Worksheet 2 Flashcards
Layers and Projects in GIS
In GIS, layers are saved together first as a project, then as a Geopackage as you progress. Each project comprises one or more ‘layers’. A layer is visible only when activated by clicking its empty box. A layer panel contains one or more layers, each representing geographic features like towns or roads. Each data frame and layer has specific properties such as colors, symbols, and labels, which can be customized.
Considerations for Setting Up a New GIS Project
When setting up a new project in GIS, consider the map projection system used. The most common international projection is WGS84, which converts the near-spherical Earth into a flat map. However, no projection can perfectly transform the Earth’s surface. This means that different projection systems may not perfectly overlap. In QGIS, using the British National Grid (BNG) may require transforming other projections like OpenStreetMap for alignment. While most conversions are automatic, data from different projection systems may pose importing challenges. Seek advice during practicals or online help sessions if needed, as some map transformations may not be solvable in a second-year module.
Saving QGIS Projects
In QGIS, projects can be saved as .qgs or .qgz files. The default is .qgz, a compressed format that includes all the data saved in a .qgs file but in a zipped (compressed) format, saving memory space.
What items are saved when you save a QGIS project? (11)
- Layers added
- Which layers can be queried
- Layer properties, including symbolization and styles
- Projection for the map view
- Last viewed extent
- Digitizing settings
- Table Relations
- Project default styles
- Plugins settings
- QGIS Server settings from the OWS settings tab in the Project properties
- Queries stored in the DB Manage
Understanding QGIS Projects
A QGIS project does not store the source files; it only records their locations and adds them each time the project is opened. Therefore, if you move or rename a source file, QGIS may be unable to open that project layer.
Customizing Map Symbols in GIS
Manually customizing map symbols provides complete control but is time-consuming, suitable mainly for single-layer projects like roads. For comprehensive maps with multiple layers (e.g., buildings, railways), it’s typical to use a pre-prepared ‘style guide’ provided by data suppliers like Ordnance Survey
Transforming Layers in QGIS
When setting the default CRS to British National Grid, the OpenStreetMap layer will require transformation each time it’s opened. A transformation wizard will pop up, where you should select the second line highlighted in blue and click ‘ok’. This window offers multiple transformation options visible upon scrolling down. Transformations use geomatics to reproject a map to a new system, often resulting in errors and imperfect alignment with other layers. However, the imperfection when transforming OpenStreetMap to British National Grid is usually negligible.
Cartographic Style in GIS
A cartographic style represents geographic data visually, providing a standard way to display map features like roads for easy comprehension by GIS map users. Style guides come in various file types, such as VML, SLD, and GML, which we’re using today.
Coordinate System in British National Grid
In the British National Grid, X corresponds to longitude, while Y corresponds to latitude. This can be counterintuitive, as in most maps, longitude varies along the x-axis. This difference often leads to confusion in dissertation projects!
Checking Sighting Records in the Database
After adding data, verify 21 sighting records in the database by checking the number of records in the attribute table. Note the explicit link between a feature on the screen and a row in the table, which forms the core of relational database systems in GIS software. Geographic databases manage GIS data through its locational information on the map
Clipping and Erasing in Spatial Analysis
Clipping and erasing are powerful spatial analysis tools. To clip or erase, you create a mask from points, lines, or polygons, and then use it to either remove or retain features underlying the mask
Erase is the exact opposite to clip
