dhis2-devs team mailing list archive

[Knut Staring <knutst@xxxxxxxxx>]

On Fri, May 21, 2010 at 3:04 PM, Bob Jolliffe <bobjolliffe@xxxxxxxxx> wrote:
> On 21 May 2010 13:58, Knut Staring <knutst@xxxxxxxxx> wrote:
>> On Fri, May 21, 2010 at 2:35 PM, Bob Jolliffe <bobjolliffe@xxxxxxxxx> wrote:
>>> On 21 May 2010 13:30, Knut Staring <knutst@xxxxxxxxx> wrote:
>>>> On Fri, May 21, 2010 at 2:25 PM, Bob Jolliffe <bobjolliffe@xxxxxxxxx> wrote:
>>>>> Generating an orgunit hierarchy in dxf is relatively straightforward.
>>>>> As long as you have some kind of parent id reference to work with.
>>>>>
>>>>> I am not sure of the algorithm to use for the simplification part but
>>>>> I guess it must be pretty standard?  You are just reducing the number
>>>>> of points on a polygon right?  My rusty maths could probably figure
>>>>> out an algorithm but this has got to already exist.  Does anyone have
>>>>> any pointers?
>>>>
>>>> Yes - there is Douglas-Peucker, Visvalingam and Special Visvalingam,
>>>> you can test them at the following link. But some of these create
>>>> cracks between polygons.
>>>> http://mapshaper.com/test/demo.html
>>>
>>> Ok.  This an online shaper.  Can't see any code to download :-)  I
>>> guess the three names above are the algorithms?  Any idea what the
>>> cracks are about?  Faulty algorithm, faulty implementation of
>>> algorithm or faulty shapefiles to start with.  Might be the shapefiles
>>> need to be pre-processed (cleaned) before transforming.
>>
>> Not quite sure, maybe it is linked to polygons not really sharing
>> borders, just overlapping, and then Switzerland's border with Germany
>> gets simplified differently from Germany's border with Switzerland.
>> Though I could be completely wrong - and it may also depend a lot on
>> the parameters one chooses. But it seems that the commercial tools
>> like FME and ArcToolbox may handle it better, or maybe the data was
>> just clearner, or that the tools do some automatic precleaning, as
>> opposed to the default versions of the algorithms as implemented in
>> the online Mapshaper and also in PostGIS (see below link) don't quite
>> preserve common borders between polygons.
>>
>> http://bostongis.org/PrinterFriendly.aspx?content_name=postgis_simplify
>>
>>>>
>>>>> If there is some reasonable java code to do this, then
>>>>> I would create a java class to do it somewhere in dhis and make that
>>>>> class available as a an extension to the xalan xslt processor.  That
>>>>> sounds complicated but its not really.  I did something similar with
>>>>> calculating dates off excel's (dodgy) date representation.
>>
>> I think this is interesting, as it would potentially allow people to
>> upload their own shapefiles without worrying about simplification and
>> conversion to GeoJSON. What will also be needed is a matching
>> algorithm - though Jan Henrik has already implemented identical
>> matching and manual visual matching in the client.
>>
>> By the way, in addition to simplification, there is "smoothing". Most
>> of this is available as part of the Java Topology Suite:
>> http://www.vividsolutions.com/jts/jtshome.htm
>> http://lists.refractions.net/pipermail/jump-users/2005-July/002564.html
>>
>
> Yes well if someone else can provide the simplification/smoothing or
> what have you I can make sure its available to the input transform.
> Other than that I'm a bit out of my depth with these processes at the
> moment.

Fair enough ;-)

Just to complete the picture, I think what is needed is what FME
provides with its option "Preserve Shared Boundaries":
http://docs.safe.com/fme/2009/html/Transformers/content/transformers/generalizer.htm

This seems to be an additional feature beyond the standard algorithms
- would be nice to know how its achieved, and especially if there are
open source implementations of this feature.

k