Nice! I did not know this!
On 19-04-2019 06:46, Naraina Damle wrote:
I would like to add my two cents to this discussion.
You need to read, study, learn and practice this stuff about
Coordinate System and projections.
You can always land up in surprise\ing situations.
One thing I would like to mention here is to Split the thing in Two
Aspects : DATUM and PROJECTION and understand the difference between
the two.
Simply stated just saying "WGS84" is not complete. It is only talking
about Datum and not projections.
DATUM DECIDES WHAT IS THE PRECISE LATITUDE AND LONGITUDE OF A PLACE.
As per WGS84 the ZERO Longitude is NOT at Greenwhich line it is a
little away.
https://gis.stackexchange.com/questions/88428/why-is-the-gps-reference-meridian-100m-to-the-east-of-the-prime-meridian
One needs to Know and use Datum even if you want to make a Map on a
Actually Sphere / Globe. But in this case you do not need projection.
PROJECTION DECIDES HOW YOU ARE FLATTENING THE SPHERE/ PART OF IT TO
SEE IT ON A FLAT MAP.
So in case of a Flat map You need to be careful about DATUM as well as
PROJECTION.
Datum is required because there were historical maps made with Datum
other than WGS84.
...... This is just a simplified way but it is more complex. I am a
permanent learner.
Regards
Naraina Damle
On Thursday, April 18, 2019 at 11:08:57 PM UTC+5:30, Sharad Lele 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.
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 <http://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
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