[R-sig-Geo] Transform decimal degrees units in km - ReadOGR & owin{spatstat}

[Aurelie C.Godin]

Thanks for the hint Tom,

I did came across this post previously: 
http://r-sig-geo.2731867.n2.nabble.com/readOGR-default-unit-td7337370.html
http://r-sig-geo.2731867.n2.nabble.com/readOGR-default-unit-td7337370.html 

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?

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 

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.

Thank you very much in advance,
Aurelie


-----
Aurelie Cosandey-Godin
Ph.D. Student, Department of Biology, Dalhousie University
Industrial Graduate Fellow, WWF-Canada
Email: godina at dal.ca | Web: wormlab.biology.dal.ca

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