Douglas-Peuker and Simplification Program

Unfortunately, the links below and my earlier research indicates that
the "preserve shared boundaries" feature may not be available outside
of proprietary tools. Still, for many purposes, it will be fine to not
have this, especially at low levels of simplification (resulting in
larger GeoJSON files, which will of course be heavier to get over the
internet and for the browser to handle). It is largely an empirical
question - I noticed the cracks when trying to create GeoJSON that was
small enough for world maps, which was a true struggle. One option
would be to provide users with a choice of whether the shapefile they
upload should be simplified or not, and hardcode a reasonable
simplification. Their other option would be to use Mapshaper (or ask
someone with a proprietary tool) to simplify before uploading.

Knut

···

---------- Forwarded message ----------
From: Johan Lemarchand <johan.lemarchand@gmail.com>
Date: Fri, May 21, 2010 at 3:15 PM
Subject: Douglas-Peuker and Simplification Program
To: Knut Staring <knutst@gmail.com>

http://www.cartoweb.org/downloads/vertexsimplification/documentation.html

http://groups.google.com/group/Google-Maps-API/browse_thread/thread/e5e865ddeeaa81ea?pli=1

--
Cheers,
Knut Staring

Or rather, there is an open soruce python implementation that will
work inside PostGIS (the first link below).

We could use this here instead of FME since we have all our data in
PostGIS, and as I said in an email last night, I actually think it is
a good idea in the longer term to provide a version of DHIS 2 which
runs on top of PostGIS (with full precision data) and also has
Geoserver somewhat integrated. So people who have this version of
DHIS2 could have a third option in addition to the two I outline
below.

···

On Fri, May 21, 2010 at 3:30 PM, Knut Staring <knutst@gmail.com> wrote:

Unfortunately, the links below and my earlier research indicates that
the "preserve shared boundaries" feature may not be available outside
of proprietary tools. Still, for many purposes, it will be fine to not
have this, especially at low levels of simplification (resulting in
larger GeoJSON files, which will of course be heavier to get over the
internet and for the browser to handle). It is largely an empirical
question - I noticed the cracks when trying to create GeoJSON that was
small enough for world maps, which was a true struggle. One option
would be to provide users with a choice of whether the shapefile they
upload should be simplified or not, and hardcode a reasonable
simplification. Their other option would be to use Mapshaper (or ask
someone with a proprietary tool) to simplify before uploading.

Knut
---------- Forwarded message ----------
From: Johan Lemarchand <johan.lemarchand@gmail.com>
Date: Fri, May 21, 2010 at 3:15 PM
Subject: Douglas-Peuker and Simplification Program
To: Knut Staring <knutst@gmail.com>

http://www.cartoweb.org/downloads/vertexsimplification/documentation.html

http://groups.google.com/group/Google-Maps-API/browse_thread/thread/e5e865ddeeaa81ea?pli=1

--
Cheers,
Knut Staring

--
Cheers,
Knut Staring

I think the biggest problem is that the underlying shape files
themselves have topological issues. This is a big problem specifically
with the WHO dataset, which has been composed of individual
submissions of country level data, and then sort of seamed together.
The GAUL dataset from FAO is supposedly topologically integral, and
should not suffer from spilters/gaps etc when simplified, IF, the
topology is preserved. This assumes that the original shape file is
topologically correct from the beginning, which is often not the case
with a lot of the data we deal with. As noted in the thread you sent
the DP algorithm does not maintain toplogy (assuming that exists from
the beginning).

You probably need to fix the topology on the underlying data, a
daunting task that is still on my old WHO to-do list from 2006. Good
luck.

Regards,
JPP

···

On Fri, May 21, 2010 at 3:40 PM, Knut Staring <knutst@gmail.com> wrote:

Or rather, there is an open soruce python implementation that will
work inside PostGIS (the first link below).

We could use this here instead of FME since we have all our data in
PostGIS, and as I said in an email last night, I actually think it is
a good idea in the longer term to provide a version of DHIS 2 which
runs on top of PostGIS (with full precision data) and also has
Geoserver somewhat integrated. So people who have this version of
DHIS2 could have a third option in addition to the two I outline
below.

On Fri, May 21, 2010 at 3:30 PM, Knut Staring <knutst@gmail.com> wrote:

Unfortunately, the links below and my earlier research indicates that
the "preserve shared boundaries" feature may not be available outside
of proprietary tools. Still, for many purposes, it will be fine to not
have this, especially at low levels of simplification (resulting in
larger GeoJSON files, which will of course be heavier to get over the
internet and for the browser to handle). It is largely an empirical
question - I noticed the cracks when trying to create GeoJSON that was
small enough for world maps, which was a true struggle. One option
would be to provide users with a choice of whether the shapefile they
upload should be simplified or not, and hardcode a reasonable
simplification. Their other option would be to use Mapshaper (or ask
someone with a proprietary tool) to simplify before uploading.

Knut
---------- Forwarded message ----------
From: Johan Lemarchand <johan.lemarchand@gmail.com>
Date: Fri, May 21, 2010 at 3:15 PM
Subject: Douglas-Peuker and Simplification Program
To: Knut Staring <knutst@gmail.com>

http://www.cartoweb.org/downloads/vertexsimplification/documentation.html

http://groups.google.com/group/Google-Maps-API/browse_thread/thread/e5e865ddeeaa81ea?pli=1

--
Cheers,
Knut Staring

--
Cheers,
Knut Staring

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--
--
Jason P. Pickering
email: jason.p.pickering@gmail.com
tel:+260968395190