[R-sig-Geo] Differences in area between R sp and ArcGIS

[Andrew Plowright]

Hello,

As my first email to this list, I just wanted to start out by thanking
everyone who has been involved in developing this excellent package, and to
everyone who has answered questions on this mailing list. I have been
consulting this list's archives for years, and the answers that have been
given have been a huge help.

My question is as follows:

I was just wondering if there was a known difference in between the way the
R sp's package computes area and the way ArcGIS does. Below is a link to a
subset of a dataset I am working on. Its attribute table contains two
columns: "Area_Arc" and "Area_R".

https://drive.google.com/folderview?id=0Bw1lKQwaoU84fmRwald5d0JqM1BEemFQX2FmVEFtdHk2RlltTzZFLU1EQ01CcURZcDdteUU&usp=sharing

"Area_R" was computed using the following lines in R:

> polySample <- readOGR("C:\\Users\\Andy\\Documents", "polySample")
> polySample[["Area_R"]] <- sapply(polyfile at polygons, function(x){x at area})

"Area_Arc" was computed using ArcMap's Calculate Geometry tool, using the
coordinate system associated with the file.

I tried reprojecting this file and repeating the process, but each time,
there is a discrepancy between the areas. Specifically, the area calculated
by ArcMap is always in the same ballpark but smaller.

> proj4string(polySample)
[1] "+proj=aea +lat_1=50 +lat_2=58.5 +lat_0=45 +lon_0=-126 +x_0=1000000
+y_0=0 +datum=NAD83 +units=m +no_defs +ellps=GRS80 +towgs84=0,0,0"
> sum(polySample[["Area_R"]])
[1] 15332220331
> sum(polySample[["Area_Arc"]])
[1] 12330204266

My understanding is that both of these methods use the same underlying
mathematics to compute area based on the projection system assigned to the
file, so I can't figure out why they would produce different results. I
apologize if this question has been addressed before, but I was unable to
come up with any answers from the R sp help file or on the mailing list's
archives.

Thank you very much
Andrew

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