By Florencia Pérez

Field trials are strategic agronomic practices that allow to evaluate at scale the use of new techniques or the performance of certain genetic materials, among other applications. In this article we will show you how to create different types of plots for a field trial using QGIS.

We will go through the following points:

• Delimiting micro plots for grid trials.
• Delimiting plots for strip trials
• Delimiting micro plots for trials by zones.
• Merge spatial objects and unite vector layers.
• Advanced digitizing panel.

NOTE: The above procedures require that both the project and the vector layers are in the same projected coordinate system.

Delimiting micro plots for grid trials

Procedure

1. Generate a grid. To do this go to Vector > Research Tools > Create Grid.

A panel will open where we will be able to configure grid type, extent, spacing and CRS.

A. First we check that the CRS that appears is the same that we have previously assigned to the project and to the field or prescription layer on which we want to carry the trial.

B. Then in grid type, in the drop-down list, select “Rectangle”.

C. Fill in the sections ‘Horizontal spacing’ and ‘Vertical spacing’ with the dimensions and units we want each microplot to have.

D. Assign a location to the file that will be generated from: Grid > Save to file (in file type select “SHP files”).

E. In Grid extension we choose “Draw on canvas”.

2. Edit the grid.

A. Toggle editing of the layer created from .

B. Select and remove unwanted elements from the grid. Activate the selection tool from the selection toolbar. Select the elements you want to delete and delete them from . 

NOTE: Holding down the Ctrl key allows you to remove elements from the selection while holding down the Shift key allows you to add elements to the selection.

C. Move and rotate the grid. With the move feature and rotate feature tools you can change the location of the element and place it where you want within the batch.  

NOTE: Previously we must select all the elements that we want to move or rotate. 

D. Merge grid elements. 

In some cases it may be necessary to combine micro-plots of the grid either because they have the same prescription/attribute or because we want to form a larger plot.  To do this, first toggle layer’s editing; then, with the selection tool select the elements you want to join. Finally, use the “Merge selected features” tool .

3. Unite the grid to the field or prescription layer.

Usually, the test design should be transferred to an application monitor, which is why the prescription layers are joined beforehand. This is done from: Vector > Geoprocessing tools > Union.

In Input layer place the batch or prescription layer and in Overlay layer place the grid. Leave the rest of the options as default. A new layer will be generated. To import the generated layer to 360 you must export it in the coordinate reference system supported by 360:EPSG:4326 – WGS 84 (Review: Importing a Layer). .

NOTE: Unlike the “Merge Selected Features” tool, the individuality of each object that makes up the layer is not lost through this merging process.

Delimiting plots for strip trials

Procedure

1. Generate a grid. Follow the steps indicated to delimit micro plots for grid trial. 

NOTE: In this case the plot dimensions will be larger, take this into account when drawing the grid on the map canvas.

2. Editing the grid. Generate a grid. Follow the steps indicated to delimit micro plots for grid trial.

3. Unite the grid to the field or prescription layer. Also check how it was done in point 3 of generating microplots for grid trial.

Delimiting micro plots for trials by zones

Procedure

1. Generate a grid. Follow the steps indicated to delimit micro plots for grid trial. 

2. Edit the grid. Do it in the same way as above. 

3. Unite the grid to the field or prescription layer. Also check how it was done in point 3 of generating microplots for grid trial. 

Advanced Digitizing Panel

In certain situations we may want to be more precise when drawing, moving or rotating an object. In order to do this, we can make use of the Advanced Digitizing Panel.

The advanced digitizing panel allows you to define and lock distances, digitize lines exactly parallel or perpendicular to a particular angle or lock lines to specific angles. Furthermore, you can enter coordinates directly so that you can make a precise definition of your new geometry. (Reference: The Advanced Digitizing panel – QGIS Documentation).

To activate it just right-click on the toolbar and check « Advanced Digitizing Panel ».

d: allows you to set a certain distance

a: allows you to set a certain angle

x: allows you to set a certain x-coordinate

y: set a specific y-coordinate

allows you to perform queries

allows to draw a parallel to a given segment

allows to draw a perpendicular to a given segment

allows you to set angles

To use this tool you must necessarily:

a. Enable layer editing

b. Enable the Snapping of the layer . This tool allows the perfect adjustment between the limits of two objects and is activated by right clicking on the toolbar.

c. Check in which projection the layer is located (if it is in geographic coordinates the tool is not enabled).

d. Enable the add polygon option , or activate any shape digitizing or advanced digitizing tool (for example: and ) and then enable the advanced digitizing option .

To deepen in the use of these two tools (Advanced Digitizing Panel and Snapping toolbar) and the editing tools it is recommended to review QGIS Documentation: Editing.