Requirements

• Install QGIS (LTR)
• Download the rawdata.
• These notes can be dowloaded here

The data

Shapefiles

Shapefiles are actually collections of files among which:

• .shp contains the geometry of the features
• .dbf contains the attribute table
• .prj contains the projection
• other extensions are beyond the scope of this tutorial

There is no notion of resolution while manipulating these objects.

→ That does not mean that they were drawn with perfect precision!

Load the data: using Layer > Add Layer... > Add Vector Layer, load ./rawdata/NPL_adm3_UTM44N.shp

Raster data

Raster data are contained in a single file .(g)tif(f), .netcdf, ... They may also have attached file like .aux.xml but that is also beyond the scope of this tutorial.

They are organized as images with pixels, where each pixel can be attached to a single or several values. Each variable correspond to a band.

As real images they have a resolution, which often need to be taken into account in the processing flow.

Load the data: using Layer > Add Layer > Add Raster Layer..., load ./rawdata/Nepal_WGS_UTM44N.tif

Visualization

In the Layers panel, change the transparency of the administrative limits layer using Right-click > Properties... > Symbology

In the Layers panel, open the attribute table using Right-click > Open Attribute Table

Processing

Extracting features

• Select the administrative limits layer in the Layers panel
• Then using the Select Features button, the an administrative unit, and
• In the Layers panel, use Right-click > Export > Save Features as and select the Save only selected features to save a single administrative unit in a new shapefile (use the format ESRI Shapefile).

Select Features button:

Clipping data

• In the Toolbox panel, search for the GDAL algorithm Clip raster by extent
• Set the parameters to extract the portion of raster overlapping with the adminstrative units, and run the algorithm

Toolbox button:

Walking distance from a given point

Creating features

• First create a new point layer using Layer > Create Layer > New Shapefile Layer
• Enable editing mode using the Toggle Editing button:
• Add a feature using the Add Point Feature button:
• Save the changes using the Save Layer Edits button:
• Disable editing mode using the Toggle Editing button

Projection

• If you right click on a given and put your mouse on Layer CRS, you can see the projection system it uses. See that the point layer you just created in the previous step is not the same as the clipped raster from the previous step.
• Reproject the administrative layer using the same projection system as the raster one (EPSG:32644) by searching Reproject layer in the Toolbox panel.

The algorithm

• In the Toolbox panel, search for the GDAL algorithm r.walk.points
• Set the parameters and run the algorithm

Zonal statistics

• In the Toolbox panel, search for the GDAL algorithm Zonal Statistics ⚠️ This algorithm modify directly the layer file
• Set the parameters to compute the average walking time to the point for the entire administrative unit

Network analysis

• Load the roads shapefile ./rawdata/roads/
• Add a secont point to the layer you created
• In the Toolbox panel, search for the GDAL algorithm Shortest Path (layer to point)
• Set the parameter in order to find the shortest paths between the points of your layer and a point that you will point in the canvas, and run the algorithm