[R-sig-Geo] Restricting geographical space for SDM analyses in R

[Micha Silver]

I would approach this as follows:

Get a polygon of the country boundary where your study area is located, 
then buffer it inside, (using a negative buffer distance). Now you can 
"difference" the full country boundary and the inner buffer to get just 
the coastal strip.


Here's an example:


library(sf)
library(maptools)
library(dplyr)
library(ggplot2)
theme_set(theme_bw())

data("wrld_simpl") # From maptools package
# Example for Egypt
egypt = st_as_sf(wrld_simpl[wrld_simpl$NAME == "Egypt",])
# Transform to Spherical Mercator for correct buffering in meters
egypt = st_transform(egypt, 3857)

# Inside buffer size 50 kilometers
d = 50000

# Setup polygon to crop only coast region
coast_corners = matrix(c(26,32,34,32,34,30,26,30,26,32),
                       ncol=2, byrow=TRUE)
coast_region = st_sf(st_sfc(st_polygon(list(coast_corners))))
st_crs(coast_region) = st_crs(4326)
# But transform this region also to Spherical Mercator
coast_region = st_transform(coast_region, 3857)

# Use negative buffer distance to get inner buffer
egypt_buffer =  st_buffer(egypt, -d)

# Now Difference the full country and buffer polygons,
# then Crop out only coast region
coastal_strip = st_difference(egypt, egypt_buffer) %>%
   st_crop(., coast_region)

# Plot result
ggplot(data = egypt) +
   geom_sf() +
   geom_sf(data = coastal_strip, fill="red")



HTH,

Micha


On 11/16/20 1:44 PM, Francisco Borges wrote:
> I am conducting SDM analysis in R, and would like to restrict my
> predictions in the geographical space. I am aware of how to do it by
> cropping the world space with latitude and longitude, but my case is a
> little bit trickier.
>
> I want to restrict my analysis to only the coastal area (i.e. from ocean
> to, let's say a certain distance inland). I have yet to find a solution
> online to this question, but I am aware that using a shapefile it could
> work.
>
> I have zero knowledge of GIS, unfortunately, as I understand that this
> could be one of the solutions.
>
> I appreciate any info or suggestion you can provide me
>
> Best,
>
> *Francisco Borges*
>
> MARE – Marine and Environmental Sciences Centre
>
> Laboratório Marítimo da Guia - Faculdade de Ciências da Universidade de
> Lisboa
>
>
> Av. Nossa Senhora do Cabo, 939
>
> 2750-374 Cascais, Portugal
>
> Tel: +351 214 869 211
>
>
>
>
> publons.com/a/1504585/
> orcid.org/0000-0002-3911-0421
> http://www.mare-centre.pt/pt/user/8232
>
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>
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-- 
Micha Silver
Ben Gurion Univ.
Sde Boker, Remote Sensing Lab
cell: +972-523-665918


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