[GRASS-user] digitisation of an adjacent polygon

GRASS GRASS user
1

Hi,
I am using grass digitisation tools and wants to know wether it is required to redigitise the common boundaries between the adjacent polygons. Also let me know how to add islands, merge two polygons into a single polygon.
Thanking in advance

ramarajua

2

No, Grass is a topological GIS, that is adjacent polygons share a single common boundary...
You can digize a layer of boundaries, then create the polygon topology with v.build.

Good luck,

Vincent

ramaraju a écrit :

Hi,
I am using grass digitisation tools and wants to know wether it is required to redigitise the common boundaries between the adjacent polygons. Also let me know how to add islands, merge two polygons into a single polygon.
Thanking in advance

ramarajua
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3

Vincent BAIN wrote:

No, Grass is a topological GIS, that is adjacent polygons share a
single common boundary...

That's right.

You can digize a layer of boundaries, then
create the polygon topology with v.build.

Correction: you don't need to run v.build. Once you exit v.digit the
topology is built automatically. To create a polygon digitize it's
boundaries, make sure their nodes are snapped, digitize centroid within
each polygon. In GRASS, a polygon is made of boundaries and a centroid
inside.

Maciek

4

ramarajua wrote:

I am using grass digitisation tools and wants to know wether it is
required to redigitise the common boundaries between the adjacent
polygons. Also let me know how to add islands, merge two polygons into a
single polygon. Thanking in advance

[...]
Maciej Sieczka wrote:

To create a polygon digitize it's boundaries, make sure their nodes are
snapped, digitize centroid within each polygon. In GRASS, a polygon is
made of boundaries and a centroid inside.

and to make an island digitize a boundary but give it no centroid.

to merge two polygons you can use v.dissolve, that only dissolves
boundaries if the areas on either size share the same category value,
or (in GRASS 6.3) you can use the column option to base the dissolve on
some attribute column in the attached database.

Hamish