How to create a ppp spatstat object from a sf object

Question

I am trying to create a ppp object from a sf object in R.

I am using the population-weighted centroids in Minnesota from 2020 census data. You can find the data here: https://www2.census.gov/geo/docs/reference/cenpop2020/blkgrp/CenPop2020_Mean_BG27.txt

library(spatstat)
library(sp)
library(sf)
library(maptools)
library(dplyr)

# pathing
setwd("My folder location")

# read data
centroids <- read.csv("./block_group_weighted_centroids.txt")

# convert to sf object
sf_df <- centroids %>%
  as.data.frame() %>%
  select(LONGITUDE, LATITUDE) %>% 
  st_as_sf(coords=c("LONGITUDE","LATITUDE"), crs=4326) %>%
  as_Spatial()

# create a ppp
sf_ppp <- as.ppp(sf_df) # This line results in an error

The error message: Error in as.ppp.SpatialPointsDataFrame(sf_df) : Only projected coordinates may be converted to spatstat class objects

How can I make sure I am using the planar projected coordinates?

Answer 1

You need to transform your points from lat-long degrees to a planar projected coordinate system appropriate for your data region.

For Minnesota I imagine there's some sort of state-designated coordinate system, or you can use the UTM zone appropriate for Minnesota's longitude.

The sf function st_transform will do this once you've got your lat-long data.

Note you don't need as_Spatial since spatstat will convert sf objects to ppp objects. You probably don't need as.data.frame either, since read.csv will return a data frame.