In this tutorial, you will learn how to use QGIS to pre-process raster data downloaded from the WaPOR portal, which involves correcting raster values with conversion factors.
Spatial data is often stored in a specific format by the provider. In order to extract information properly, we need to understand the data format and metadata provided with the data we have collected. The first step is to find this information from the WaPOR data catalog, understand how it applies to the data layer.
In the next step, you will need to multiply the value in each pixel by the conversion factor given. For annual and monthly data, the raster data provided is already calculated for the year and month, respectively. This makes the conversion quite simple to calculate. For AETI, for example, the calculation is:
average annual value * 0.1 = mm/year
average monthly value * 0.1 = mm/month
For dekadal data, the given data is an average daily value (mm/day). To convert it to an average value per dekad (mm/dekad), you need to multiply by the number of days in the dekad of analysis. For AETI, for example, the calculation is:
average daily value * 0.1 * number of days in the dekad = mm/dekad
In order to multiply the value in each pixel by the conversion factor, we can use QGIS 'Raster calculator' tool. This video will show you how to do it.