[R-sig-Geo] inconsistent as.data.frame(SpatialPointsDF)

[Edzer Pebesma]

There is some logic: sp tries to track coordinate names when it can, but
if coordinates are set as a nameless matrix, as in the last example
below, it will choose names itself. coordnames() helps you discover how
the coordinates of a SpatialPointsDataFrame are called:

> df = data.frame(x=1:2, y=2:1, z = 3:4)
> df1 = df
> library(sp)
> coordinates(df1) = ~x+y
> as.data.frame(df1)
  x y z
1 1 2 3
2 2 1 4
> coordnames(df1)
[1] "x" "y"
> df1=df
> coordinates(df1) = df[1:2]
> coordnames(df1)
[1] "x" "y"
> df1=df
> coordinates(df1) = cbind(df$x,df$y) # nameless
> df1
  coordinates x y z
1      (1, 2) 1 2 3
2      (2, 1) 2 1 4
> coordnames(df1)
[1] "coords.x1" "coords.x2"


a bit of a semantic trap is this: if your coordinates are longitude and
latitude and carry these names, after you project the object with
rgdal::spTransform they're still called longitude and latitude, although
they are no longer understood as such. You can then solve this yourself by

> coordnames(df1) = c("x", "y")

after which

> coordnames(df1)
[1] "x" "y"


On 03/20/2015 05:01 PM, MacQueen, Don wrote:
> In my experience, relying on column names to extract the coordinates is
> not at all a good idea. I would strongly recommend that you take the time
> to update all of your scripts to use the coordinates() function. I think
> it will be worth it in the long run.
> 
> It's not a good idea because the column names of the coordinates depend on
> how the SpatialPointsDataFrame was originally created, and in my own
> applications that is highly variable.  Sometimes ('x','y'), sometimes
> ('lon','lat'), or any of several other variations of how to spell or
> abbreviate latitude and longitude (with or without capitalization). Or
> ('easting','northing'). Or, or, or... Trying to carefully control all that
> is more trouble than it's worth; I just use, for example,
> coordinates(obj)[,1] and coordinates(obj)[,2] if I want to pull them out
> as vectors. Ugly, but I can count on it.
> 
> That said, if
>   as.data.frame(locs)
> produces different names for the coordinates when used in different
> contexts, then you've got something else going on that should not be going
> on. This is where Frede's suggestions might help. You will need to
> carefully track the construction of your locs object and see if it is
> somehow different in the two situations. I don't know of any "designed
> circumstance" that would explain this.
> 
> -Don
> 

-- 
Edzer Pebesma
Institute for Geoinformatics (ifgi),  University of Münster,
Heisenbergstraße 2, 48149 Münster, Germany; +49 251 83 33081
Journal of Statistical Software:   http://www.jstatsoft.org/
Computers & Geosciences:   http://elsevier.com/locate/cageo/
Spatial Statistics Society http://www.spatialstatistics.info

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 490 bytes
Desc: OpenPGP digital signature
URL: <https://stat.ethz.ch/pipermail/r-sig-geo/attachments/20150320/4b2333c1/attachment.bin>