[Qgis-user] [QGIS-Developer] Geodesic Line Simplification

Tue Oct 8 23:31:11 PDT 2019

Hi Calvin,
IMHO having simple algs with few options is less confusing for users,
and small bricks are easier to reuse. So I'm +1 with the split.
Cheers.

On 08/10/19 17:41, C Hamilton wrote:
> I have currently implemented a geodesic decimation processing routine,
> but I have a question for the community. I am thinking about making 3
> algorithms to simplify the process rather than one algorithm with more
> options where some options would not be applicable to points and other
> not applicable to lines so would you agree with my reasoning or not? If
> I break it into 3, these would be the algorithms.
> 
> 1. Geodesic distance and time decimation for point layer.
> This would calculate the distance between consecutive points and if they
> don't meet the minimum distance then a point would be removed.
> Additionally, if a time field is selected it would also calculate the
> difference in time between two points and if it is less then the minimum
> time then the point would be removed. When both distance and time are
> selected and either condition is below the minimum, then the point would
> be removed. This algorithm would include an order field and group by
> field. There would be the option of preserving the last point despite
> the fact it might not meet the criteria.
> 
> 2. Geodesic distance decimation for lines and perhaps polygons.
> Time would not be a part of the algorithm because the individual
> vertices don't have time associated with them. This would calculate the
> distance between consecutive vertices and if they don't meet the minimum
> geodesic distance then the vertex would be removed. There would be an
> option of preserving the final vertex despite it might not meet the
> criteria.
> 
> 3. Geodesic Simply Algorithm for lines and polygons.
> This would implement geodesic versions of algorithms such the
> Douglas-Peucker algorithm for lines and polygons. Instead of using
> Cartesian math it would use geodesic math.
> 
> I have #2 implemented and #1 without time. I think it is probably better
> to have a separate algorithm for points and another for lines and
> polygons. Would you agree?
> 
> Thanks,
> 
> Calvin
> 
> On Wed, Oct 2, 2019 at 7:51 PM Nyall Dawson <nyall.dawson at gmail.com
> <mailto:nyall.dawson at gmail.com>> wrote:
> 
>     On Wed, 2 Oct 2019 at 01:30, C Hamilton <adenaculture at gmail.com
>     <mailto:adenaculture at gmail.com>> wrote:
>     >
>     > I am guessing this is working on Cartesian coordinates and not
>     geodesic which is fine if you have a local projection. I think I
>     will go ahead and implement some similar geodesic algorithms. Thank
>     you for the GRASS link. It will help.
> 
>     Hi Calvin,
> 
>     I think there's a strong use case for adding this to the c++ API.
>     Would you be willing to put together a proof of concept in Python code
>     and I'll port it to c++ for you?
> 
>     Nyall
> 
> 
>     >
>     > Thanks,
>     >
>     > Calvin
>     >
>     > On Tue, Oct 1, 2019 at 7:45 AM Áron Gergely
>     <aron.gergely at rasterra.nl <mailto:aron.gergely at rasterra.nl>> wrote:
>     >>
>     >> Hi Calvin and list,
>     >>
>     >> Have you looked at the GRASS GIS v.generalize tool?
>     https://grass.osgeo.org/grass64/manuals/v.generalize.html
>     >>
>     >> The Douglas-Peucker algorithm is implemented there and I have
>     used that with (topologic)success recently on many touching
>     polygons. Should work for polylines too!
>     >>
>     >> Best regards,
>     >> Aron
>     >>
>     >> On 01/10/2019 08:19, Tim Sutton wrote:
>     >>
>     >> Hi
>     >>
>     >> On 26 Sep 2019, at 17:13, C Hamilton <adenaculture at gmail.com
>     <mailto:adenaculture at gmail.com>> wrote:
>     >>
>     >> Are there any geodesic QGIS algorithms that simplify lines?
>     "Simplify" just uses the projection units of measure. If there isn't
>     I thought I would add this to the Shape Tools plugin. I could easily
>     implement the Douglas-Peucker algorithm using geodesic math.
>     >>
>     >> For GPS tracks I also see a need to throw out all points that are
>     less than a certain distance or less than a certain time interval. I
>     don't see any simplification algorithms that make use of time. Am I
>     just missing them?
>     >>
>     >>
>     >> No I don’t think we have any time based simplifiers - would be a
>     great addition!
>     >>
>     >> Regards
>     >>
>     >> Tim
>     >>
>     >>
>     >>
>     >>
>     >> Thanks,
>     >>
>     >> Calvin
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>     >>
>     >> —
>     >>
>     >>
>     >>
>     >>
>     >>
>     >>
>     >>
>     >>
>     >> Tim Sutton
>     >>
>     >> Co-founder: Kartoza
>     >> Ex Project chair: QGIS.org
>     >>
>     >> Visit http://kartoza.com to find out about open source:
>     >>
>     >> Desktop GIS programming services
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>     >> GIS Training
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>     >>
>     >> Skype: timlinux
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>     >>
>     >> I'd love to connect. Here's my calendar link to make finding time
>     easy.
>     >>
>     >>
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-- 
Paolo Cavallini - www.faunalia.eu
QGIS.ORG Chair:
http://planet.qgis.org/planet/user/28/tag/qgis%20board/