[GRASS-user] v.patch fix/workaround needed

Peter.Lowe · August 21, 2009, 6:37am

Hi,

I am using GRASS6.3 (on SuSE) to create a polygon layer out of several individual polygons.

In a first step, a polygon ("A") is created using v.patch from an outline and a centroid. This works fine.

In a second step, the newly created polygon is added to a larger set of polygons ("SET") using v.patch again.

In a few cases (reproducable) the "SET" polygon is NOT extended by the proper polygon "A" but the centroid of "A" and the polyline of "A" (the process works correctly in 9 out of 10 cases).

Since this process is to run as a script, how can the disjoint tuple of controid-A and polyline-A in the "SET" vector layer be mended again ?

Peter

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<peter.loewe@gmx.de>

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neteler · August 21, 2009, 1:37pm

On Fri, Aug 21, 2009 at 8:37 AM, <peter.loewe@gmx.de> wrote:

Hi,

I am using GRASS6.3 (on SuSE) to create a polygon layer out of several individual polygons.

In a first step, a polygon ("A") is created using v.patch from an outline and a centroid. This works fine.

In a second step, the newly created polygon is added to a larger set of polygons ("SET") using v.patch again.

In a few cases (reproducable) the "SET" polygon is NOT extended by the proper polygon "A" but the centroid of "A" and the polyline of "A" (the process works correctly in 9 out of 10 cases).

Since this process is to run as a script, how can the disjoint tuple of controid-A and polyline-A in the "SET" vector layer be mended again ?

Before we start hunting bugs for nothing, could you please try
GRASS 6.4? AFAIR there have been implemented some fixes in the
topology engine... perhaps it already works?

Markus

Peter.Lowe · August 24, 2009, 12:12pm

Hi

>
> In a first step, a polygon ("A") is created using v.patch from an
outline and a centroid. This works fine.

v.patch: "A-outline" + "A-centroid" => "A-polygon" (-a Option)

>
> In a second step, the newly created polygon is added to a larger set of
polygons ("SET") using v.patch again.
>

v.patch -a "A-polygon" + "Set" => "Set"

> In a few cases (reproducable) the "SET" polygon is NOT extended by the
proper polygon "A" but the centroid of "A" and the polyline of "A" (the
process works correctly in 9 out of 10 cases).
>
> Since this process is to run as a script, how can the disjoint tuple of
controid-A and polyline-A in the "SET" vector layer be mended again ?

Before we start hunting bugs for nothing, could you please try
GRASS 6.4? AFAIR there have been implemented some fixes in the
topology engine... perhaps it already works?

Markus

The results are the same on Grass6.4RC5.

I was able to track down to different phenomena.

Some background info:
The dataset "Set" consists of many squares (all imported as as "A-polygons"). SOME OF THEM OVERLAP.

1)
As long as the centroids of the affected squares are OUTSIDE of the overlapping area, everything is OK.

If the would centroids fall INSIDE the overlapping zone, the newly to be added polygon is only vpatched as a line and a controid.

Since the intentional use of overlapping polygons is surely a bad idea, this makes sense. However, some sort of feedback to the user would be a good thing to have if such a case occurs.

2)
Yet, there are cases without "critical overlap" resulting in centroid/line-pairs: In each case two identical centroids (holding identical attributes but different categories) were created.

This could be a bug in v.patch.

Peter

--
Dr. Peter Löwe
<peter.loewe@gmx.de>