reading KML files in R

Excuses for the quite adjacent question, does anybody know if kml files require
using the google spherical projection?
My understanding is that, in theory, you can use any projection, but,
in fact, you must use
google spherical to get a correct display in google maps

Agus

2010/5/28 Wade Wall <wade.wall at gmail.com>:

Thanks Dr. Bivand,

I was able to import my kml file using your example.  I opened up the
kml file in a text editor to find out the layer name, but
system(paste("ogrinfo", paste(td, "cities.kml", sep="/")),
intern=TRUE) also works to find the name.

As for modifying ogrinfo(), I will take a look and see if it is above
my skill level or not.

Wade

On Fri, May 28, 2010 at 3:29 AM, Roger Bivand <Roger.Bivand at nhh.no> wrote:

On Thu, 27 May 2010, Wade Wall wrote:

I am not exactly sure what you mean by identify the correct layer
name.  Using R under Linux, this is what happens.

I have a file in the current directory named ASMI_GA.kml

library(rgdal)
tmp<-readOGR(".", layer="ASMI_GA.kml")

However, I get the following message.

Error in ogrInfo(dsn = dsn, layer = layer, input_field_name_encoding =
input_field_name_encoding) :
 Cannot open file

That is what I said. Each driver has its own understanding of what the dsn=
and layer= arguments are, and your usage doesn't match what the driver is
expecting. The dsn= for the KML driver seems to be the file name (possibly
with its complete path), and the layer= is the string in the:

<Document><Folder><name>cities</name>

line (conditional on the writing software using a similar logic to OGR),
here in this example:

library(rgdal)
cities <- readOGR(system.file("vectors", package = "rgdal")[1], "cities")
is.na(cities$POPULATION) <- cities$POPULATION == -99
summary(cities$POPULATION)
td <- tempdir()
writeOGR(cities, paste(td, "cities.kml", sep="/"), "cities", driver="KML")
xx <- readOGR(paste(td, "cities.kml", sep="/"), "cities")

How one discovers this name is not obvious, since the KML driver in OGR
needs it to access the file. One possibility is:

system(paste("ogrinfo", paste(td, "cities.kml", sep="/")), intern=TRUE)

which lists the layer name(s). Perhaps ogrInfo() could be revised to access
the dsn in the same way when not given a layer= argument to discover layer
names - anyone with lots of time on their hands like to help?

Roger

PS. ogrDrivers() lists the drivers seen by OGR as running in rgdal, but for
drivers with write but not read capability, it cannot distinguish. In that
case, you only find out if a read driver is present and has its external
dependencies properly satisfied by using it, I'm afraid. The example above
will show whether your rgdal/GDAL/Expat do work or not.

I have expat XML parser installed and gdal.  This probably sounds
rather stupid, but I am not sure what the following means in the link
that Dr. Bivand provided: "KML reading is only available if GDAL/OGR
is built with the Expat XML Parser, otherwise only KML writing will be
supported."  I installed both as debian packages (using get-apt).
Should I install both from source code, Expat XML Parser first and
then GDAL?

Sorry if I seem dense here, but it is not real clear to me.  Also,
even though this may be beyond the scope of R-sig-geo, I think that
other people are also struggling with this.

Thanks for any help.

Wade


On Thu, May 27, 2010 at 2:15 PM, Roger Bivand <Roger.Bivand at nhh.no> wrote:

On Thu, 27 May 2010, rick reeves wrote:

I do note that the example I provided does not demonstrate how to READ a
KML
file into R. But, SOON, I plan to modify the example to demonstrate
reading of KMLs
into some R-compatible format.

readOGR() does read KML files, but you do have to identify the correct
layer
name - see:

http://www.gdal.org/ogr/drv_kml.html

Roger

RR

On 5/27/2010 10:15 AM, Wade Wall wrote:

Hi all,

Does anyone have experience reading and writing KML files in R?  I
have seen several threads that seem to suggest that it is difficult if
not impossible, but it is convenient to use KML files when working
with Google Earth and inconvenient to read and write shapefiles and
then convert to KML using other software.

If anyone is able to read and write KML files, if they could provide a
description of how to do it, I am sure a lot of people would
appreciate it.  I have tried to do so under both Windows and Linux to
no avail.  The Windows approach looks more difficult, judging by the
readme file associated with rgdal.

Thanks,

Wade

--
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no

--
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no