Interpolating irregular spatial data - land mass only

Question

I have irregular spatial data I want to plot, and it's on a somewhat coarse grid of points, so I'm using linear interpolation from the interp library in order to do so. However, I only want to visualize the data on land, not on water (water should just be white space). And the interp function is interpolating over some of the water, which I don't want to know. Any ideas on how to avoid this, or how to override this orange region with white space? Here is the code I have tried and the accompanying plot:

data_for_plot <- interp(df$x, df$y, df$z, nx = 500, ny = 500) |> 
  interp2xyz() |> 
  as.data.frame()

ggplot(data = data_for_plot , aes(x, y)) +
  geom_raster(aes(fill = z)) +
  scale_fill_distiller(palette = "Spectral", na.value = NA) + 
  theme_classic() 

Also, here is the code used to generate the data frame df:

df <- structure(list(x = c(-82.4858388134, -82.3769388134, -82.3703388134, 
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Answer 1

So, i have two ways to filter away or assign all the unwanted points, depending on what is availably. The trick in both is to have a definition of the unwanted area.

1. If the water you are talking about is the ocean there is a simple if not slow approach. with the package "ggOceanMaps"

library(ggOceanMaps)

#get the distance to land, if the distanced is greater than 0 it is on water and should be NA'ed Blockquote

pts <- dist2land(data_for_plot)
pts[pts$ldist > 0, "z"] <- NA

2. you need a shapefile that contains the defenition of the water

library(sf)
library(dplyr)
#convert to spatial
pts <- data_for_plot %>% 
  sf::st_as_sf(coords = c("x","y")) %>% 
  sf::st_set_crs(4326) #projection

#load in your shapefile of water
poly <- st_read(...)

#finde points in polygon
points_in_polygons <-lengths(st_intersects(pts, poly)) > 0
pts$in_polygon <- points_in_polygons

#points in the polygon set to NA
pts[pts$in_polygon, "z"] <- NA