A bit over a year ago I got useful advice from Gabor Grothendieck and
Duncan Temple Lang in this thread:
http://finzi.psych.upenn.edu/R/Rhelp02/archive/117140.html
If the coordinates are nested deeply, then it probably safer to search
for a specific tag or tags that are just above them . You probably
want to search for the "LinearRing" tag and then store the coordinates
along with its "id". Perhaps some of my mistakes can be avoided as you
work on your methods.
--
David Winsemius
On Mar 1, 2009, at 10:42 PM, Don MacQueen wrote:
I have an XML file that has within it the coordinates of some
polygons that I would like to extract and use in R. The polygons are
nested rather deeply. For example, I found by trial and error that I
can extract the coordinates of one of them using functions from the
XML package:
doc <- xmlInternalTreeParse('doc.kml')
docroot <- xmlRoot(doc)
pgon <- xmlValue(docroot[[52]][[3]][[7]][[3]][[3]][[1]][[1]])
but this is hardly general!
I'm hoping there is some relatively straightforward way to use
functions from the XML package to recursively descend the structure
and return the text strings representing the polygons into, say, a
list with as many elements as there are polygons. I've been looking
at several XML documentation files downloaded from http://www.omegahat.org/RSXML/
, but since my understanding of XML is weak at best, I'm having
trouble. I can deal with converting the text strings to an R object
suitable for plotting etc.
Here's a look at the structure of this file
graphics[5]% grep Polygon doc.kml
<Polygon id="15342">
</Polygon>
<Polygon id="1073">
</Polygon>
<Polygon id="16508">
</Polygon>
<Polygon id="18665">
</Polygon>
<Polygon id="32903">
</Polygon>
<Polygon id="5232">
</Polygon>
And each of the <Polygon> </Polygon> pairs has <coordinates> as per
this example:
<Polygon id="15342">
<outerBoundaryIs>
<LinearRing id="11467">
<coordinates>
-23.679835352296,30.263840290388,5.000000000000001
-23.68138782285701,30.264740875186,5.000000000000001
[snip]
-23.679835352296,30.263840290388,5.000000000000001
-23.679835352296,30.263840290388,5.000000000000001 </coordinates>
</LinearRing>
</outerBoundaryIs>
</Polygon>
Thanks!
-Don
p.s.
There is a lot of other stuff in this file, i.e, some points, and
attributes of the points such as color, as well as a legend
describing what the polygons mean, but I can get by without all that
stuff, at least for now.
Note also that readOGR() would in principle work, but the underlying
OGR libraries have some limitations that this file exceeds. Per info
at http://www.gdal.org/ogr/drv_kml.html.
--
---------------------------------
Don MacQueen
Lawrence Livermore National Laboratory
Livermore, CA, USA
925-423-1062
m...@llnl.gov
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