[Qgis-user] 200717 - How do I enclose an orthogonal grid of points with a polygon boundary.

[Håvard Tveite]

Hi Paul,

I don't think you will find a function in QGIS to do this,
but you should be able to achieve it by using the following
procedure:

1) Create a polygon grid with the same "spacing" as your point grid.
Use the "Create grid" algorithm (from the Processing Toolbox), with
Grid type: Rectangle (Polygon)
Grid extent: Click on the ... pull-down and select "Calculate from 
layer", and select your point layer
Horizontal spacing: 25 meters
Vertical spacing: 100 meters

2) Run the "Join attributes by location (summary)" algorithm with
Input layer: the generated polygon grid from the first step
Join layer: your point grid
Geometric predicate: "intersects"

3) Run the algorithm "Select by Attribute" with
Input layer: your "Joined layer" (from step two)
Selection attribute: one of the *_count attributes
Operator: =
Value: 4
Modify current selection by: creating new selection

4) Run the "Dissolve" algorithm with
Input layer: your "Joined layer" from step 2-3
check "Selected features only"

This should work (but rounding errors may cause problems in
step two and four)

Håvard

On 17.07.2020 07:21, Paul T A Drea wrote:
> Hello list,
> 
> I hope this finds everyone safe and healthy during these trying times.
> 
> I am using a MacBook Pro running macOS 10.15 (Catalina).
> 
> QGIS version 3.14.0-Pi. QGIS code revision 9f7028fd23.
> 
> I have a shp file containg 2 653 337 points in an orthononal grid with 
> 25m x 100m spacing of the points.
> 
> The grid is oriented north south. These points are in lines spaced 25m 
> in the easting (X) direction and 100m northing (Y) direction.
> 
> I would like to apply a polygon as a boundary (red) as shown in the 
> screen grab below.
> 
> 
> I have not been able to achieve this using convex hull as the final 
> output does not take into acount the sawtooth steps.
> 
> Any help or pointers would be appreciated thanks.
> I am not a coder and have no knowledge of writing any scripts.
> 
> Thanks and Best regards,
> Paul
> 
> 
> -- 
> 
> */Paul T. A. Drea/*
> 
> *Geodetic Consultant*
> 
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> 
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> 
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> 
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> 
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> 
> *e**-mail 2: **ptadrea at otenet.gr* <mailto:ptadrea at otenet.gr>
>