ANINTRODUCTIONTOGIS


Effect of past land use on crop development

For this tutorial, we will see how we can create a vector summarizing past land use for a given field, and then evaluate the effect of land use evolution on crop wthin-field heterogeneity. More specifically, we will study the effect of previously forested areas.

Past land use map will be created from the historical aerial images georeferenced in this tutorial.

Within-field heterogeneities will be evaluated using the Normalized Difference Vegetation Index (NDVI) maps created in this tutorial.

1. Creating a map of past land use

This folder contains the different layers needed to complete this tutorial, including the borders of a field that could be used as an example.

But it is also possible to perform this tutorial for the field of your choice. To do so: - Locate a field on the recent aerial image; - Make sure that this field is cultivated with only one crop on the NDVI map.

You may then create the vector of the borders of this field in the following way: - Layer > Create Layer > New Shapefile Layer


Figure 1 Create vector for field borders


Then, toggle Editing (right click on the name of the new layer) and draw the borders of the field with:

  • Edit > Add Polygon Feature

You can then stop editing to save the vector.

For simplicity, you can also do this tutorial using the “Field_Borders_L93” vector file available in this folder.

By changing the order of the layers in the left panel, project the historical image below the field borders.

To add transparency to the field borders, right click on the name of the layer then:

  • Properties > Symbology > Fill color > Transparent fill

Before creating new entities designating the previously forested areas, we will create a new field in the attribute table:

  • Open the attribute table of the vector layer
  • In the top bar, select the icon “New field”
  • Fill the new window as follows:

Figure 2 Add field to designate forested areas


To crop the field borders vector to designate wood areas:

  • Toggle Editing
  • Edit > Split features
  • Cut out the wooded areas (starting/ending with a click outside the polygon)
  • Change the ‘wood’ field of the attribute table to differenciate forested/non forested areas

Figure 3 Designate forested areas


Practise: Mark the forested areas for your field. Once finished, save the vector layer by closing the editing mode.

Note that the figure above represents a simple example of map editing, where the new entities (i) are at the edge of the map and (ii) are not adjacent. More complex cases require more care to ensure the validity of the map topology (no overlapping entities…). For example, the creation of two adjacent entities requires the activation of snapping (Project > Snapping Options) to guarantee the proper connection of the entities.


II. Compare current crop development with an NDVI map

First, you can create the NDVI map of the area by following this tutorial.

The Sentinel-2 rasters made available for this tutorial date from March 6, 2021. That year, the field highlighted in the first part was cultivated with soft winter wheat.

We will now assign an average value NDVI per zone (previously foresetd or non-forested).

Open the Toolbox:

  • Processing > Toolbox

In the search bar of the Toolbox, look for Zonal statistics. If this extension is not available, it is possible to install it using the Plugins manager.

Open Zonal statistics, then fill the window as follows:


Figure 4 Compute mean NDVI per zone


From the vector layer, you may now evaluate the effect of past forested areas.