#134 Create hull from point layer

open
nobody
5
2014-11-19
2011-06-29
Arnd
No

In the geometry functions of OJ (Tools > Edit Geometry) are three methods to create a hull or border:
- convex hull
- hull
- border
All these methods only work with lines or polygon but no method works with points.
It will be nice to get method for point layers

Discussion

  • michael michaud

    michael michaud - 2011-06-29

    According to my test, it works for envelope and convex hull and not for boundary, which seems "ok" to me :

    -envelope of a point is the point itself
    -convex hull of a point is the point itself
    -boundary of a point is the empty set (see ogc and C2 point in http://tsusiatsoftware.net/jts/jts-faq/jts-faq.html\). OpenJUMP does not produce a feature with empty geometry in this case which seems fine to me, but which could be discussed...

    Can you give a test case of what does not work for you

    Michaël

     
  • Arnd

    Arnd - 2011-06-30

    An example case is having a point layer with lots of points (e.g. from gps measurements) describing a border of a field. Now I want to create a linestring or polygon from these points to have polygon for further geometry caluclations.

    I can imagine two options:
    - creating a convex hull around all points
    - connecting the points by the order of features ID (choosing if internal OJ FID or if there is an attribute with the user defined ID)

     
  • michael michaud

    michael michaud - 2011-06-30

    OK,

    I see your point now,
    It is not a standard geometry operation as it needs extra information (to order the points)
    Can be developped as a plugin (not too difficult, except if one want an intelligent option to order points a a clever way without usinf the id or an attribute).

    Would be a useful addition.

    Michaël

     
  • Jukka Rahkonen

    Jukka Rahkonen - 2014-05-15

    The convex hull for the whole layer is implemented and it works also for point features.
    I don't know where the "hull" and "border" operations are nowadays (OJ v1.7).

     
  • michael michaud

    michael michaud - 2014-05-15

    I think original features are still there in OJ 1.7, in
    Tools > Analysis > Geometry functions...
    - Convex Hull
    - Envelope
    - Boundary
    This plugin works on a per feature basis. Convex hull makes more sense for a set of features, and 2 plugins have been added in Tools > Analysis
    - convex hull of a layer
    - convex hull of a selection
    (maybe both tools should be unified as for union plugin which is context sensitive)

     

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No, thanks