# [QGIS-Developer] Geodesic Line Simplification

Tue Oct 8 08:41:01 PDT 2019

```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> wrote:

> On Wed, 2 Oct 2019 at 01:30, C Hamilton <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>
> 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> 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
> >>
> >> 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
> >> Geospatial web development
> >> GIS Training
> >> Consulting Services
> >>
> >> Skype: timlinux
> >> IRC: timlinux on #qgis at freenode.net
> >>
> >> I'd love to connect. Here's my calendar link to make finding time easy.
> >>
> >>
> >> _______________________________________________
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