Tutorial on using QGIS for marine data CMEMS Service Desk March 2017 Table of contents Introduction ............................................................................................................................................. 2 I / Creation of maps showing global surface temperatures and salinities .............................................. 3 II/ Creation of global maps representing sea surface height (SSH) ....................................................... 16 1. Using the regular Points tool ..................................................................................................... 16 2. Using the Point Sampling Tool .................................................................................................. 19 3. Using the Contour tool .............................................................................................................. 20 III/ Producing global maps that display ocean currents ........................................................................ 23 1.1. Using the Vector Field Calc tool to obtain the magnitude and orientation of u and v .............. 23 1.2 Using the regular Points tool ................................................................................................. 26 1.3 Using the Point Sampling Tool .............................................................................................. 26 1.4 Processing the arrows representing the intensity and orientation of currents .................... 26 2.1 Using the Vector Field Calc tool to obtain the magnitude from u and v............................... 30 2.2 Using the regular Points tool ................................................................................................. 30 2.3 Using the Point Sampling Tool located in the toolbar ........................................................... 30 2.4 Processing the arrows representing the intensity dots and orientation of currents ............ 30 IV/ Overlaying multiple variables on a single map ................................................................................ 32 1. Overlaying SSH with current speeds ......................................................................................... 32 2. Overlay of two or more raster layers ........................................................................................ 33 VI/ Producing regional maps ................................................................................................................. 35 1. Differences in the representation of SSH for a regional area at 1/12° ..................................... 35 2. Differences in the representation of currents for a regional area at 1/12° .............................. 36 VII/ Producing coastal maps .................................................................................................................. 36 1. Differences in the representation of SSH for a coastal area at 1/36° ....................................... 36 2. Differences in the representation of currents for a regional area at 1/36° .............................. 37 VIII/ Tips for QGIS .................................................................................................................................. 38 1. Clipping a raster ......................................................................................................................... 38 2. Zooming on the map area ......................................................................................................... 39 3. Copying and pasting styles ........................................................................................................ 40 4. Viewing data on a raster and on the vector/utilisation part of the tool tip on a vector .......... 41 5. Loading a new extension/plugin ............................................................................................... 44 IX/ Useful links and documents for using QGIS ..................................................................................... 45 1 Introduction QGIS is a popular, free, open source Geographic Information System (GIS), which runs on all major operating systems. People often use QGIS to view, edit, and analyse geospatial data. QGIS has a comprehensive website (http://qgis.org ), which makes it easy to download, install, and use. There is also an international community of users and developers which is very helpful and reactive (mailing list of QGIS users: [email protected]; QGIS developers: [email protected] ). Installing and running QGIS : If you haven't already installed QGIS, click on the Download Now button on the main QGIS web page to download the QGIS software (http://www.qgis.org/fr/site/). What you do next depends on which operating system you are running on your computer: - For MS Windows, you can download a double-clickable installer that installs QGIS and all the required libraries in one go. Make sure you use the OSGeo4W installer, QGIS itself, and all the required libraries. - For Mac OS X, you'll need to visit the Kyngchaos website (http://www.kyngchaos.com/software/qgis ) to download and install the GDAL library, before installing a version of QGIS specially built for your operating system. All the required packages are available from the Kyngchaos site. - For Unix-like systems, you'll use a package manager to download, compile, and install QGIS and the required libraries from an appropriate package repository. More information about installing on a Unix-like system can be found at http://qgis.org/en/site/forusers/alldownloads.html#linux . Once you have installed the QGIS system, you can run it just like any other application on your computer, for example, by double-clicking on the QGIS icon in your Applications folder. If everything goes well, the QGIS application will start up. Then, you will need the following plugins that you can download on the QGIS repository (https://plugins.qgis.org/plugins/) or directly through the software interface: - VectorFieldCalc (Calculates vector field parameters and pathlines, given two rasters representing x and y velocities) - VectorFieldRenderer (Draws a point layer with arrow symbols) - Point Sampling Tool (Samples polygon attributes and raster values from multiple layers at specified sampling points) - Contour plugin (Generates contours of a set of data points) - NetCDF Browser (Allows you to browse multi-variable and multi-dimensional netCDF files) In addition, you will need several files and images before launching the software: - Basemaps, - Colour palettes in .xml - Grids with several mesh sizes - A template in .qpt for the map composer 2 This tutorial provides a detailed explanation of how to produce maps of oceans of different areas, representing one or more variables from netcdf files (.nc), which are available in the CMEMS catalogue (http://marine.copernicus.eu/). I / Creation of maps showing global surface temperature and salinity Open QGIS 2.xx There are different ways to open a netCDF file in QGIS. Either click on the icon on the top left, as shown below, or drag and drop a file directly onto the map canvas. Choose a netCDF file (.nc), then click on the variable you want; for example, temperature. Or, using the NetCDF Browser plugin: 3 Using this plugin, it is possible to directly extract one or more variables, one or more time steps, and one or more depths. This can be useful when the file is large and includes these concatenated parameters. Once you have selected the file, select the projection system, WGS 84 (EPSG: 4326). The file will then appear. Double click on the Temperature layer to display the properties. The "Metadata” tab provides all information related to the file: name, date, size, units, etc. The unit of temperature is degrees Celsius. Sometimes, though, it may be in Kelvin. The "Style" tab allows the appearance of the layer to be changed: 4 Go to Render type and choose Singleband pseudo colour then Generate new colour map. To generate a customised colour palette, you must import your own palettes by going to the "Project" menu, choosing Project properties and then the Default styles tab. In the Colour Ramp field, edit your settings by clicking on the icon on the right or on the Style Manager button. 5 In the new window, click on the Colour ramp tab and the Share icon on the lower right, then choose the Import option. Browse for the file in .xml format from a directory or an url, then import it. Each palette has to be imported individually. You will therefore, need to carry out this operation for each palette you want to import. 6 If you want to modify a particular palette, simply select it by double clicking it, or by clicking on Edit. A wizard, with graduated colour displays, opens in a new window. You can modify the start or end colour, as well as that for multiple stops, along with their offsets. You can also add or remove interval stop. Close the Style Manager window when you have finished. Once the new palettes have been imported, return to the Style tab with the layer properties. Choose the most appropriate colour palette for the temperature variable. To increase or decrease the number of classes, choose the equal interval mode. Generally, the number of classes varies from about 20 to 40. The minimum and maximum values are loaded by default, but it is best to adapt them as a function of the area chosen and what you want to highlight. The colour settings can be further customised by changing the brightness, saturation and contrast. Generally, resampling