Parse .geojson file in R

Question

I am new to R and geospatial data analysis. I would like to extract the coordinates from a .geojson file in R. For example, a line from the .geojson file is as follows:

{ "type": "Feature", "properties": { "id": 36084.000000, "osm_id": 471120302.000000, "type": "tertiary", "name": null, "tunnel": 0, "bridge": 0, "oneway": 0, "ref": null, "z_order": 4.000000, "access": null, "service": null, "class": "highway" }, "geometry": { "type": "LineString", "coordinates": [ [ 117.152985627930335, 39.199392977596688 ], [ 117.154272752981669, 39.197016121313759 ], [ 117.156671150755756, 39.193602255970234 ] ] } }

I used 'readOGR' to read the file.

data <- readOGR(dsn = "test.geojson", layer = "OGRGeoJSON")

I could extract features like osm_id using data[['osm_id']], but that does not work with coordinates. How to extract the coordinates? Can anyone give me a hint how to proceed?

Answer 1

You could try the geojsonio package, which i've used for a few tasks involving sp/sf conversions.

# read in the geojson data and specify that you want to parse it
data <- geojson_read("C:\\filepath\\test.geojson", method = "local", parse = TRUE)

It has a strange old listed structure after parsing, here's my dummy polygon:

$type
[1] "FeatureCollection"

$crs
$crs$type
[1] "name"

$crs$properties
$crs$properties$name
[1] "urn:ogc:def:crs:OGC:1.3:CRS84"


$features
     type id dummy geometry.type
1 Feature  1     0       Polygon

geometry.coordinates
1 16.4844, 22.5928, 24.7461, 17.4951, 16.4844, 59.7363, 61.1426, 55.0342, 55.1221, 59.7363

you can see the area of interest; list item named "features".

data$features is actually a data.frame, but the 4th column (so to speak) is another data.frame consisting of a character and a list. The list, our coordinates, is in fact an array. It's all rather nested.

# this accesses the geometry
data$features$geometry
 type
1 Polygon

coordinates
1 16.4844, 22.5928, 24.7461, 17.4951, 16.4844, 59.7363, 61.1426, 55.0342, 55.1221, 59.7363

# and this accesses the list in which the array in which the coordinates are stored
data$features$geometry$coordinates[[1]]

, , 1

        [,1]    [,2]    [,3]    [,4]    [,5]
[1,] 16.4844 22.5928 24.7461 17.4951 16.4844

, , 2

        [,1]    [,2]    [,3]    [,4]    [,5]
[1,] 59.7363 61.1426 55.0342 55.1221 59.7363

So there we go. To access your coords, you can either call the entire array as a matrix, the individual coord pairs or all of the x/y coordinates:

# all coordinate pairs as matrix
data$features$geometry$coordinates[[1]][1,,]

# individual coordinate pairs (i've put 1:5 here illustratively, which is the same as above)
data$features$geometry$coordinates[[1]][,1:5,]

# all x or y coordinates (dont use word or!)
data$features$geometry$coordinates[[1]][,,1 or 2]

Answer 2

Converting the data to a fortified tidy dataframe does the trick. See below:

mydf_fortified <- tidy(mydf) #mydg is a geojson file

The ids in the mydf_fortified dataframe will correspond to the relevant list items, you can then join to a new data frame if needed.