Dear Sharad, My apologies for the delay in responding.(The week was blissfully busy and I lost track of responding to this email). My response is inline.
On Thu, Apr 18, 2019 at 11:09 PM Sharad Lele <sharad.l...@gmail.com> wrote: > Devdatta's summary was short and sweet, and practical. To add to it: > > 1. The NNRMS CRS suggested seems to be in Lambert Conformal Conic > projection (with WGS84 ellipsoid and datum). An alternative to it is the > UTM projection system, which keeps longitudes parallel, and gives > reasonably accurate area estimate. The particular UTM zone one may use > varies by which part of India one is in. > 2. In common parlance, 'unprojected' means having no Coordinate Reference > System at all. In GIS parlance, 'unprojected' may be understood as > 'geographic' CRS, that is, in lat-long rather than in metres. I presume > Ashim means 'having no CRS' at all. Whether one's map needs a CRS or not > depends upon what use one wants to put it to (and hence how accurate it > must be) and its size. A map of a 60'x40' piece of land does not require a > CRS if the purpose is house construction. It may require a CRS if one is > setting up a telescope on it which will do interferometry with another > telescope 500km away. A map of even a layout or a village or small town may > not need a CRS if one wants approximate locations/directions to navigate > visually (old style). It will need a CRS if one wants to use a GPS to > navigate. > If, Ashim, you mean 'geographic' vs 'projected' system, the answer is > different: geographic is useful for representation, easy to understand > (lat-long concept), but not to be used for calculating distances and areas. > I think when the data is "unprojected", it simply plots the points in a plane based on the co-ordinates( long / lat) . When it is projected it uses a transformation to go from points ( long / lat ) -> 2D map. > Hope this helps. Devdatta and others: please correct/supplement. > Sharad > > On Tuesday, April 16, 2019 at 3:38:08 PM UTC+5:30, Ashim wrote: >> >> 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 <ashim...@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 <nikh...@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. 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