https://github.com/defuneste/utile_comme_du_pq

small shiny app to understand various to correct geometries
https://github.com/defuneste/utile_comme_du_pq

Last synced: about 1 month ago
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small shiny app to understand various to correct geometries

• Host: GitHub
• URL: https://github.com/defuneste/utile_comme_du_pq
• Owner: defuneste
• Created: 2020-04-30T10:47:09.000Z (over 4 years ago)
• Default Branch: master
• Last Pushed: 2022-12-11T04:15:41.000Z (about 2 years ago)
• Last Synced: 2023-03-05T03:35:27.091Z (almost 2 years ago)
• Language: R
• Homepage:
• Size: 314 KB
• Stars: 3
• Watchers: 2
• Forks: 0
• Open Issues: 0
• Metadata Files:
  • Readme: README.md

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README

        
# Utile Comme du PQ

This used to be a small repo with few files I needed to grab(yeah for french reader I created it when people were hoarding toilette papers...) but I guess now it is more about checking polygons geometries! 

The goal is to make a small shiny app that help myself (and maybe other see [here](https://github.com/Robinlovelace/geocompr/issues/811)) understand how various algorithms and their implementations are used to correct non valid geometries.

The source of inspirations started with this great post from Paul Ramsey (see [here](https://www.crunchydata.com/blog/waiting-for-postgis-3.2-st_makevalid) and [here](http://s3.cleverelephant.ca/invalid.html)) about a new implementation of `MakeValid()` in postGIS. 

I tried to reproduce some of this post with R in `erreur_topo.R` but some are still missing. 

## What is a valid polygons ?

First I should clarified what is a valid polygons: 

> Polygons have the concept of *validity*. The rings of a valid polygon may only intersect at distinct points -- rings can't overlap, and they can't share a common boundary. A polygon whose inner rings partly lie outside its exterior ring is also invalid  (p.38 [PosGIS in Action](#References))  

This is specified by the [Simple Features](#References) set of standard

## List of tools that can be used 

- GEOS: https://libgeos.org/doxygen/geos__c_8h.html#acb2987c643bda31b1fcaecff8b62ce98  

- GRASS with V.clean : https://grass.osgeo.org/grass82/manuals/v.clean.html

    * See: https://github.com/rsbivand/rgrass

    

## Tools inplemeted to correct geometries

### sf::st_buffer(x, m)

A great way of "cleaning some mess" but sometimes it feels like we are hammering stuff! 

### [{terra}](#References) and [{sf}](#References)

Both have implementation of [GEOS](https://libgeos.org/) but {sf} can also use s2. The engine use in {sf} depend of your version of GEOS (currently mine is 3.10) and if you are using planar or spherical geometry.   

- `terra::makeValid()` : https://rspatial.github.io/terra/reference/isvalid.html

- `sf::st_make_valid()` : https://r-spatial.github.io/sf/reference/valid.html

Both implementation have the option to provide you with more insight  (see `reasons` in `st_make_valid` and `messages` in `makeValid()`).

### [{polyclip}](#References) 

Github: https://github.com/baddstats/polyclip

It use [clipper](http://angusj.com/clipper2/Docs/Overview.htm) v1 now but maybe v2 at one point. 

Implementation in {spatstat} is in the `owin` [function:]( https://github.com/spatstat/spatstat.geom/blob/d90441de5ce18aeab1767d11d4da3e3914e49bc7/R/window.R#L230-L240)

`polyclip` works on **closed polygon** and take as input a list of `x` and `y` but the last vertex should not repeat the first (as in simple feature standard). It can also take a list of list for several polygons (see: `?polyclip::polyclip`). Polygon `B` is created in the `spatstat.geom::owin` as a larger rectangle.

My implementation seems not to be perfect as converting to polyclip object to sf's class maybe bring some errors.

### [{prepr}](#References)

> Automatically repair broken GIS polygons using constrained triangulation and returns back a valid polygon.

it can be find [here](https://gitlab.com/dickoa/prepr) or [here](https://github.com/dickoa/prepr)

More about here the algorythm in this publication: Ledoux, H., Arroyo Ohori, K., and Meijers, M. (2014). A triangulation-based approach to automatically repair GIS polygons. Computers & Geosciences 66:121–131.

## References:

1 Leo S., et Regina Obe. 2021. PostGIS in Action, Third Edition. Simon and Schuster.  

2 https://en.wikipedia.org/wiki/Simple_Features  

3 Hijmans R (2022). _terra: Spatial Data Analysis_. R package version 1.6-17,

  .  

4 Pebesma, E., 2018. Simple Features for R: Standardized Support for Spatial Vector Data. The R

  Journal 10 (1), 439-446, https://doi.org/10.32614/RJ-2018-009  

5 Johnson A, Baddeley A (2019). _polyclip: Polygon Clipping_. R package version 1.10-0,

  .  

6 Dicko A (2020). _prepr: Automatic Repair of Single Polygons_. R package version 0.1.9000,

  .