Re: [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  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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> List info: http://lists.osgeo.org/mailman/listinfo/qgis-user
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>
>
> --
> Micha Silver
> Arava Drainage Authority
> +972-523-665918
>
>
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Re: [Qgis-user] Minimum bounding circle from cluster of points

[Micha Silver]

  
  
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






  
  
  
  
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-- 
Micha Silver
Arava Drainage Authority
+972-523-665918
  

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

[SEGGIE Graeme]

I don’t have enough experience of coding steps in QGIS yet, but could you do 
the following?

For each set of points sharing a plot ID attribute value:
Identify the distances between all points.
Take the furthest apart pair (or one set of if more pairs are 
equal), which would become a diameter.
From this chosen pair, get the mid-point, which becomes the midpoint of the 
circle.
Buffer the identified midpoints as circles with size of radius (half max 
distance identified) to get circle containing all others in each set.

I think that would get all of them, or at least I can’t think of a situation 
where any longest pair identified could be further away from another point to 
be left out.

Graeme.


From: Qgis-user [mailto:qgis-user-boun...@lists.osgeo.org] On Behalf Of Tina 
Cormier
Sent: 08 July 2016 14:50
To: qgis-user@lists.osgeo.org
Subject: [Qgis-user] Minimum bounding circle from cluster of points

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


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

[Nicolas Cadieux]

Hi,
This could work but there may be a more efficient way.   Here are my ideas.
1) Split vector layer using the  ID for each plot/cluster.2) Create a polygone 
from the file extent. (use the processing toolbox), merge all the polygone into 
1 file.3) Find the centroid of this shape and the maximum extent of the file or 
the maximum extent of the shape (field calculator). 4) Use the Create Buffers 
tools (on the centroid file) in the MMQGIS plugin.  You can create a circle 
based on a Radius specified in a column (the max file extent).
Step 1 could also be create convex hull for every group of points. Perhaps this 
can be done by aggregating the points into a multi point files. 
(vector/geometry tool/single part to multipart).
Nicolas
From: ml-node+s1560n5275434...@n6.nabble.com
To: nicolas.cadi...@archeotec.ca
Date: Fri, 8 Jul 2016 07:33:36 -0600
Subject: Minimum bounding circle from cluster of points



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




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