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@polygons, function(x){x@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 [[alternative HTML version deleted]] _______________________________________________ R-sig-Geo mailing list [email protected] https://stat.ethz.ch/mailman/listinfo/r-sig-geo
