[GRASS-user] Geographic Mean

GRASS GRASS user
1

Hi!

I have a vector map of points. I'd like to generate this into an area which covers all points and places the centroid in the geographic mean. Is there any way to do that?

--Wolf

--

<:3 )---- Wolf Bergenheim ----( 8:>

2

Il giorno lun, 22/05/2006 alle 14.56 +0300, Wolf Bergenheim ha scritto:

Hi!

I have a vector map of points. I'd like to generate this into an area
which covers all points and places the centroid in the geographic mean. Is
there any way to do that?

Yes, v.hull should do that.

HTH, Steko

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Stefano Costa
http://www.iosa.it Software Open Source per l'Archeologia
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3

On Mon, 22 May 2006, Stefano Costa wrote:

Yes, v.hull should do that.

Indeed! thanks!

--

<:3 )---- Wolf Bergenheim ----( 8:>

4

Is the centroid of the hull equivalent to the centre of gravity of all the points or is it the area centroid of the hull? Are these the same?

I have a similar application. I have a series of point data with a numeric attribute. I would like to get centroid weighted with respect to the attribute. Can v.hull do this?

The application is a mass haul problem. I have multiple sites each generating a different mass of material. I wish to find the centre of generation of the set of sites.

Thanks in advance.

Dave

On May 22, 2006, at 8:19 AM, Wolf Bergenheim wrote:

On Mon, 22 May 2006, Stefano Costa wrote:

Yes, v.hull should do that.

Indeed! thanks!

--

<:3 )---- Wolf Bergenheim ----( 8:>

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5

On Tue, 23 May 2006, Dave Kent wrote:

Is the centroid of the hull equivalent to the centre of gravity of all the points or is it the area centroid of the hull? Are these the same?

To my (admitably limited) understanding the definition of centroid is that it lies in the centre of gravity of an area. If I'm wrong, please correct me.

--Wolf

--

<:3 )---- Wolf Bergenheim ----( 8:>

6

> Is the centroid of the hull equivalent to the centre of gravity of
> all the points or is it the area centroid of the hull? Are these
> the same?

To my (admitably limited) understanding the definition of centroid is
that it lies in the centre of gravity of an area. If I'm wrong, please
correct me.

I would not make that assumption. e.g., where to put the centroid in a
2D toroid?

see Vect_get_point_in_area() in lib/vector/Vlib/poly.c

/*!
\fn int Vect_get_point_in_area ( struct Map_info *Map, int area, double
*X, double *Y)
\brief get point inside area and outside all islands
        Take a line and intersect it with the polygon and any islands.
        sort the list of X values from these intersections. This will
        be a list of segments alternating IN/OUT/IN/OUT of the polygon.
        Pick the largest IN segment and take the midpoint.
\return 0 on success, -1 on error
\param Map_info structure, area number, x, y
*/

Then look in Vect_get_point_in_poly_isl() (same file)

Hamish