# Having problems calculating polygon area from coordinates

Hi.

I'm trying to calculate area of polygons in km^2, but I dont know how to interpret the values or how to translate the values to km^2. I've been using the polyarea function: polyarea function - RDocumentation

As you can see in my data below, the polygon area has been calculated to 163. What unit is it? It certainly isn't km^2.

Any ideas?

Here's a small subset of my data:

data.frame(
stringsAsFactors = FALSE,
Currentid = c("Acraea issoria",
"Acraea issoria","Acraea issoria","Acraea issoria","Acraea issoria",
"Acraea issoria","Acraea issoria","Acraea issoria",
"Acraea issoria","Acraea issoria","Acraea issoria",
"Acraea issoria"),
Lat = c(30.0033,30.0033,30.0033,
30.5226,30.5226,30.5226,22.4191,22.4191,22.4191,22.4191,
22.4191,22.4191),
Lon = c(119.8,119.8,119.8,77.8575,
77.8575,77.8575,103.852,103.852,103.852,103.852,
103.852,103.852),
polyarea = c(163.1910525,163.1910525,
163.1910525,163.1910525,163.1910525,163.1910525,
163.1910525,163.1910525,163.1910525,163.1910525,163.1910525,
163.1910525)
)

Thank you!!!

The polyarea() function does not calculate area on a sphere or ellipse. It is intended for coordinates in the plane. Try using the sf library and the st_area() function. But, you will need to work out how to convert your coordinates (which I presume are in latitude and longitude) into an sf object.

I see, thank you for your comment.

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