.. _8.1:

Visualization of GHSL-JRC population data in QGIS
=================================================

In this lecture we need to import into QGIS the following layers:

+ GHS_POP_E2020_GLOBE_R2023A_4326_3ss_V1_0_R5_C19.tif
+ ObservedEvent_UTM
+ AreaOfInterest_UTM

The first one has been downloaded in :ref:`lecture 6 <6.1>` while the second and third one have been obtained in :ref:`lecture 5 <5.1>` .

Data preprocessing
------------------
The first thing we will do is to reproject the “GHS_POP_E2020_GLOBE_R2023A_4326_3ss_V1_0_R5_C19.tif” (:numref:`8.1.1`) . Click on Raster **(1)**, Projection **(2)** and then select **Warp (Reproject) (3)**. Set “GHS_POP_E2020_GLOBE_R2023A_4326_3ss_V1_0_R5_C19.tif” as the **Input layer** of the tool **(4)**. Now we want to set **Target CRS** as “WGS 84 / UTM zone 32N EPSG 32632”, to do so you can click on the drop down menu **(5)**, if you recently used this CRS it will appear in the menu allowing you to select it. Otherwise click on the **Select CRS** icon **(6)** and search for it in the same way as shown in :numref:`5.1.2.3` . Once done click on **Run (7)**.
.. warning:: Make sure that the output file is saved with the .tiff extension

.. _8.1.1:

.. figure:: /img/8/8.1.1.png
   :align: center

   -- Reproject Raster Layers

Due to their size, our data requires a high computational burden to be managed, and it will be better to clip them to our area of interest (:numref:`8.1.2`) . In order to do so, click on **Raster (1)**, **Extraction (2)**, **Clip Raster by Mask Layer… (3)**. From the menu that just opened choose as Input layer the layer that we want to clip (“Reprojected”) **(4)** and as **Mask layer** the layer corresponding to our area of interest (“AreaOfInterest_UTM”) **(5)**. Set **Assign a specific nodata value to output bands [optional]** to “-9999” **(6)** and scroll down using the lateral bar **(7)**, until you reach the button of the menu **(8)**. Click on the three dotted button **(9)**, select **Save to File (10)** and choose a folder to save your resulting layer, for the purpose of this lecture we will call it “Pop_UTM_Clipped”. Once done, click on **Run (11)**.

.. _8.1.2:

.. figure:: /img/8/8.1.2.png
   :align: center

   -- Clipping raster by mask layer in QGIS

Now we will visualize in a better way our clipped layer (:numref:`8.1.3`) . Open the **Layer Properties** menu of the clipped layer **(1)**, click on **Symbology (2)** and choose the “Singleband Pseudocolor“ **Render type (3)**. Choose the Interpolation type as “Discrete” **(4)**, change the Color ramp to one you like (5) and set the **Label precision** to “0” **(6)**. Set the **Mode** field as “Continuous” (7) and click on **Ok (8)**.

.. _8.1.3:

.. figure:: /img/8/8.1.3.png
   :align: center

   -- Visualization of GHSL-JRC population data in QGIS

If you’d like to know the population value in a specific point, you can use the Identify Features tool (:numref:`8.1.4`) , click on it **(1)**. Zoom-in on the area with the highest population density and click on a point where you are interested to know the population value **(2)**. A menu called **Identify Result** will pop up on the right of the screen, from there you can read the population value **(4)**.

.. _8.1.4:

.. figure:: /img/8/8.1.4.png
   :align: center

   -- identify tool in QGIS