[postgis-users] Creating touching depth areas from gridded data

Nicolas Ribot nicolas.ribot at gmail.com
Thu Feb 4 03:12:53 PST 2016

Hi Hemant,

You may find some tips to simplify polygons while keeping topology on
Postgis wiki here:
ยง Examples of Spatial SQL


On 3 February 2016 at 19:34, Hemant Bist <hemantbist at gmail.com> wrote:

> Hi,
> I have gridded data for ocean/lake(that gives depth of water at
> equidistant intervals) and I am trying to extract polygon and multipolygons
> that represent different water depth range.
> example of gridded data set is  here
> <http://www.gebco.net/data_and_products/gridded_bathymetry_data/> , and
> the polygons  I am trying to create are ST_Multipolygon approximation of
> different color areas in this image
> <https://www.soest.hawaii.edu/gmt/gmt/html/images/example_02.png>.
> I tried the simple approach of st_union grid rectangles with
> st_snap_to_grid as mentioned here
> <http://gis.stackexchange.com/questions/31895/what-is-the-best-way-to-join-lots-of-small-polygons-to-form-a-larger-polygon>
> .
> The problem is that the resulting polygons have zig-zag edges. If I do
> st_simplify or smoothening (grass v.generalize) I am left with polygons
> that are not touching anymore (there is a gap between polygons that were
> touching earlier).
> Is there something in Postgis/Some other open source tool that I can use
> to avoid this problem: get smooth polygons that touch each other. I will
> appreciate any pointers.
> There was some reference that "topology" may be able to help, but I am
> unable to find a tutorial/example that would load the data in postgis and
> extract smooth polygons.
> Best,
> HB
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