Writing Space-Time Point Data To KML

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1 Writing Space-Time Point Data To KML Barry Rowlingson December 3, Introduction KML is a file format for vector map data such as points, lines and polygons. It has come to wide use since the rise of Google Earth as a map data viewer, and is now an OGC standard file format. This makes it easy for other software to to read and write KML files. However, KML is a very rich format and can do a lot of interesting things with the data. This makes it hard for software to implement the export functionality to create KML files that do exactly what you want. Fortunately KML is an XML format, and so it is easy to create from any programming language that can do a bit of text manipulation. In this report I show how to create a KML file of space-time points from R that can be imported into Google Earth and used in an animation. 2 KML File Format Definition The OGC standard documents are available from org/standards/kml, and Google s slightly friendlier documentation is online at A simple KML document for a single point with a begin and end date looks like this: <?xml version="1.0" encoding="utf-8"?> <kml xmlns=" <Document> <Folder> <name>simple Dataset</name> <open>1</open> <description> Here are some points </description> <Placemark> <Name>point-1</Name> <Point> 1

2 <coordinates> , ,0</coordinates> </Point> <TimeSpan> <begin> </begin> <end> </end> </TimeSpan> </Placemark> </Folder></Document> </kml> This generates KML with one point at those lat-long coordinates with the given begin and end date. What is needed is a function that takes a number of points and creates a file with all the points in <Placemark> sections. 3 Export Function Walkthrough The function will take three compulsory arguments giving the coordinates and beginning time points. It will have defaults for everything else which the user can override: stkml <- function(x,y,start,len=1, names=paste("point-",1:npts,sep=""), title="r Output",description="Some points from R", template, output=stdout()){ The x and y arguments are vectors of longitude and latitude coordinates as decimal numbers. The point events start at the time in the start argument, which should be a vector of R Date objects. The duration of the events is given by len, which defaults to one day, but can be a vector if all the events have different durations. To generate the KML it could use R s cat and paste functions to string everything together, but this makes for very messy code. I have used the brew package which enables you to write a clear template and embed loops and variable substitutions: require(brew) Next we get the number of points (further error checking here could test the lengths of the other arguments) and compute the end times of the events. The starts and ends are then formatted as dates in the right format for KML files: npts = length(x) end = start + as.difftime(len,units="days") start = format(start,"%y-%m-%d") end = format(end,"%y-%m-%d") 2

3 To enable some flexibility the function includes a simple template but lets the user specify an external file for more customisation: if(missing(template)){ template = '<?xml version="1.0" encoding="utf-8"?> <kml xmlns=" <Document> <Folder> <name><%=title%></name> <open>1</open> <description> <%=description%> </description> <% for(i in 1:npts){ %> <Placemark> <name><%=names[i]%></name> <Point> <coordinates><%=x[i]%>,<%=y[i]%>,0</coordinates> </Point> <TimeSpan><begin><%=start[i]%></begin><end><%=end[i]%></end></TimeSpan> </Placemark> <% } %> </Folder></Document> </kml> ' brew(text = template, output=output) }else{ brew(file = template, output=output) } } 4 Usage A simple test function was written to illustrate the function. This generates points within a box on a random day in June, Two KML files are written, one to test the fixed-length event code, and one to test the variable-length event code: tests <- function(n=50){ xyt <- function(n){ data.frame(x=runif(n, , ), y=runif(n, , ), t=as.date( paste(2008,7,sample(30,n,replace=true),sep="/"), format = "%Y/%m/%d")) } 3

4 } a = xyt(n);b=xyt(n) stkml(a$x,a$y,a$t,output="test1.kml", title="rescues", description="locations of Rescues in June" ) dur = sample(10,n,replace=true) stkml(b$x,b$y,b$t,dur,output="test2.kml", title="fires", description="fires occurred here") The two KML files created can be loaded into Quantum GIS and styled to produce a pretty map: They can also be loaded into Google Earth and viewed using the animation slider: 4

5 By right-clicking on the layer it is possible to access the style of the points and change the icon to get something like this: To do further styling, it will be necessary to write a brew template file similar to the one embedded in the function and to specify that as an argument. For example, to produce KML with points styled with the fire icon, create styledfire.brew in a text editor like this: 5

6 <?xml version="1.0" encoding="utf-8"?> <kml xmlns=" <Document> <Style id="fire"> <IconStyle> <Icon> <href> </Icon> </IconStyle> </Style> <Folder> <name><%=title%></name> <open>1</open> <description> <%=description%> </description> <% for(i in 1:npts){ %> <Placemark> <styleurl>#fire</styleurl> <Name><%=names[i]%></Name> <Point> <coordinates><%=x[i]%>,<%=y[i]%>,0</coordinates> </Point> <TimeSpan><begin><%=start[i]%></begin><end><%=end[i]%></end></TimeSpan> </Placemark> <% } %> </Folder></Document> </kml> And then, given the data in x, y, t and dur, run stkml like this: > stkml(x,y,t,dur,output="fires.kml",title="fires", description="fires occurred here", template="styledfire.brew") For more customisation, check the KML documentation or find examples of KML files that do what you want to do and read the source. 6