[prev in list] [next in list] [prev in thread] [next in thread] 

List:       postgis-users
Subject:    Re: [postgis-users] Implementation of postgis interoperability with R
From:       Paul Ramsey <pramsey () cleverelephant ! ca>
Date:       2020-03-13 2:36:23
Message-ID: CACowWR1NFDNf2K0tQaUYV-_6j=4bK8=sYWx59S92P4XueEhEAQ () mail ! gmail ! com
[Download RAW message or body]

proj_identify:

https://proj.org/development/reference/functions.html#_CPPv413proj_identifyP10PJ_CONTEXTPK2PJPKcPPCKcPPi


On Thu, Mar 12, 2020 at 7:35 PM Paul Ramsey <pramsey@cleverelephant.ca> wrote:
> 
> (a) do you already know the authority and code? Then first use those
> to look-up the SRID in spatial_ref_sys (select srid from
> spatial_ref_sys where auth_name = 'EPSG' and auth_code = 14234)
> (b) do you only have WKT input? Then, sInce you have access to proj on
> the R side, I'd suggest instantiating your const PJ *obj, and then
> using proj_identify() to see if proj can look up your projection with
> a good match. Maybe it can!
> (c) try a text lookup, it will probably fail, since as you note
> there's a million ways text representations can differ while still
> being the same.
> 
> P.
> 
> On Thu, Mar 12, 2020 at 8:20 AM Etienne B. Racine <etiennebr@gmail.com> wrote:
> > 
> > Dear list,
> > 
> > I'm currently updating the simple feature package for R to use the latest proj6 \
> > to interact with postgis. One of the things we perform behind the scene is \
> > synching the projections between R and postgis. 
> > The current version of the package provides interoperability in two ways: read \
> > and write data to postgis, and allow the user to write R code that is executed, \
> > as SQL, on the remote database, just like a normal data.frame (R tables). 
> > Our current implementation relies on epsg code and proj4string matching to synch \
> > database srid with the local crs, but with PROJ6, we need to add proj wkt \
> > matching. 
> > Two questions:
> > 
> > This is how I plan to adapt the projection synching, any suggestion for \
> > improvement or pointer to other implementations would be much appreciated: 1. \
> > Match local epsg code and srid, create a crs in R using the `srtext` and check \
> > that the two projections are equivalent using `IsSame` from OGRSpatialReference. \
> > 2. No matching code, so check `spatial_ref_sys` table to see if a proj WKT would \
> > match any `srtext` (slow and not very robust, I have a follow up question) 3. No \
> > matching code and no WKT match, so try to match proj4string. 4. Clearly no \
> > matching projection, so insert new srid in `spatial_ref_sys` and set `srid = \
> > max(srid) + 1`, `auth_name = 'sf'` (the simple feature package in R is called \
> > sf). 5. If it fails (e.g. lack of permission), then error and ask the user to \
> > change the projection, or use srid = 0. 
> > Is string matching `=` the best way to match a `srtext` and `proj4string` in  \
> > `spatial_ref_sys`? ``` sql
> > select * from spatial_ref_sys where srtext = {wkt};
> > ```
> > But `=` obviously rejects matches if parameters are ordered differently (and I \
> > suspect we could have issues with encoding). One solution is to read \
> > `spatial_ref_sys` to R, and match there, but it is very very slow since it \
> > requires to parse all the wkt to create a projection, and then perform a \
> > comparison with OGRSpatialReference->IsSame on every projection. 
> > Thanks for your help!
> > Etienne
> > _______________________________________________
> > postgis-users mailing list
> > postgis-users@lists.osgeo.org
> > https://lists.osgeo.org/mailman/listinfo/postgis-users
_______________________________________________
postgis-users mailing list
postgis-users@lists.osgeo.org
https://lists.osgeo.org/mailman/listinfo/postgis-users


[prev in list] [next in list] [prev in thread] [next in thread]