Worksheet 1: Warm up Qgis

Getting started with complex software is tedious and requires a certain level of frustration tolerance. Due to the similarity of software tools and concepts, it makes sense to build up analogue knowledge and access the workflow in a logically structured way, rather than learning by heart the individual steps. This is not a matter of course, especially as GIS software packages such as ArcGIS or QGIS are among the most complex software architectures.

What we want to do in this unit

In addition to the difficulty of learning new conceptual and technical problem areas, there is also the challenge of dealing with a software architecture with an unfamiliar graphical user interface (GUI). To alleviate this initial hurdle, you will take a step-by-step approach, starting in this first unit with some basic and typical tasks that will essentially familiarise you with the new software and data formats.

Learning Objectives

After completing this tutorial you will be able to * Install QGIS * Create a QGIS project file * Distinguish between data formats and data models * Open raster and vector data in QGIS * Check the properties of data layers * Create (digitise) geometries

Materials needed

Data

  • Digital aerial image of Marburg and surrounding area (example RGB image as raster dataset)
  • OSM Forest Objects Extract from the latest (11/2020) Open Streetmap (OSM) dataset.
  • OSM Street Objects Extract from the latest Open Streetmap (OSM) dataset.
  • [OSM POIs] (https://raw.githubusercontent.com/gisma-courses/geoinfo-basis-qgis/master/docs/assets/data/mr_objects.xls) OSM Points of Interest (POI) as Excel spreadsheet.

Software

Introductory materials

  • A gentle introduction to GIS](https://docs.qgis.org/3.28/de/docs/gentle_gis_introduction/index.html){.external target=“_blank”}: provides a successful QGIS-centred overview of GI concepts. Highly recommended additional learning materials
  • QGIS User Manual 3.28 EN](https://docs.qgis.org/3.28/de/docs/user_manual/index.html){.external target=“_blank”}: The current English version. It is the reference for users.

Tasks 1

Task 01-01

This worksheet serves as an introduction to the different data models in GIS. You will also learn how to import your own spatial data into the GIS or how to create spatial data in the GIS, e.g. by digitising.

Task 01-01
  • Create a directory that is named without special characters and spaces (throughout the path/directory tree).
  • Create a QGIS project
  • Download the marburg_RE.tif file.
  • Find out about the properties of the dataset. (projection, data model, range of values)
  • Create a subdirectory called Data as a subfolder of your project directory. Download the dataset containing Hesse from the geofabrik download page. Unzip this archive into the Data subfolder.
  • After unzipping the archive, load the file gis_osm_pois_free_1.shp into your QGIS project.
  • Read about the properties of this dataset. (projection, data model, range of values).
  • Crop the gis_osm_pois_free_1 dataset to the size of the marburg_RE.tif aerial image.
  • Export this point data in geopackage format under the name mr_points.
  • Import the mr_objects.xls table (Spatial Objects dataset (https://raw.githubusercontent.com/gisma-courses/geoinfo-basis-qgis/master/docs/assets/data/mr_objects.xls)) as a spatial layer into your QGIS project.
  • Create a spatial layer based on the aerial image marburg_RE.tif with the coordinate system EPSG:4326 WGS84:
    • any three surfaces (filename mr_poly),
    • any three polylines (filename mr_line)
    • Finally, add 3 arbitrary positions (points) of your choice to the mr_points layer.

Task 01-02

Unfortunately, we cannot completely bypass the topic of the correct location of geodata sets - i.e. the appropriate projection. In Task 01-01 you imported raster and vector data into QGIS and created your own vector data. The spatial information of the data was available in geographic coordinates. The QGIS software you used always performs a real-time projection to project these spherical coordinates onto the flat monitor. This has nothing to do with a cartographically correct projection. As a rule of thumb, almost all spatial analysis methods and geometric calculations can only be performed on correctly projected data.

Warning

To begin with, we can only urge you to keep the CRS (Coordinate Reference System) or KBS (Koordinatenbezugssystem) of your project and each data layer identical. For Germany the official system is ETRS89 UTM 32. This helps to avoid wrong positioning, one of the most common mistakes in the GIS world.

Task 01-02
  • Create a new QGIS project. First load the raster file marburg_RE.tif and then the vector datasets mr_roads and mr_nat.
    • What are the projections of each dataset?
    • In which projection are the data displayed?
    • Where can you define the projection used to display the data?
    • What are the possible problems of an on-the-fly or real-time projection?

Assistance

We would like to offer you some introductory help on QGIS as a concrete introduction to working on Task 1. QGIS is a complex software product that is not only available in different versions for all common operating systems, but is also completely customisable. For this reason, a large number of help files, manuals and tutorials have been circulating on the internet for the past 20 years - by no means will every Google hit be appropriate, nor will every tutorial be suitable for the version you are using.

The first port of call for specific information is the built-in help section of the software package. Extensive documentation is also available. The current QGIS Landing Page is the central access point to all documentation officially provided by the project.

The following applies to all help pages: be sure to look at the help for your version of QGIS. If it is not (yet) available, try the previous version and preferably the English help, as this is the reference for all translations.

Warning

The transition from QGIS2.x to QGIS3.x has brought fundamental changes. QGIS 2.x tutorials and help are virtually useless as a concrete guide for the current version.

Setting up a QGIS project

For computers with a home directory in the cloud/network (e.g. in university pools) it is strongly recommended to store the data on a local hard drive. To do this, you must first create a folder structure for your project. Help on the project file can be found in Working with project files.

Warning

Make sure you do not use any special characters, umlauts or spaces.

Digitising Geometry Data

The manual creation of vector data is commonly referred to as digitising. You create your own vector-based datasets from raster image files such as satellite imagery, aerial photographs, thematic and topographic maps, simple screenshots or other templates. See the QGIS help for digitising. For a detailed application example, see Digitising forest stands.

Cropping vector data

This task is a good example of how difficult it can be to find help. Of course, you can always search the web. For example, try “crop vector data qgis”. You will get a lot of hits, but on closer inspection you will not find anything about cropping vector data using a raster file. What happens when you crop? You obviously need to tell a tool what you want to crop. Start again with this knowledge. Only this time use the toolbar and type “clip” or “clip”. Several hits will be displayed. One of them is Clip vector to extent. If you select this, you can choose three options for the extension…

Importing tables into QGIS

There are a number of pitfalls when importing tables. In general, table data is only useful in QGIS if it contains either coordinates (i.e. geographic information) or a key such as an ID that can be assigned to existing geometry data. In general, however, they can follow the following scheme Importing spreadsheets or CSV files.

Working with projections

The chapter Working with projections is not easy for beginners. The correct assignment of projections is particularly important. For reprojecting vector data it is worth having a look at Reprojecting and transforming data.