Dear R-listers, I have a data set which combine population (site name), longitude, latitude, altitude, initial (for flowering of a plant), and end (also for flowering of the same plant) of each site. Now, I want to make contour plot to display spatial pattern of the initial or end of the flowering especially, I still can not plot that, although I have gotten some helps from this list. I need to know how can I process my data so as to meet the ascending order of X and Y. And, I need to know if if contour can handle Date variable. Please give me some directions on that. Thank you in advance. data and codes used are as followed: 1. data Population Lon Lat Alt Initial End ZDPd01 98.8763 28.4419 3449 2009-4-15 2009-4-25 GSPy01 98.63446667 28.01211667 1736 2009-3-15 2009-3-28 KDPd01 101.9054667 30.18946667 2914 2009-4-18 2009-4-29 MKPd01 102.2027833 31.90801667 2526 2009-4-22 2009-5-1 LXPd01 102.8014833 31.66243333 2746 2009-4-9 2009-4-21 LXPd02 103.0387 31.40156667 2158 2009-4-7 2009-4-19 BXPd01 102.75395 30.71616667 2180 2009-4-5 2009-4-14 XJPd01 102.5218 31.02406667 2467 2009-4-2 2009-4-9 BJPt01 115.95 40.45 80 2009-4-19 2009-4-26 KMPd01 102.6225761 24.96880965 1900 2009-3-22 2009-4-11 NYPd06 94.08488239 29.73718216 3379.591549 2009-5-1 2009-5-7 PLPd02 95.49816933 29.90361244 2650.12 2009-4-26 2009-5-5 CYPd01 97.01336034 28.50232574 1631.607595 2009-4-15 2009-4-23 CYPd02 97.47131095 28.67033436 2431.802469 2009-4-21 2009-5-2 CYPd03 97.22515032 29.13309676 3263.019417 2009-4-25 2009-5-3 NCPt01 118.97 42.28 1300 2009-5-25 2009-6-1 QLPd01 103.6905774 37.44786532 2403.370968 2009-5-15 2009-5-23 LKSPt01 112.0359546 36.61293704 1664.597938 2009-4-21 2009-5-11 2. code phen<-read.delim("2009phenology.txt", header=T) str(phen) phen.1<-phen[,1:7] phen.1[,6]<-as.Date(phen.1[,6]) phen.1[,7]<-as.Date(phen.1[,7]) str(phen.1) phen.2<-phen.1[order(phen.1[,3], phen.1[,4]),] fix(phen.2) contour(phen.2$Lon, phen.2$Lat, phen.2$Initial) # failed contour(phen.2$Lon, phen.2$Lat, phen.2$Alt) # failed Regards, Mao J-F