# Calculating distance (km) between points by sea

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## Calculating distance (km) between points by sea

 Hi, I'm trying to calculate the shortest distances by sea between points in the Meditteranean sea.   I've been trying to do this using a costDistance approach in the gdistance package.    (See code below). I have imported maps from ArcGIS in which sea has a value of 1 and land 10,000 there are also NA values at edges of the ascii matrix.  I have set the transition values to resistance so that points should choose travel by sea.  The projection is WCS 1984 (lat long) and the cell size (after aggregate command) is 0.1 Hopefully the steps along the way are correct, but the output of the costDistance command is confusing to me.  The actually distances in km should be in the region of from <10 to 100km.  Could anybody help me to find a way of converting this?  I have tried a lot of variations on the code and hope it is correct.  I would appreciate someone looking over it for me and letting me know if it's right. Many thanks Ruth ################### code library(maptools) y <- readAsciiGrid("C:\\R\\Marine_algae\\westmed1.asc", proj4string = CRS("+proj=longlat + ellps=WGS84")) ############# vector of values from ascii grid ########## v1 <- y@data\$westmed1.asc ########## create raster ####### library(raster) med <- raster(y) setValues(med, v1) med2 <- aggregate(med, fact=10, fun=min) ############### g distance ########## library(gdistance) tr2 <- transition(med2, transitionFunction="mean", directions = 8) tr2 <- geoCorrection(tr2, type = "c") matrixValues(tr2) <- "resistance" ########## test points ############## p1 <- read.table("C:\\R\\Marine_algae\\test_points_med.csv", header = T, sep =",") p1 <- unique(p1) p1 <- as.matrix(cbind(p1\$x, p1\$y)) cost1 <- costDistance(transition = tr2, fromCoords = p1, toCoords = p1)  cost1          [,1]      [,2]      [,3]      [,4]      [,5]     [,6]     [,7] [1,] 0.000000 2.7774957 2.7774957 2.7774957 2.0402271 2.306833 3.386609 [2,] 2.777496 0.0000000 0.0000000 0.0000000 0.7372686 2.479186 3.576727 [3,] 2.777496 0.0000000 0.0000000 0.0000000 0.7372686 2.479186 3.576727 [4,] 2.777496 0.0000000 0.0000000 0.0000000 0.7372686 2.479186 3.576727 [5,] 2.040227 0.7372686 0.7372686 0.7372686 0.0000000 1.741917 2.839459 [6,] 2.306833 2.4791858 2.4791858 2.4791858 1.7419172 0.000000 3.106073 [7,] 3.386609 3.5767274 3.5767274 3.5767274 2.8394588 3.106073 0.000000 ########## ??? conversion to km???         [[alternative HTML version deleted]] _______________________________________________ R-sig-Geo mailing list [hidden email] https://stat.ethz.ch/mailman/listinfo/r-sig-geo