I have a shapefile containing polygons (area of mangrove) per year for 35 years (=35 shapefiles).

How do I map changes of mangrove extent over time?

  1. I want to map gains and losses (colour coded) between 1984 and 2019.

  2. I would like to illustrate changes over time like this map for example.


For overall change, I have been trying to union the mangrove_2019 with mangrove_1984, creating a field 'difference' and calculate the difference area. Though I can't figure out how to union layers based on a corresponding field/column. And then I want to somehow visualise the gains and losses - perhaps by categorising the positive and negative observed changes and then symbolising these. Though this is not exactly working for me.

For yearly changes, my only idea is to rasterise the dataset first. But I've tried to do this and my output raster is blank.

  • 1
    Why not use the tool difference? – Erik Jun 22 '20 at 7:10
  • This tool would be perfect, except it seems to just illustrate change and does not define specifically positive and negative changes which is important for me to show. – Lola Jun 22 '20 at 21:38
  • Then run symmetric difference. – Erik Jun 23 '20 at 7:03
  • The difference to difference is (haha), that this tool keeps attributes from both layers, so if you simply add a column with "1984" to one and "2019" to the other layer, you know, which area stems from growth and which from loss. – Erik Jun 23 '20 at 7:12

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