Getting longest distance between multiple points and creating multiple polygons from lines using QGIS?

Question

I am still a beginner with GIS.

I can't find an easy way to get a maximum distance from multiple points.

I have a layer of up to 100 points (without a unique ID - maybe that is part of the problem? but I could easily add that) and I want to get the length of maximum distance between any two of them.

I tried plenty of line-points plugins, but nothing seems to work, I also tried to make a minimum convex polygon (basically convex hull of all the points) and get a diagonal of that but failed in all tries again.

How do I do this without using a programming language?

I am using QGIS 2.18.5

Answer 1

You will need an id of each record. Then a Spatialite self cross join SQL query of a virtual layer will give the distance between every point.

Adding an id column

In the Field Calculator:

Make a new integer column called id and update it with row_number:

Make a virtual layer:

Write the SQL:

select st_distance(t1.geometry, t2.geometry) as dist, t1.id as id1, t2.id as id2
from testpnt t1
cross join testpnt t2
order by 1 desc

Like:

Result:

Note:

This solution returns duplicated records (distance) for every two points, since id 1 is compared to id 2 and id 2 is also compared to id 1 in a cross join.

Update: The original table looks like this: