Determine what geoJSON polygon a point is in

Question

I have two data frames, one that contains lat / long points and another that contains geojson data to draw multiple polygons.

The first dataframe (countyDF) is imported from a CSV and the 2nd dataframe (basinData) is imported from geoJSON using readLines() (should I be using getJSON() instead for the json data?

i.e. (Over simplified lat and long, let me know if a more realistic example would help) countyDF

pointNum Lat Long
1        100 251    
2        150 175
3        50  -330
4       -150 100

and geoJSON formatted like this(basinData):

{
"type": "FeatureCollection",
"crs": { "type": "name", "properties": { "name": "urn:ogc:def:crs:OGC:1.3:CRS84" } },

"features": [
{ "type": "Feature", "properties": { "Basin_ID": "9-19", "Basin_Subb": "9-19", "Basin_Name": "TIA JUANA", "Subbasin_N": null }, "geometry": { "type": "MultiPolygon", "coordinates": [ [ [ [ -117.0679768595155, 32.574291366336219 ], [ -117.06716593683379, 32.573400317874729 ], [ -117.06341997467541, 32.569091397019029 ], [ -117.06194315333148, 32.566356289257612 ], [ -117.0590009947062, 32.565022113434004 ], [ -117.05426277324793, 32.561991889800737 ], [ -117.04796901017907, 32.559156827796407 ], [ -117.03949416454982, 32.555722526353854 ], [ -117.03658670964059, 32.55260623337746 ], [ -117.03547505795123, 32.551186682532702 ], [ -117.03106791328587, 32.547633284977714 ], [ -117.02453341519558, 32.542766715943806 ], [ -117.03096733611001, 32.542203418941845 ], [ -117.0324558562139, 32.542073586201752 ], [ -117.03444148063983, 32.541897693349327 ], [ -117.03508530679723, 32.541840746762325 ], [ -117.03893675722108, 32.541503138385245 ], [ -117.04563848478635, 32.540916090254136 ], [ 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{ "type": "Feature", "properties": { "Basin_ID": "9-18", "Basin_Subb": "9-18", "Basin_Name": "OTAY VALLEY", "Subbasin_N": null }, "geometry": { "type": "MultiPolygon", "coordinates": [ [ [ [ -117.09094993890868, 32.619563585315568 ], [ -117.09024988989258, 32.616130450534968 ], [ -117.08912052342582, 32.606956836308925 ], [ -117.08936013555311, 32.601840287378998 ], [ -117.08875364427725, 32.598575478052986 ], [ -117.08680278644947, 32.595962898606246 ], [ -117.08441502669503, 32.595310423365085 ], [ -117.08156822333883, 32.595469231360795 ], [ -117.07566674287644, 32.595559902113109 ], [ -117.06972154230749, 32.596914588847568 ], [ -117.06432122834885, 32.598375496484095 ], [ -117.05964154380946, 32.598503627441907 ], [ -117.05616095566411, 32.597465378923445 ], [ -117.05457753753359, 32.596282762027229 ], [ -117.05073717232032, 32.594157791168556 ], [ -117.04589484061222, 32.592852213251177 ], [ -117.04312260910274, 32.593353146236865 ], [ -117.03899112955028, 32.593759832413419 ], [ 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}

I want to test point numbers 1,2,3,4 against all "Basin_ID"'s and if they are within that basin, add that as a column to countyDF.

For example if point 1 were in basin 9-18 and none of the other points fell within polygons contained in basinData the returned data frame would look like the following...

returnedDF:

pointNum Lat Long   Basin
1        100 251    9-18
2        150 175     n/a
3        50  -330    n/a
4       -150 100     n/a

Might anybody suggest a specific library / method / solution of accomplishing this? I imagine if there's a library that tests if a point is in a polygod I can loop over the 2nd dataframe for each point?

Answer 1

Here is one way you could do it, using rgdal to read the geojson (see this answer for more details on this) and sp and/or rgeos to test if a point lies within a polygon or not.

Note, I adjusted your coordinates, since none of them was located within a polygon.

First, read the data:

countyDF <- read.table(textConnection("
pointNum Lat Long
1        32.6 -117.1    
2        90 175
4       -90 100"), header = TRUE)


basinDF <- rgdal::readOGR("basin.json", "OGRGeoJSON")

Make sure points and polygons have the same projection:

sp::coordinates(countyDF) <- ~Long+Lat
sp::proj4string(countyDF) <- sp::proj4string(basinDF)

Here we use sp::over to extract the attributes of basinDF at each point. If points are not located within a polygon of basinDF NA is returned.

sp::over(countyDF, basinDF)

# Basin_ID Basin_Subb  Basin_Name Subbasin_N
#  1     9-18       9-18 OTAY VALLEY       <NA>
#  2     <NA>       <NA>        <NA>       <NA>
#  3     <NA>       <NA>        <NA>       <NA>

Alternatively, you could also use rgeos, which tells you that point 1 is located in poygon 1.

rgeos::gWithin(countyDF, basinDF, byid = TRUE)

#   1     2     3
# 0 FALSE FALSE FALSE
# 1  TRUE FALSE FALSE