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I have created a set of isochrones (3, 5, 10 minute walk times) from a point layer. I now want to dissolve the layer to make it more readable. However when I try to dissolve based on the field that specifies walk times I do not get the result I want. I have a pretty clear example I am trying to imitate but they do not explain how they did it.

I want to go from this:
before dissolved isochrones

To this:
after dissolved isochrones

However, when I try dissolve and select the MINS field as the dissolve field I get this:
wrong output

Is there a way to do this in one step? I know I could select the polygon from the isochrones layer for each distance (3, 5, 10 mins) and then dissolve only selected features but I am looking for a way to do this in one step with the dissolve tool. Is this possible or should i just stick with the slower method of selecting and then dissolving individually?

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first, select all yellow polygons and do union. second, select all the blue ones and do union.and lastly for the gray ones.

Now you should have three features. Then give a unique attribute to each. e.g. unionYellow = 0 , unionBlue = 1, unionGray = 2 then use merge shapefiles tool or any other merger tool. You can use the numbers to change their order. Now you are ready to go. Good Luck.

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For such tasks, the Aggregate function is an easy way to do it: Menu Processing / Toolbox / Aggregate.

Use the field that specifies walk times as input for Group by expression, define the desired Aggregate function for each field (or delete fields you don't use) and run the tool.

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