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2022 GeOnG - QGIS Workshop

Step 3: Calculate density of high fire

A - Add Reproject layer algorithm, rename it: “Reproject layer grid”

  • Input layer: Count from algorithm “Count point in grid low fire”
  • Target CRS: EPSG: 3857 - WGS 84 / Pseudo-Mercator

B - Add Field Calculator algorithm, rename it: “Field Calculator_grid”

  • Input layer: Reprojected from algorithm “Reproject layer grid”
  • Field name: h_fire_density
  • Type: Decimal
  • Length: 30
  • Precision: 10
  • Expression: “nb_high_fire” / $area
  • Output: fire_grid_density

C - Manage dependencies

  • “Reproject layer admin” depend of “Count point in grid low fire”
  • “Field Calculator h_fire_density grid” depend of “Reproject layer grid”

D - Understand density value

Depending on the Project properties you have, the function $area will calculate area in m² or km², or other units. As all grid squares have the same area, these steps are not really useful, but let keep it mind: we should represent the polygon with graduated colour value from a ratio and not an integer according to semiological rules.

E - Run the model to test results consistency.

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F - Save as you model, in case further developments break its execution. It will help you to get back on a solid base, if you mess up the processing.

Model output: Model_on_fire_v3.3.model3