Unexpected convertion output from Latlong to UTM in R

Question

I'm trying to convert Latlong to UTM but, for some reason, the results coming from R are very different from what I looking for.

The dataset is just one observation (the city of São Paulo):

LatLong

Longitude of São Paulo: -46.633309

Latitude of São Paulo: -23.550520

UTM coordinates (WGS84) of São Paulo

The UTM coordinates (WGS84) of São Paulo are: Zone 23K E: 333287.02 N: 7394586.09

Source: http://www.gps-latitude-longitude.com/gps-coordinates-of-sao-paulo

library(rgdal)
x<-c(-46.633309)
y<-c(-23.550520)
zone<-23
xy<-data.frame(ID = 1:length(x), X = x, Y = y)
coordinates(xy)<-c("X", "Y")
proj4string(xy)<-CRS("+proj=longlat +datum=WGS84")  ## for example
res<-spTransform(xy, CRS(paste("+proj=utm +zone=",zone,"ellps=WGS84",sep='')))
res

> res
         coordinates ID
1 (333287, -2605414)  1

Easting term seems to be right, but Northing is very diferent from the expected value (7394586.09 not equal to -2605414).

Can anyone tell me what is going on? Thanks in advance.

aichao aichao · Accepted Answer · 2016-07-27T20:16:03

The issue is that you are in the southern hemisphere and the origin for Northing is no longer the equator but the south pole. Therefore, use

res<-spTransform(xy, CRS(paste("+proj=utm +south +zone=",zone,"ellps=WGS84",sep='')))
##        coordinates ID
##1 (333287, 7394586)  1

Note the +south addition to the CRS specification.

Hope this helps.

Unexpected convertion output from Latlong to UTM in R

1 Answers