[R-sig-Geo] rgdal_1.1-1: dsn path string problems

[Rainer Hurling]

Hi Roger,

many thanks for your quick answer.


Am 08.11.15 um 18:02 schrieb Roger Bivand:
> On Sun, 8 Nov 2015, Rainer Hurling wrote:
> 
>> Dear Roger, dear list,
>>
>> many thanks for the newest update 1.1-1 of package rgdal. It is really
>> helpful.
>>
>> I am using rgdal with R-devel and GDAL 2.0.1, both on FreeBSD
>> 11.0-CURRENT and Windows7. On both platforms, I have problems with path
>> strings in dsn.
>>
>>
>>
>> (a) The minor problem is on Windows. While a dsn="C:/some/path" works
>> fine, the same path with a slash at the end (dsn="C:/some/path/") fails.
>>
>> On most other software it is conventional to allow a closing slash.
>> Wouldn't it be nice, if rgdal would be more tolerant about it?
>>
> 
> R base::file.exists does include this in its help file: "However,
> directory names must not include a trailing backslash or slash on Windows".
> 
> so this isn't just rgdal. The dsn= argument for many drivers is a file
> name, not a directory; a work-around seems like overkill.

OK, that seems reasonable. Of course, rgdal should handle this like R in
total. Thanks you for clarification.

>>
>> Now my main problem:
>> (b) On FreeBSD, for some time now, the dsn path can not be used anymore
>> to check for existence of a file.
>>
>> I want to use writeOGR to overwrite an existing shapefile. I use
>> something like dsn="/path/to/somewhere" and layer="layername".
>>
>> Within writeOGR(), R/ogr_write.R calls the C++ function
>> 'ogrCheckExists'. ogrCheckExists is not able to return TRUE, if a
>> shapefile already exists. Every time its result is FALSE:
>>
>> R/ogr_write.R
>> 61:        ogrI <- .Call("ogrCheckExists", as.character(dsn),
>> s.character(layer), PACKAGE = "rgdal")
>> [1] FALSE
>>
>>
>> As far, as I can see, 'ogrCheckExists' comes from src/ogrsource.cpp.
>> Something must go wrong in that C++ file, at least for FreeBSD :(
>>
>> Unfortunalety, I have no skills how to debug such a C++ code within an R
>> package ...
>>
> 
> I do not have access to such a system. As a first step, does
> rgdal::ogrListLayers work correctly?

The syntax, shown in the helpfile, does not work for me:

rgdal::ogrListLayers(dsn="/path", layer="layername")
Fehler in rgdal::ogrListLayers(dsn = "/path", layer = "layername") :
  unbenutztes Argument (layer = "layername")    # unused argument


The same, with layer integrated in the dsn, works fine:

rgdal::ogrListLayers(dsn="/path/layername.shp")
[1] "layername"
attr(,"driver")
[1] "ESRI Shapefile"
attr(,"nlayers")
[1] 1


This leads me to the idea, that the same could work for writeOGR or
ogrWrite.R. And yes, this works on FreeBSD:

writeOGR(gis.layer, dsn="/path/layername.shp",
         layer="layername", driver="ESRI Shapefile",
         check_exists=TRUE, overwrite_layer=TRUE)

So, I have to double 'layername' here, once in dsn and once in layer.
But it should be ok as a workaround until I found the real cause.

> 
> Beyond that, you'd need to insert Rprintf() statements into the C++ file
> to see whether the FALSE result comes from a failure to open the dsn
> with a known driver, or from the fact that such a layer is not found in
> that dsn.

Thanks for the tip. I will investigate into it.

A first question to this: If I want to integrate some Rprintf into
ogrsource.cpp in the section of ogrCheckExists (lines 832 ...), what is
a good syntax to output for example 'ogrSource' and 'Layer'?

What I tried at ogrsource.cpp:l834 without success is:

Rprintf( "ogrCheckExists: OGRSource %s %p\n", OGRLayer, (void *)
ogrSource) ;

It think, I have to wrap 'ogrSource' with some function here? (I am not
a C/C++ programmer)

> 
> Does using a different driver help? Does using the shapefile.shp as the
> dsn help (I think this works better in GDAL2)?

Because using the shapefile.shp as the dsn helps, I did not try a
different driver. If this is important for you, I can catch it up later.

Thanks again for your help,
Rainer

> 
> Hope this helps,
> 
> Roger
> 
>> Any help is really appreciated. Please tell me, if I could try and test
>> something. My box has FreeBSD, R-devel, gcc-4.8.5, gdb-7.1.0 ...
>>
>> Greetings,
>> Rainer Hurling
>>
>>
>