Support other types of summaries

[krlmlr]

Are union and combine the only types of summary ever required? How about e.g. pairwise intersections, stored in a multipolygon? Or combinations of several single polygons into a multipolygon? Perhaps the do_union argument to summarise.sf() should also accept a summary function? Alternatively, how about permitting custom summaries: if the result of summarise.data.frame() already contains an sfc, don't summarize the geometry column.

Related to this, I noticed that st_as_sfc.list() doesn't work for lists of "sfc" objects, which might occur when doing a manual summary. (It returns NULL invisibly, I believe this is a mistake.) I haven't found a straightforward way to convert a list of "sfc" to a single "sfc" other than . %>% purrr::map(1) %>% st_as_sfc(); this loses the geometry metadata, e.g. the coordinate system.

[edzer]

We union or combine because that gives the geometry for which summarise acted over corresponding attributes (non-geometry feature properties). Could you give a concrete use case for which one would want something else?

I committed a change that accepts lists of sfg objects (geometries), like an sfc that lost its class. You can combine a list with sfc objects with do.call(c, lst):

> do.call(c, list(st_sfc(st_point(0:1), st_point(1:2), crs=4326), st_sfc(st_point(4:5))))
Geometry set for 3 features 
geometry type:  POINT
dimension:      XY
bbox:           xmin: 0 ymin: 1 xmax: 4 ymax: 5
epsg (SRID):    4326
proj4string:    +proj=longlat +datum=WGS84 +no_defs
POINT (0 1)
POINT (1 2)
POINT (4 5)

[krlmlr]

Perhaps area covered by more than one geometry might be useful when attempting to repair slivers. Or convex hull/bounding box, without materializing the union first? I agree that union/combine are by far the most useful operations, but maybe not the only ones?

[edzer]

As an alternative to do_union = TRUE we could use an argument that takes a function, like union = st_union that returns a geometry, both to sf::summarise.sf and sf::aggregate.sf.

[krlmlr]

What if we looked at the results of the summary and held off doing union/combine if the result already contains a geometry?

[krlmlr]

Explicitly creating a geometry column in summarise() would mean we don't do anything ourselves.

[edzer]

Who is "we"? sf:::summarise.sf calls NextMethod(), which is a call to the dplyr method, and that returns an object without geometry afaics.

[krlmlr]

I'm thinking about supporting the case where the user manually creates a geometry column. That would be returned by dplyr:::summarise.tbl_df() and processed later by sf.

[edzer]

Ah, yes, that makes sense. Could you provide an example how this can be done in dplyr::summarise?

[krlmlr]

Not sure if this is what you're asking about, but I would expect the output of the two examples to be identical. I think it's more about that summarise.sf() detects if the result of NextMethod() already has a geometry column.

library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union
library(sf)
#> Linking to GEOS 3.5.1, GDAL 2.2.1, proj.4 4.9.3

df <- tibble(a = 1:3, pt = st_sfc(st_point(1:2)))
df %>%
  summarize(a = mean(a), ptx = st_union(pt)) %>%
  st_sf()
#> Simple feature collection with 1 feature and 1 field
#> geometry type:  POINT
#> dimension:      XY
#> bbox:           xmin: 1 ymin: 2 xmax: 1 ymax: 2
#> epsg (SRID):    NA
#> proj4string:    NA
#> # A tibble: 1 x 2
#>       a     ptx
#>   <dbl> <POINT>
#> 1     2   (1 2)
df %>%
  st_sf() %>%
  summarize(a = mean(a), ptx = st_union(pt))
#> Simple feature collection with 1 feature and 1 field
#> Active geometry column: pt
#> geometry type:  POINT
#> dimension:      XY
#> bbox:           xmin: 1 ymin: 2 xmax: 1 ymax: 2
#> epsg (SRID):    NA
#> proj4string:    NA
#> # A tibble: 1 x 3
#>       a     ptx      pt
#>   <dbl> <POINT> <POINT>
#> 1     2   (1 2)   (1 2)

Created on 2018-05-07 by the reprex package (v0.2.0).

[edzer]

Like that?

[krlmlr]

Looks good, thanks for supporting arbitrary aggregations!