the data in unique classes. Here we will use the salinity values and will classify them into 3 classes: low, medium, and high salinity. There are 5 modes available in the Graduated style to do this: Equal interval, Quantile, Natural breaks, Standard deviation and Pretty breaks. You can read more about these options in qgis documentation.
In this tutorial, for simplicity we’ll use the Quantile option. This method will decide the classes such that number of values in each class are the same; for example, if there are 100 values and we want 4 classes, the quantile method decide the classes such that each class will have 25 values.
In the Style section: Select->Graduated, in Column->salinity psu, and in color ramp we’ll do colors ranging from yellow to red.
In the classes box write down 3 and select mode–>Quantile. Click on classify, and QGIS will classify your values in different ranges.
Now, I am going to get rid of the black outline to make the points easy to visualize. Select the point by clicking on Simple Marker and in Outline style select the No Pen. Do the same for the remaining two points.
We can start to assemble the final version of our map. QGIS has the option to create a Print composer where you can edit your map. Go to Project -> New Print composer
In the Print composer window, we want to bring the map view that we see in the QGIS canvas to the composer. Go to Layout-> Add a Map. Once the Add map button is active, hold the left mouse and drag a rectangle where you want to insert the map. You will see that the rectangle window will be rendered with the map from the main QGIS canvas.
You can see in the far right end the Items box; this shows you the map you just added. If you want to make changes, you’ll select the map and edit it under item properties. Sometimes it is useful to edit the scale until you are happy with the map.
We can also add a second map of the location of Cayapas Mataje in South America as a geographic reference. Go to the main qgis canvas and zoom out the map until you can see where in South America the reserve is located.
Now go back to Print Composer and add the map of the entire region. You’ll do the same as with the first map. Go to Layout–> Add map. Drag a rectangle where you want to insert the map. You will see that the rectangle window will be rendered with the map from the main QGIS canvas. In Items box, you can see you have Map 0 and Map 1. Select Map 1, and add a frame under Item properties, click on Frame to activate it and adjust the thickness to 0.40mm.
On the right-hand panel, click on the Item Properties tab and expand the Search directories and select the north arrow image you like the most. Once you’ve selected your image, you can always edit the arrow under SVG parameters.
Now we’ll add a scale bar. Click on Layout–> Add a Scale bar. Click on the layout where you want the scale bar to appear. Choose the Style and units that fit your requirement. In the Segments panel, you can adjust the number of segments and their size. Make sure Map 0 is selected under main properties.
I’ll add a legend to the map. Go to Layout–> add a Legend. Hold on the left mouse button, and draw a rectangle on the area you want the legend to appear. You can make any changes such as adding a title in the item properties, changing fonts and renaming your legend points by clicking on them and writing the text you want.
It’s time to label our map. Click on Layout ‣ Add Label. Click on the map and draw a box where the label should be. In the Item Properties tab, expand the Label section and enter the text as shown below. You can also make additional changes to your font, size by editing the label under Appearance.
Here is a list of links that could help with QGIS:
-QGIS blog with various tutorials and new info on functions to use: here.
-If you want more information on how QGIS handles symbol and vector data styling: here is a good tutorial.
-If you need data, a good place to start is Natural Earth: Free vector and raster basemap data used for almost any cartographic endeavor.
If you have specific questions please don’t hesitate to ask.