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

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!

Inline image 2

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Micha Silver
Arava Drainage Authority

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