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I'm trying to draw points on a map and I have a problem using the projections. I have no problem getting points from longlat to UTM. But it seems that there is a problem when specifying UTM zone 15M and then try to plot them. I figure out that there is a difference of 10 000 000 meters between the 2 coordinates that I had. Why is that? 

library(sp)
library(mapview)

gps.pt = data.frame(lon = -90.22139,
                    lat = -0.6893730)
lonlat= sp::SpatialPoints(gps.pt, proj4string=CRS("+proj=longlat +datum=WGS84"))
spgeo <- sp::spTransform(lonlat, CRS("+proj=utm +zone=15M +datum=WGS84"))
# OK! 
mapview::mapView(spgeo)

# Wrong map (the easting and northing values were collected from [memento][1] on an Android smartphone)
gps.pt2 = data.frame(easting = 809289.5,
                     northing = 9923713)
spgeo2 = sp::SpatialPoints(gps.pt2, proj4string=CRS("+proj=utm +zone=15M +datum=WGS84"))
mapview::mapView(spgeo2)

# Now OK! 
gps.pt3 = data.frame(easting = 809289.5,
                     northing = 9923713-10000000)
spgeo3 = sp::SpatialPoints(gps.pt3, proj4string=CRS("+proj=utm +zone=15M +datum=WGS84"))
mapview::mapView(spgeo3)

Do I actually have to subtract 10 000 000 meters or I've just specified my CRS wrong? When should I apply a 10 000 000 meters correction?

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    try proj4string=CRS("+proj=utm +zone=15M +south +datum=WGS84") to tell the projection you're in the southern hemisphere.
    – TimSalabim
    Sep 17, 2019 at 12:34
  • Worked beautifully! Thanks a lot! I haven't found a "cheatsheet" for all the CRS things we can put inside a CRS. Do you have something like that?
    – M. Beausoleil
    Sep 17, 2019 at 13:30
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    My goto reference is epsg.io. I am not aware of a cheatsheet as such.
    – TimSalabim
    Sep 17, 2019 at 13:38
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    For more general doc on the proj library see, proj.org. "M" is the latitude band, 0 to -8°, but I'm not sure that the proj library uses the latitude band to determine if the zone needs the +10million false northing. Thus, the +south parameter.
    – mkennedy
    Sep 17, 2019 at 16:32
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    I think it's ignored, sf strips it but rgdal::CRSargs does not: sf::st_crs("+proj=utm +zone=15M +datum=WGS84")
    – mdsumner
    Sep 17, 2019 at 18:25

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