[Freegis-list] Which OS GIS package to edit/query shapefiles?

[Ugo Taddei]

Hello Brent,

Brent Wood wrote:

>
>I have a coastline as a shapefile.
>
>I want to draw lines across various bays.estuaries & harbours to determine
>their areas.
>
>
>Is there an OS GIS tool that will let me graphically interact with the data,
>draw lines with the mouse, merge these lines with existing lines/polygons
and
>define the new polygons to derive the areas?
>
>I can see a way of using PostGIS by gridding the regions with small cells &
>running a query to sum the areas of the cells outside the coast &
>left/right/below/above certain locations but this is a very cumbersome
>approach, & is very non-graphical/interactive.
>
>I can't seem to do this with JUMP, in that I close a bay by moving one
vertex,
>but can't get a clean topology with a new area (the bay) out of it.

That's right! By doing so you create an invalid geometry (a bow tie
polygon!).

My suggestions are:

1) You digitize (some of the) points along the bay margin, closing the
bay; then you let JUMP compute the area (a bit cumbersome...but you get a
lot of help from the snap feature);

2) You copy the poly into a new layer, closing the bay as you did (moving
a vertex) and then deleting the other vertices (*not* the ones forming the
margin) till you get a valid gemoetry (a bit of work...)

or

3) Send your question directly to the JUMP users-list

http://www.jump-project.org/ (see resources)

Hope that helps,

Ugo

>
>This is frustrating as the operation is trivial with my commercial GIS
package,
>which I'm trying to replace with OS GIS software.
>
>
>Any advice/help appreciated.
>
>Brent Wood
>
>_______________________________________________
>Freegis-list mailing list
>Freegis-list at intevation.de
>https://intevation.de/mailman/listinfo/freegis-list
>

-- 
l a t / l o n  GmbH
Aennchenstrasse 19           53177 Bonn, Germany
phone ++49 +228 184960       fax ++49 +228 732153
http://www.lat-lon.de        http://www.deegree.org