having trouble testing point/geometry intersections in sf - r

Question

i am trying to figure out how to use sf_intersects() to test whether or not point data that i have falls inside the geometries of some map data i have.
data i'm working with: https://osfm.fire.ca.gov/media/5818/fhszs19sn.zip
other data i'm working with too: https://osfm.fire.ca.gov/media/7564/c19fhszl06_5.zip
for now, i'm just trying to see if this data falls in the polygons of the above shapefile:
la_test_points <- data.frame(y = runif(1000, 33.6, 34.8), x = runif(1000, -119, -117.6))
when i put my map data and point data together, this is what it looks like:
so far, so good. now i attempt to test point/geometry intersections. as the figure suggests, i should be able to get quite a few.
# changing coordinate system of map created by shape file
la_fire_sra <- st_transform(st_as_sf(la_fire_sra), crs = 3857)
# merging test points with map data
la_test_points_merged <- st_as_sf(la_test_points, coords = c('y', 'x'), crs = st_crs(la_fire_sra))
# seeing if points fall within any of the geometries in the shapefile
la_test_points_merged <- la_test_points_merged %>%
mutate(intersection = st_intersects(geometry, la_fire_sra))
that last bit is where it all goes wrong. rstudio doesn't throw an error, but when i print la_test_points_merged to see my results, this is what i see:
> la_test_points_merged
Simple feature collection with 1000 features and 1 field
Geometry type: POINT
Dimension: XY
Bounding box: xmin: 33.60155 ymin: -118.9959 xmax: 34.79907 ymax: -117.6015
Projected CRS: WGS 84 / Pseudo-Mercator
First 10 features:
Error in xj[i, , drop = FALSE] : incorrect number of dimensions
the last line above is in red.
when i try using st_intersection() instead of st_intersects(), i get a different error:
> la_test_points_merged <- la_test_points_merged %>%
+ mutate(intersection = st_intersection(geometry, la_fire_sra))
Error in `stopifnot()`:
! Problem while computing `intersection = st_intersection(geometry, la_fire_sra)`.
x `intersection` must be size 1000 or 1, not 0.
Run `rlang::last_error()` to see where the error occurred.
i would like to end up with a result like this that tells me whether or not each of the points in la_test_points is contained by any of the geometry values in la_fire_sa.
how can i fix this to make my code work? i have looked at lots of other similar questions, but i can't seem to find any answers that apply to my current situation.
thanks in advance for any help.

Answer 1

You can join the points to the shapefile, and the result will show you the fire hazard for each point that falls within a polygon. The default for an st_join is st_intersects, but you can change it if you'd like.
Below I've used one of the shapefiles you linked. If you need to use both you can combine them for a single dataframe with all the polygons. Looks like they have different columns though, so some cleaning might be needed.
library(tidyverse)
library(sf)
set.seed(3) #to make la_test_points reproducible
a <- read_sf('fhszs06_3_19.shp')
# Create synthetic data, make it an sf object, and set the crs
la_test_points <- data.frame(y = runif(1000, 33.6, 34.8), x = runif(1000, -119, -117.6)) %>%
st_as_sf(coords = c('x','y')) %>%
st_set_crs(4326) %>%
st_transform(st_crs(a))
# join the points with the fire hazard area
joined <- st_join(la_test_points, a)
# the sf dataframe, lots of NA's so they're removed for a look:
joined %>% filter(!is.na(HAZ_CODE)) %>% head()
#> Simple feature collection with 6 features and 5 fields
#> Geometry type: POINT
#> Dimension: XY
#> Bounding box: xmin: 125951 ymin: -433789.6 xmax: 177186.8 ymax: -369094
#> Projected CRS: NAD_1983_Albers
#> SRA HAZ_CODE HAZ_CLASS Shape_Leng Shape_Area geometry
#> 1 SRA 3 Very High 613618.0 686671532 POINT (163249.3 -395328.4)
#> 2 SRA 3 Very High 250826.8 233414399 POINT (127980.6 -433789.6)
#> 3 SRA 3 Very High 613618.0 686671532 POINT (167675.9 -386506.6)
#> 4 SRA 3 Very High 391522.6 297194108 POINT (143421.2 -369094)
#> 5 SRA 2 High 208122.8 211364977 POINT (177186.8 -388738.9)
#> 6 SRA 3 Very High 613618.0 686671532 POINT (125951 -399105.6)
# Plotting points, colored according to fire hazard code
ggplot() +
geom_sf(data = a) +
geom_sf(data = joined, aes(color = HAZ_CODE)) +
scale_color_gradient(low = 'yellow', high = 'red')
Created on 2022-11-08 with reprex v2.0.2
Edit to address joining the example shapefiles:
# Keeping the columns that the example shapefiles have in common,
# and joining them together.
ax <- a %>% select(HAZ_CODE, HAZ_CLASS, Shape_Leng, Shape_Area)
bx <- b %>% select(HAZ_CODE, HAZ_CLASS, Shape_Leng, Shape_Area)
fires <- rbind(ax, bx)
head(fires)
Simple feature collection with 6 features and 4 fields
Geometry type: MULTIPOLYGON
Dimension: XY
Bounding box: xmin: 151597.9 ymin: -470591.9 xmax: 198216 ymax: -443900.4
Projected CRS: NAD83 / California Albers
# A tibble: 6 × 5
HAZ_CODE HAZ_CLASS Shape_Leng Shape_Area geometry
<int> <chr> <dbl> <dbl> <MULTIPOLYGON [m]>
1 3 Very High 5415. 1355567. (((152996.8 -469302.2, 152996.9 -469302.2, 152965.9 -469339.9, 152957.5 -…
2 3 Very High 2802. 423658. (((153701.7 -468506, 153703.9 -468590.6, 153708 -468758.1, 153707.6 -4687…
3 3 Very High 802. 32272. (((191491 -449977.1, 191494.3 -449973.2, 191517.3 -449946.5, 191521.5 -44…
4 3 Very High 1097. 40800. (((182453.8 -445649.1, 182216.3 -445706.6, 182215.4 -445655.7, 182170.4 -…
5 3 Very High 59226. 9379764. (((198201 -446611.2, 198199.9 -446580, 198199.1 -446551.3, 198200 -446580…
6 3 Very High 1255. 70800. (((186617.7 -444161.6, 186619 -444164.5, 186630.5 -444192.8, 186561.8 -44…