R - sf package - custom proj4string

Question

I am doing some geospatial analysis and am having a difficult time finding an appropriate CRS to transform the data to. I am using two resources to look up CRS codes.

1. https://resources.arcgis.com/en/help/main/10.1/018z/pdf/projected_coordinate_systems.pdf
2. https://epsg.io/

The proj4string of a CRS code, for example, EPSG:6566, can be obtained using the code below:

sf::st_crs("EPSG:6566")$proj4string

# "+proj=lcc +lat_0=17.8333333333333 +lon_0=-66.4333333333333 +lat_1=18.4333333333333 +lat_2=18.0333333333333 +x_0=200000 +y_0=200000 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs"

My data contains latitudes and longitudes for places that I want to use for performing proximity analysis.

library(sf)
library(dplyr)

data %>% 
  st_as_sf(coords = c("nh_lon", "nh_lat"), crs = 4326) %>% 
st_transform(crs = "EPSG:6566")

If I wanted to use a Equidistant Conic (eqdc) projection instead of a Lambert Conformal Conic (lcc) projection, could I do it by simply replacing the "+proj=" portion of the proj4string or is it more nuanced than this?

library(sf)
library(dplyr)

prj <- "+proj=eqdc +lat_0=17.8333333333333 +lon_0=-66.4333333333333 +lat_1=18.4333333333333 +lat_2=18.0333333333333 +x_0=200000 +y_0=200000 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs"
    data %>% 
      st_as_sf(coords = c("nh_lon", "nh_lat"), crs = 4326) %>% 
st_transform(crs = prj)

Answer 1

Replacing lcc with eqdc does change a conic projection to an equidistant one. But you should make sure all the other parameters are relevant and correct for your use case.

The general parameters are documented here:

https://proj.org/en/9.3/usage/projections.html

and specific parameters for those projections are on their pages:

• https://proj.org/en/9.3/operations/projections/lcc.html
• https://proj.org/en/9.3/operations/projections/eqdc.html

You might notice that eqdc doesn't have a lat_0 parameter, which in this case probably doesn't matter (I assume it gets ignored, haven't tested) so you have probably made a valid PROJ string and you will get a eqdc projection. Does it look right when you test it? Check it on a few features. Then maybe adjust the parameters to get it rotated the right way up for your data, adjust the offset parameters if you want the projected origin (0,0) to be elsewhere and so on.