Dear All, I have a small query on similar lines. When do we use UNPROJECTED maps ? I understand that to go from 3D to 2D we need a projection. When is it reasonable to use an unprojected map?
Many thanks, Ashim On Tue, Apr 16, 2019 at 12:12 PM Ashim Kapoor <ashimkap...@gmail.com> wrote: > Dear Devdatta and Nikhil, > > I attended a R GIS school where the teachers were from France and they > were using CRS 2154. They told us that they were not familiar with the CRS > for Indian maps. > > I used 2154 thinking that it would not make much of a difference, but I > was wrong. My apologies for the confusion. When I used that on Indian ( > datameet ) shapefiles,I got this : map1.png ( see attachment ) > > I have also attached the R code to create these maps from the Datameet > District level ( 2011 ) shapefiles. I have created map2.png and map3.png > from crs = 3857 / 7755 respectively. > > I also found this : > https://epsg.io/?q=india > > Which is confusing because many CRS are there for India. Why do we have so > many choices of CRS for India ? > > I guess for the time being I will use 7755. > > Many thanks to Devdatta and Nikhil for their help. > Best Regards, > Ashim > > > > On Tue, Apr 16, 2019 at 11:42 AM Nikhil VJ <nikhil...@gmail.com> wrote: > >> Hi Ashim, >> >> Pls provide the exact link of what you're referring - this stuff is never >> as much on the top of people's minds as we assume it to be. >> Do try loading your shapefiles on other tools and check there : >> mapshaper.org is a site that does a lot of quick, cool things with >> shapefiles; QGIS is a software that does everything with shapefiles. Both >> are free and open source. >> >> The latter will help you re-save the shapefile into any CRS you want. >> Right-click the layer > Save As (or "Export") and remember to choose your >> preferred CRS in the dropdown. >> >> Can you give background on what this CRS : *2154* : is and why you want >> to transform to that? Because I've only come across two main CRS's both >> being under the "WSG 84" category : >> - EPSG 4326 : this makes everything in latitude longitude >> - EPSG 3857 : this makes everything in meters from the equator (I think) >> and we need to get the data in this format when we want to do things in >> physical distance terms like making a 1km distance buffer or measuring areas >> >> I haven't learned GIS stuff from theory, I just use it, so don't know >> more details about CRS. I do understand that the dizzying multitude of >> CRS's out there are so because apparently GIS folks like to re-orient the >> center of the world (geometrically speaking) to where their data is to >> ensure least distortion of their shapes in the rendering. When I come >> across anything that's in a non-standard CRS, my first move is to transform >> it to either lat-longs (4326) or meters (3857). >> >> Regards >> Nikhil VJ >> >> >> On Monday, April 15, 2019 at 1:57:20 PM UTC+5:30, Ashim wrote: >>> >>> Dear All, >>> >>> I am referring to 2011_Dist. * set of shapefile. (District level >>> shapefiles). >>> >>> When I read them as simple features in R, ( like this ) >>> >>> map <- st_read("2011_Dist.shp") %>% st_as_sf() >>> >>> ( it's unprojected because it says 4326 in the epsg when I read the >>> above ) >>> >>> plot(map$geometry) >>> >>> it looks OK. >>> >>> But when I do : >>> >>> map <- st_read("2011_Dist.shp") %>% st_as_sf() %>% st_transform(crs= >>> 2154) >>> >>> plot(map$geometry) >>> >>> the map is TILTED. >>> >>> What is the correct projection to use for this dataset? Please clarify. >>> >>> Best Regards, >>> Ashim >>> >> -- >> Datameet is a community of Data Science enthusiasts in India. Know more >> about us by visiting http://datameet.org >> --- >> You received this message because you are subscribed to the Google Groups >> "datameet" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to datameet+unsubscr...@googlegroups.com. >> For more options, visit https://groups.google.com/d/optout. >> > -- Datameet is a community of Data Science enthusiasts in India. Know more about us by visiting http://datameet.org --- You received this message because you are subscribed to the Google Groups "datameet" group. To unsubscribe from this group and stop receiving emails from it, send an email to datameet+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
