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Home - General / All posts - Finding an Area Covered by Overlapping Polygons
#06-Nov-18 16:57

Hi there,

I'm an absolute novice trying to teach myself Manifold, and I've come up against an issue that I need some help with. I'm hoping that somebody can lend a hand.

I have a shape file containing a number of circular polygons that represent tree crowns, and I am trying to calculate the total canopy coverage. The problem is that many of the crowns overlap with each other. I understand that I can find the area of each individual polygon in its row on the data table, but I can't just add all of these areas together, as I don't want to include the overlapping parts multiple times - I just need the area of ground that is covered by tree canopies.

Is there a way to calculate this figure?

All the best,

Leyland

tjhb

8,410 post(s)
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#06-Nov-18 17:24

Manifold 8 or Manifold 9.

One way is to use a side-effect of the Normalize Topology transform, since it removes overlaps between areas.

That is destructive, so apply the transform to a duplicate of the drawing. Then you can simply sum the remaining area values (perhaps using a ViewBot if it is Manifold 8, otherwise SQL).

Even simpler: again create a duplicate of the drawing, then apply the Union transform. This gives you a single area, with overlaps resolved, and a single area value for the whole canopy.

StanNWT89 post(s)
#06-Nov-18 17:24

Hi there,

Based on some novice viewing, and what I would do in a different GIS application, and finding the corresponding ones in Manifold 9, I would think the following two transforms would set you up nicely to get what you're looking for:

  • Buffer
  • Union Areas

Then calculate your area(s)?

Your buffer could have a distance that is 0.0001 cm?

Or make a duplicate of that drawing of the tree canopies and do a "Union Areas".

ColinD


1,886 post(s)
#07-Nov-18 00:19

Sounds like you are using M8. Because you only want the total cover simply Union all of the polygons. Open the drawing then at the bottom transform bar [All Objects] Union Apply. There will then be one large combined drawing with no overlaps and in the table Area (I) will give the combined area in whatever units you are using.


Aussie Nature Shots

#13-Nov-18 12:06

Thanks all - Union worked perfectly.

All the best,

Leyland

adamw


8,259 post(s)
#07-Nov-18 09:39

If you just need the area of the ground that is covered by tree canopies, then simply do Union Areas and use the area of the result.

If you want to turn overlapping circles into rings, either use Normalize Topology - or, perhaps simpler, convert areas to their boundaries (lines) and run Bounded Areas to create areas bounded by lines.

#13-Nov-18 12:06

Many thanks. Union seems to have done the job.

Leylan

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