[Qgis-user] Minimum bounding circle from cluster of points

[Tina Cormier]

Thank you all for your help! Graeme and Nicholas were on the same track I
think, and Micha - I knew this would likely be very simple (and already a
tool) in PostGIS, but it's not my go-to approach, so I'm actually going to
try this out bc it will solve my problem AND give me a bit more experience
with postgreSQL and PostGIS. Additionally, I received a handy function for
completing this task in R. So many ways to tackle the same problem, and
I've very grateful that all of you took the time to respond. I have the
concept down of what to do now, and that's really what I needed. You guys
are the best!

Thank you again!
Tina


On Sun, Jul 10, 2016 at 2:37 AM, Micha Silver <micha at arava.co.il> wrote:

> This can be achieved using the PostGIS function
> ST_MinimumBoundingCircle(). If you can import your subplots point layer
> into a PostGIS database, then (assuming the points have an attribute
> "subplot_id", and geometry column "the_geom") you could do something like:
>
> SELECT s.subplot_id, ST_MinimumBoundingCircle(ST_Collect(s.the_geom)) as
> the_geom
>     FROM subplots AS s
>     GROUP BY s.subplot_id;
>
> (Based on the example from
> http://postgis.net/docs/ST_MinimumBoundingCircle.html)
>
>
> I would also point out that there's a concept "Standard Deviational
> Ellipse" for encircling points by an ellipse at a certain stdev distance
> from the "center of gravity" of the points. If this is of interest, you can
> implement it in GRASS-GIS with the v.ellipse addon.
>
>
> On 08/07/2016 16:49, Tina Cormier wrote:
>
> Hi all,
>
> Looking for help aggregating some field data subplots to the plot level.
> In the attached screenshot, you can see clusters of 4 points (4 subplots =
> 1 plot). I'd like to create a circle around each cluster that is the
> smallest circle that would encompass all 4 points. Sort of like a convex
> hull, but a circle (convex hull, in this case, would give me a triangle).
> Is there a tool or a series of steps to accomplish this without doing it
> manually? In the attribute table, I have a unique ID for each plot/cluster.
> So for each plot ID (in this case, consisting of 4 subplots), I'd like to
> build a circle around all of the subplots.
>
> I should also mention that it's not always 4 points (subplots), and they
> aren't always covering the same size area on the ground.
>
> I have QGIS 2.14.2 as well as one of the nightly builds from a few weeks
> ago (can download the newest one if necessary).
>
> Thanks in advance for any ideas/insights! Google has not helped me so much
> yet with this one!
> Tina
>
>
> [image: Inline image 2]
>
>
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>
> --
> Micha Silver
> Arava Drainage Authority
> +972-523-665918
>
>
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