[R-sig-Geo] Spatial joins between sf and stars objects

[Edzer Pebesma]

On 03/02/2021 20:14, Julian M. Burgos wrote:
> Although if I use bind_cols the result is not an sf object....

right, this is because bind_cols is not a generic (its first argument is 
... , which you can't dispatch on) so doesn't have an associated sf method.

dplyr offers a mechanism to work around this by providing an sf method 
for dplyr_reconstruct - something we could try, but then the dplyr 
authors mention that this is experimental and considered a stop-gap 
measure, so that might work now but stop working any time.

However,

sf3 <- cbind(st_extract(st1, sf1), st_drop_geometry(sf1))

does return an sf object (note the changed order of arguments)

> 
> Julian M. Burgos writes:
> 
>> Thanks Edzer, I should have checked if st_extract kept the order of the rows. In this case, a way to do it more generally is:
>>
>> sf1 <- bind_cols(st_drop_geometry(sf1),
>>                   st_extract(st1, sf1))
>>
>>
>>
>> Edzer Pebesma writes:
>>
>>> st_join does a spatial join, which comes at a cost; since st_extract
>>> doesn't change the row order of the sf object, you could as well use
>>>
>>> st_extract(st1, sf1) %>% mutate(nums = sf1$nums)
>>>
>>> On 03/02/2021 17:10, Julian M. Burgos wrote:
>>>> Dear list,
>>>>
>>>> What would be the best way to do a spatial join between a sf object (of POINT geometry) and a star object, so the sf object gets the values of the corresponding pixels in the star object?  I have done using a combination of st_join and st_extract, but it feels a bit clunky.  Is there a better way?
>>>>
>>>> Here is what I have done:
>>>>
>>>> ## --------------------------------------------------------------
>>>> ## Load a stars object
>>>> st1 <- read_stars(system.file("tif/L7_ETMs.tif", package = "stars")) %>%
>>>>     slice(band, 1)
>>>>
>>>> ## Create an sf object
>>>>
>>>> set.seed(100)
>>>> bb <- st_bbox(st1)
>>>>
>>>> sf1 <- tibble(lon = runif(n = 10, min = bb[1], max = bb[3]),
>>>>                 lat = runif(n = 10, min = bb[2], max = bb[4]),
>>>>                 nums = 1:10) %>%
>>>>     st_as_sf(coords = c("lon", "lat"), crs = st_crs(st1))
>>>>
>>>> ## Do the spatial join
>>>>
>>>> sf1 <- st_extract(st1, sf1) %>%
>>>>       st_join(sf1)
>>>> ## --------------------------------------------------------------
>>>>
>>>> Thanks,
>>>>
>>>> Julian
>>>>
>>>> --
>>>> Julian Mariano Burgos, PhD
>>>> Hafrannsóknastofnun, rannsókna- og ráðgjafarstofnun hafs og vatna/
>>>> Marine and Freshwater Research Institute
>>>> Botnsjávarsviðs / Demersal Division
>>>>     Fornubúðir 5, IS-220 Hafnarfjörður, Iceland
>>>> http://www.hafogvatn.is/
>>>> Sími/Telephone : +354-5752037
>>>> Netfang/Email: julian.burgos using hafogvatn.is
>>>>
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> 
> 
> --
> Julian Mariano Burgos, PhD
> Hafrannsóknastofnun, rannsókna- og ráðgjafarstofnun hafs og vatna/
> Marine and Freshwater Research Institute
> Botnsjávarsviðs / Demersal Division
>    Fornubúðir 5, IS-220 Hafnarfjörður, Iceland
> www.hafogvatn.is
> Sími/Telephone : +354-5752037
> Netfang/Email: julian.burgos using hafogvatn.is
>