Using st_join for a spatial join using largest intersection

Question

I am using the sf_package to work around spatial data in r. At this stage, I want to make a spatial join so that the tax lots of my area of study inherit the attributes of the floodplain on which they are located. For example, taxlots may be located in a floodplain categorized as X, VE, A, A0, or V (these are codes that relate to the intensity of the flood in each area).

To do this, I tested the sf function st_join, which will by default rely on st_intersects to determine the spatial join for each entity of my tax lots.

However, I am trying to figure out the criteria used by the function when a tax lot intersects with two different floodplain areas (as in the photo below, in which several lots intersect both with an A floodplain and an AE floodplain). Does it take the value of the area that covers the largest area of the lot? or is it a matter of which area is located upper in the dataframe?

Note that I am not interested in partitioning the intersecting lots so that I divide them according to their areas intersecting one and other floodplain zones.

Photo of tax lots intesecting with more than one floodplain category

Answer 1

By default, st_join(x, y, join = st_intersects) duplicates all features in x, that intersect with more than one features from y.

If you set the argument largest = TRUE, st_join() returns the x features augmented with the fields of y that have the largest overlap with each of the features of x.

See https://r-spatial.github.io/sf/reference/st_join.html and https://github.com/r-spatial/sf/issues/578 for more details.