[GRASSLIST:10743] How to extract polygons from vector map

GRASS GRASS user
1

I've got a vector map where I can visually identify one outline which is a lake. How can I extract that one vector from the layer and turn it into a filled blue polygon in a new layer?

You've all been a huge help to me so far, and I hope that the good will continues until I am able to contribute my own answers. If anyone is interested in what I'm up to, I've been keeping a sketchy blog at http://mjr42thoughts.blogspot.com/

Regards,
Michael

2

In the new GIS Manager (gism2), if you can display only that polygon with an
attribute query, you can extract it to a new vector. (See new vector panel
with button to extract queried vector).

Michael
__________________________________________
Michael Barton, Professor of Anthropology
School of Human Evolution and Social Change
Arizona State University
Tempe, AZ 85287-2402

phone: 480-965-6213
fax: 480-965-7671
www: http://www.public.asu.edu/~cmbarton

From: Michael Rensing <michael.rensing@shaw.ca>
Date: Sun, 05 Mar 2006 11:18:12 -0800
To: <GRASSLIST@baylor.edu>
Subject: [GRASSLIST:10743] How to extract polygons from vector map

I've got a vector map where I can visually identify one outline which is
a lake. How can I extract that one vector from the layer and turn it
into a filled blue polygon in a new layer?

You've all been a huge help to me so far, and I hope that the good will
continues until I am able to contribute my own answers. If anyone is
interested in what I'm up to, I've been keeping a sketchy blog at
http://mjr42thoughts.blogspot.com/

Regards,
Michael

3

Yeah, that's one way I figured it out. Problem is, I'm generating contours from a DEM for a large region, and I wanted to pick one to extract. There are no attributes assigned by r.contours that I can use.

Unless a better idea presents itself, I'm going to create small local contours around the lakes, then collect them into a single vector layer, add an attribute, and use the form mode of the mouse query to edit the new attribute. THEN I can use an attribute query to extract the vectors I want.

There should be a better way, but this does keep me amused...

Michael

Michael Barton wrote:

In the new GIS Manager (gism2), if you can display only that polygon with an
attribute query, you can extract it to a new vector. (See new vector panel
with button to extract queried vector).

Michael
__________________________________________
Michael Barton, Professor of Anthropology
School of Human Evolution and Social Change
Arizona State University
Tempe, AZ 85287-2402

phone: 480-965-6213
fax: 480-965-7671
www: http://www.public.asu.edu/~cmbarton

From: Michael Rensing <michael.rensing@shaw.ca>
Date: Sun, 05 Mar 2006 11:18:12 -0800
To: <GRASSLIST@baylor.edu>
Subject: [GRASSLIST:10743] How to extract polygons from vector map

I've got a vector map where I can visually identify one outline which is
a lake. How can I extract that one vector from the layer and turn it
into a filled blue polygon in a new layer?

You've all been a huge help to me so far, and I hope that the good will
continues until I am able to contribute my own answers. If anyone is
interested in what I'm up to, I've been keeping a sketchy blog at
http://mjr42thoughts.blogspot.com/

Regards,
Michael
    

4

Michael,

You should have SOME kind of value attached to the vector--either as cat or
label. You can have r.to.vect create this. You can also run v.category to
add unique cat values to each vector object. Then you can use a query to
extract the one you want.

Michael
__________________________________________
Michael Barton, Professor of Anthropology
School of Human Evolution and Social Change
Arizona State University
Tempe, AZ 85287-2402

phone: 480-965-6213
fax: 480-965-7671
www: http://www.public.asu.edu/~cmbarton

From: Michael Rensing <michael.rensing@shaw.ca>
Date: Tue, 07 Mar 2006 19:53:12 -0800
To: Michael Barton <michael.barton@asu.edu>
Cc: Multiple recipients of list <GRASSLIST@baylor.edu>
Subject: Re: [GRASSLIST:10743] How to extract polygons from vector map

Yeah, that's one way I figured it out. Problem is, I'm generating
contours from a DEM for a large region, and I wanted to pick one to
extract. There are no attributes assigned by r.contours that I can use.

Unless a better idea presents itself, I'm going to create small local
contours around the lakes, then collect them into a single vector layer,
add an attribute, and use the form mode of the mouse query to edit the
new attribute. THEN I can use an attribute query to extract the vectors
I want.

There should be a better way, but this does keep me amused...

Michael

Michael Barton wrote:

In the new GIS Manager (gism2), if you can display only that polygon with an
attribute query, you can extract it to a new vector. (See new vector panel
with button to extract queried vector).

Michael
__________________________________________
Michael Barton, Professor of Anthropology
School of Human Evolution and Social Change
Arizona State University
Tempe, AZ 85287-2402

phone: 480-965-6213
fax: 480-965-7671
www: http://www.public.asu.edu/~cmbarton

From: Michael Rensing <michael.rensing@shaw.ca>
Date: Sun, 05 Mar 2006 11:18:12 -0800
To: <GRASSLIST@baylor.edu>
Subject: [GRASSLIST:10743] How to extract polygons from vector map

I've got a vector map where I can visually identify one outline which is
a lake. How can I extract that one vector from the layer and turn it
into a filled blue polygon in a new layer?

You've all been a huge help to me so far, and I hope that the good will
continues until I am able to contribute my own answers. If anyone is
interested in what I'm up to, I've been keeping a sketchy blog at
http://mjr42thoughts.blogspot.com/

Regards,
Michael
    

5

Aha! A slightly better way. r.contour seems to create a unique category for each altitude, but all contours at that altitude have the same category, which was part of my problem. Your suggestion to use v.category to add a sequence of unique cat values should do the trick nicely. Thanks very much.

Regards,
Michael

Michael Barton wrote:

Michael,

You should have SOME kind of value attached to the vector--either as cat or
label. You can have r.to.vect create this. You can also run v.category to
add unique cat values to each vector object. Then you can use a query to
extract the one you want.

Michael
__________________________________________
Michael Barton, Professor of Anthropology
School of Human Evolution and Social Change
Arizona State University
Tempe, AZ 85287-2402

phone: 480-965-6213
fax: 480-965-7671
www: http://www.public.asu.edu/~cmbarton

From: Michael Rensing <michael.rensing@shaw.ca>
Date: Tue, 07 Mar 2006 19:53:12 -0800
To: Michael Barton <michael.barton@asu.edu>
Cc: Multiple recipients of list <GRASSLIST@baylor.edu>
Subject: Re: [GRASSLIST:10743] How to extract polygons from vector map

Yeah, that's one way I figured it out. Problem is, I'm generating
contours from a DEM for a large region, and I wanted to pick one to
extract. There are no attributes assigned by r.contours that I can use.

Unless a better idea presents itself, I'm going to create small local
contours around the lakes, then collect them into a single vector layer,
add an attribute, and use the form mode of the mouse query to edit the
new attribute. THEN I can use an attribute query to extract the vectors
I want.

There should be a better way, but this does keep me amused...

Michael

Michael Barton wrote:
    

In the new GIS Manager (gism2), if you can display only that polygon with an
attribute query, you can extract it to a new vector. (See new vector panel
with button to extract queried vector).

Michael
__________________________________________
Michael Barton, Professor of Anthropology
School of Human Evolution and Social Change
Arizona State University
Tempe, AZ 85287-2402

phone: 480-965-6213
fax: 480-965-7671
www: http://www.public.asu.edu/~cmbarton

From: Michael Rensing <michael.rensing@shaw.ca>
Date: Sun, 05 Mar 2006 11:18:12 -0800
To: <GRASSLIST@baylor.edu>
Subject: [GRASSLIST:10743] How to extract polygons from vector map

I've got a vector map where I can visually identify one outline which is
a lake. How can I extract that one vector from the layer and turn it
into a filled blue polygon in a new layer?

You've all been a huge help to me so far, and I hope that the good will
continues until I am able to contribute my own answers. If anyone is
interested in what I'm up to, I've been keeping a sketchy blog at
http://mjr42thoughts.blogspot.com/

Regards,
Michael