Re-projecting a SpatialPointsDataFrame does not change the extent - r

Question

I'm trying to overlay data related to bat locations (SpatialPointsDataFrame) on to the state of Colorado (SpatialPolygonsDataFrame). The CRS of the two objects are different:
crs(colorado)
#CRS arguments:
# +proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0
crs(BatRange_data)
#CRS arguments:
# +proj=utm +zone=13 +datum=NAD83 +units=m +no_defs +ellps=GRS80 +towgs84=0,0,0
When I reproject the bat data, the CRS does indeed change, but the extent is unaffected.
Before:
BatRange_data
#class : SpatialPointsDataFrame
#features : 2456
#extent : 139996.3, 748812, 4094998, 4535103 (xmin, xmax, ymin, ymax)
#coord. ref. : +proj=utm +zone=13 +datum=NAD83 +units=m +no_defs +ellps=GRS80 +towgs84=0,0,0
#variables : 17
After:
geo_proj = "+proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0"
crs(BatRange_data) = geo_proj
BatRange_trnsfrmd = spTransform(BatRange_data,geo_proj)
BatRange_trnsfrmd
#class : SpatialPointsDataFrame
#features : 2456
#extent : 139996.3, 748812, 4094998, 4535103 (xmin, xmax, ymin, ymax)
#coord. ref. : +proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0
#variables : 17
I can't plot the points on the polygon, as the extent of my polygon object is dissimilar to that of my points object:
colorado
#class : SpatialPolygonsDataFrame
#features : 1
#extent : -109.0608, -102.042, 36.99223, 41.00561 (xmin, xmax, ymin, ymax)
#coord. ref. : +proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0
#variables : 13
Why is the extent of the SpatialPointsDataFrame not changing when re-projected and transformed?
Reproducible example(I tested this myself, and sent it to a friend; it IS reproducible with what is provided):
#Libraries
x = c('raster','sp','rgdal')
# If you don't know if you've installed the packages for some of these
# libraries, run this:
# install.packages(x)
lapply(x, library, character.only=TRUE)
rm(x)
# Read in and format data -------------------------------------------------
BatRange_data = new("SpatialPoints", coords = structure(c(179935.719907205, 179935.938979813, 179935.938979813, 176598.335967664, 176598.335967664, 4499963.43180688, 4499963.30060606, 4499963.30060606, 4489332.4211975, 4489332.4211975), .Dim = c(5L, 2L), .Dimnames = list(NULL, c("coords.x1", "coords.x2"))), bbox = structure(c(176598.335967664, 4489332.4211975, 179935.938979813, 4499963.43180688), .Dim = c(2L, 2L), .Dimnames = list(c("coords.x1", "coords.x2"), c("min", "max"))) , proj4string = new("CRS" , projargs = NA_character_))
#Use state bounds from gadm website:
us = getData("GADM", country="USA", level=1)
#Extract states (need to uppercase everything)
co = "Colorado"
colorado = us[match(toupper(co),toupper(us$NAME_1)),]
#Re-project bat data to 'longlat'
geo_proj =
"+proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0"
crs(BatRange_data) = geo_proj
BatRange_trnsfrmd =
spTransform(BatRange_data,geo_proj)
plot(BatRange_trnsfrmd)
plot(colorado,add=TRUE)

Answer 1

In your code:
geo_proj = "+proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0"
crs(BatRange_data) = geo_proj
BatRange_trnsfrmd = spTransform(BatRange_data,geo_proj)
BatRange_trnsfrmd
you are overwriting the original projection of BatRange_data to lat/long before trying to reproject it. spTransform therefore is doing nothing, since it's trying to reproject from epsg:4326 to epsg:4326. This is why your extent is not changing.
You should therefore probably just remove this line:
crs(BatRange_data) = geo_proj
and all should work.
HTH.