I am trying to figure out how I can transform my unit of length from decimal degree to km when I import the following simple ArcGIS polygon shape. Ultimately, I would like to have this information available when I assign this spatial polygon to my window in spatstat [i.e., Window area = xx square unit & Unit of length: xx km].
I did came across this post: R-sig-geo
This is what I've done so far, but I don't understand why/if I need to do so. Doesn't my div0A SpatialPolygonsDataFrame already has my CRS argument defined? Why do I need to spTransform then?
library(spatstat) library(maptools) library(lattice) library(sp) library(foreign) require("rgdal") div0A <- readOGR(dsn="Archive", layer="Projections") > summary(div0A) Object of class SpatialPolygonsDataFrame Coordinates: min max x -80.00 -57.63430 y 66.25 78.16666 Is projected: FALSE proj4string : [+proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0] Data attributes: Id NPAzimutha UTM20 UTM19 AlberEA Min. :0 Min. :348877 Min. :349232 Min. :349162 Min. :348656 1st Qu.:0 1st Qu.:348877 1st Qu.:349232 1st Qu.:349162 1st Qu.:348656 Median :0 Median :348877 Median :349232 Median :349162 Median :348656 Mean :0 Mean :348877 Mean :349232 Mean :349162 Mean :348656 3rd Qu.:0 3rd Qu.:348877 3rd Qu.:349232 3rd Qu.:349162 3rd Qu.:348656 Max. :0 Max. :348877 Max. :349232 Max. :349162 Max. :348656 proj4string(div0A) <- CRS("+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs") xxlcc <- spTransform(div0A, CRS("+init=epsg:4326")) res1 <- lapply(slot(xxlcc, "polygons"), function(x) sapply(slot(x, "Polygons"), slot, "area")) > summary(unlist(res1)) Min. 1st Qu. Median Mean 3rd Qu. Max. 89.72 89.72 89.72 89.72 89.72 89.72 newW<-as(xxlcc, "owin") > summary(newW) Window: polygonal boundary single connected closed polygon with 29 vertices enclosing rectangle: [-80, -57.6343]x[66.25, 78.16666]units Window area = 89.7204 square units
Does this look correct? How can I specify/know my unit of length in km? Moreover, this window is ~Baffin Bay, west of Greenland and I am wondering if I should be using a different datum? E.g. EPSG Projection 4747 - GR96? Although, I will have to create ppp object, for which the coordinates have been taken with GPS satellite navigation system.
I would appreciate if someone can walk me through this or suggest an online example or further readings. I read chapter 4: Spatial Data Import and Export in Applied Spatial Data Analysis with R (2008), but I'm obviously very new to all of this.