[R-sig-Geo] reading KML files in R

[Wade Wall]

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
>>>>>
>>>>> _______________________________________________
>>>>> R-sig-Geo mailing list
>>>>> R-sig-Geo at stat.math.ethz.ch
>>>>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>>>>>
>>>>>
>>>>
>>>>
>>>>
>>>
>>> --
>>> 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
>