QGIS document from the previous exercise: worldmap.qgs

Transcription

1 MAP PROJECTION 1. Introduction: All data in a GIS view must be in the same projection in order to correctly align with other datasets. In QGIS this is often done in the background. QGIS will use the projection of the first dataset loaded to set the projection for the view. NEEDED HERE: o Installed and Running - QGIS RESOURCES: QGIS document from the previous exercise: worldmap.qgs 2. Data Fame Projection: All work in these exercises will be done using the WGS84: EPSG 4326 (Geographic Coordinates Systems) map projection. Open QGIS. Start using with an existing map (worldmap.qgs) Name of the document file should already be listed, as it was previously accessed. If not, navigate to C:/GISWorkshop/Exercise If you examine closely (zoom in) in the map you will see both map not align properly. To check the projection of the data frame click on the CRS Status button in the Status Bar along the bottom-right of the QGIS window. You will see the default projection is WGS84 Pseudo Mercator: EPSG 3857 (Projected Coordinate System).

2 To change the projection to WGS84 (Geographic Coordinates Systems), simply click on WGS 84: EPSG You will find an error will occur. It is because the open layers plugin use projected coordinate system as their reference projection. Remove google physical map. Change data frame projection to WGS 84: EPSG 4326 Click OK You will see a message (Phyton error) appear on top of the window. This is because image from google usually use Projected Coordinate System (WGS 8/Pseudo Mercator: EPSG 3857) Close QGIS without saving the project. 3. ESRI Online World Imagery: The code adds an ESRI Online World Imagery base map to QGIS. It has a number of advantages over the popular OpenLayers Plugin that adds various Google, Bing and OpenStreetMap image layers to QGIS. Unlike images downloaded by the OpenLayers plugin the ESRI World Imagery base map is a true Raster who s attributes are fully editable e.g. brightness, blending mode and transparency can be adjusted. World Imagery can also be printed at a very high resolution with other QGIS layers on a map and without it shifting relative to other layers; a conspicuous problem with OpenLayers that does not use On the Fly PHYTON SCRIPT FOR ESRI ONLINE WORLD IMAGERY: qgis.utils.iface.addrasterlayer(" r.arcgisonline.com/arcgis/rest/services/esri _Imagery_World_2D/MapServer?f=json&pr etty=true","raster")

3 re-projection and only prints Google, Bing layers at a low resolution. It is an ideal aerial base map. Open new QGIS document. Simply copy and paste the following code into the Python Console (plugins>phyton console) in QGIS and press Enter. Rename the layer to World Map (Geographic) by clicking on layer, choose properties, simply change the name on Layer Name column. As this layer is the geographic coordinate system (WGS 84: EPSG 4326), 4. Projections: The CRS that all the data as well as the map itself are in right now is called WGS84. This is a very common Geographic Coordinate System (GCS) for representing data. But there s a problem, as we will see. Use your current map. Zoom in to South Africa by using the Zoom In tool. Try setting a scale in the Scale field, which is in the Status Bar along the bottom of the screen. While over South Africa, set this value to 1: (one to five hundred thousand). Pan around the map while keeping an eye on the Scale field. Scale Bar

4 Notice the scale changing? That s because you re moving away from the one point that you zoomed into at 1:500000, which was at the center of your screen. All around that point, the scale is different. To understand why, think about a globe of the Earth. It has lines running along it from North to South. These longitude lines are far apart at the equator, but they meet at the poles. In a GCS, you re working on this sphere, but your screen is flat. When you try to represent the sphere on a flat surface, distortion occurs, similar to what would happen if you cut open a tennis ball and tried to flatten it out. What this means on a map is that the longitude lines stay equally far apart from each other, even at the poles (where they are supposed to meet). This means that, as you travel away from the equator on your map, the scale of the objects that you see gets larger and larger. What this means for us, practically, is that there is no constant scale on our map! To solve this, let s use a Projected Coordinate System (PCS) instead. A PCS projects or converts the data in a way that makes allowance for the scale change and corrects it. Therefore, to keep the scale constant, we should reproject our data to use a PCS.

5 5. On the Fly Reprojection: QGIS allows you to reproject data on the fly. What this means is that even if the data itself is in another CRS, QGIS can project it as if it were in a CRS of your choice. Zoom out so you can see the whole Africa continent. To enable on the fly projection, click on the CRS Status button: In the dialog that appears, check the box next to Enable on the fly CRS transformation. Type the word global into the Filter field. One CRS (NSIDC EASE-Grid Global) should appear in the list below. Click on the NSIDC EASE-Grid Global to select it, then click OK. Notice how the shape of South Africa changes. All projections work by changing the apparent shapes of objects on Earth. Zoom in to a scale of 1: again, as before. Pan around the map. Notice how the scale stays the same! On the Fly setting On the fly reprojection is also used for combining datasets that are in different CRSs. Deactivate on the fly re-projection again.

6 Save the map as an QGIS project file (.qgs) to C:/GISWorkshop/Exercises/ projection.mxd.