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I'm using QGIS 2.6 and have 4 layers of polygons. Each of these polygons describes the outline of a tree, based on airborne imagery.

My goal is to find the relations between every synchronously sequential couple of layers. For every tree in every couple of layers I need to tell whether it survived, was born or died with respect to previous layer.

How can I do this?

enter image description here

closed as unclear what you're asking by Chris W, PolyGeo Dec 15 '14 at 3:31

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    COuld you be a little more specific about what your data represents. the way I read your post it sounds like you have separate layers from different dates (when the imagery was taken). you then want to compare them to see if a tree in date 1 is still there at date 4 and so on, is this correct? if so I can think of a thing or two. – dave_does_not_understand Dec 5 '14 at 18:50
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    Welcome to GIS.se! It will really help if you describe more about what you have already tried (and where you're stuck), and/or more about how your data are structured; as it stands, your question is too broad. – Simbamangu Dec 5 '14 at 19:10
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    We need more information to help. Do your polygons have any attributes, and if so what are they as far as being relevant (like a live/dead field or something, or a unique ID)? Is simple existence of the shape/geometry indication of that (meaning if it's new it was born, if it's gone it died, if it's there and was it survived)? Any solution is going to depend on the details of what you have to work with. – Chris W Dec 5 '14 at 19:48
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I see this more as a statistical question than a GIS one. Or maybe it's just my training. Assuming there is data representing each tree (labelled something like Tree1, Tree2, etc. in the database) and there are also fields for each of the states (survived, born, died) in each layer. I would write up a research question for each, something like this:

What is the difference in survival status for each tree between layer 1 and 2? What is the difference in survival status for each tree between layer 2 and 3? What is the difference in survival status for each tree between layer 3 and 4? What is the difference in survival status for each tree between layer 4 and 1?

Swap out "survival" for "born" then "died" for your remaining research questions. Then query the database for each combination and permutation and dump the answer into a new field with a code depending on the status outcome. For example, for the new field representing survival status between layer 1 and 2 a tree survival could be a 1, no survival would be a 2. Repeat for the other two states and the difference between the layers.

Others with more GIS experience can tell you how to render these new fields on a map such that all four layers (do they represent time?) and all three conditions (survival, born and died) for each tree can be best visually represented.

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    I'm not really sure this answers the question (at least in the body - the title is admittedly ambiguous/broad). Your 'research' questions seem to basically restate the asker's question, as does the third paragraph. How would you query the difference between layers, as that is essentially what the OP is asking? You mention adding attributes but don't really give any specific indication on how to create, assign, or use them afterward. – Chris W Dec 5 '14 at 20:00

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