[R-sig-Geo] [SoC v.autokrige2.py] autoKrige() and projected data

[Anne Ghisla]

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Edzer Pebesma ha scritto:
> If newdata is created automatically (and this makes sense), it should
> recieve the projection information of data.

hello all,

@Paul: I added the projection of obj to new_data before return. Please
test if it works as part of the package.

create_new_data <- function (obj)
{
    convex_hull = chull(coordinates(obj)[, 1], coordinates(obj)[,
        2])
    convex_hull = c(convex_hull, convex_hull[1])
    d = Polygon(obj[convex_hull, ])
    new_data = spsample(d, 5000, type = "regular")
    gridded(new_data) = TRUE
    attr(new_data, "proj4string") <-obj at proj4string
    return(new_data)
}

> On another issue: what happens if both are provided but have different
> projection attributes?
> 
> And what happens if data has long/lat coordinates (is.projected(data) ==
> FALSE)? In that case I would prefer an error -- the current
> interpolation methods (covariances) used are not appropriate.

Totally agree. The tests on projection are mandatory.

best regards,
Anne

> 
> Best regards,
> --
> Edzer
> 
> Anne Ghisla wrote:
>> Hello Paul, Edzer,
>>
>> the integration of automap package in v.autokrige2 is quite completed
>> [0], thanks Paul for this clean interface to gstat!
>> Sadly, I'm getting an error when autoKrige() receives no new_data
>> argument. Examples in documentation all involve meuse dataset, that has
>> no projection information. I browsed the Web for this situation with
>> little success. I don't know precisely where the error occurs, so I send
>> this report to Edzer too.
>>
>> Here is the code and a traceback of the error:
>>
>> ---code---
>> require(spgrass6)
>> require(automap)
>>
>> # rs in a vector map containing locations with relative elevation value,
>> in GRASS spearfish60 dataset. Obtained following this tutorial [1]
>>> rsmap <- readVECT6('rs', type='point')
>> Exporting 300 points/lines...
>>  100%
>> 300 features written
>> OGR data source with driver: ESRI Shapefile
>> Source: "/home/anne/gis/spearfish60/PERMANENT/.tmp/galadriel", layer: "rs"
>> with  300  rows and  2  columns
>> Feature type: wkbPoint with 2 dimensions
>>
>>> str(rsmap)
>> Formal class 'SpatialPointsDataFrame' [package "sp"] with 5 slots
>> [cut]
>>   ..@ proj4string:Formal class 'CRS' [package "sp"] with 1 slots
>>   .. .. ..@ projargs: chr " +proj=utm +zone=13 +ellps=clrk66
>> +datum=NAD27 +units=m +no_defs
>> +nadgrids=@conus, at alaska, at ntv2_0.gsb, at ntv1_can.dat"
>>
>> # not shown: meuse object has the same structure and contents but
>> projargs==NA.
>>
>>> autoKrige(elev~1, rsmap)
>> Errore in predict.gstat(g, newdata = newdata, block = block, nsim =
>> nsim,  :
>>   var1 : data item in gstat object and newdata have different coordinate
>> reference systems
>>
>> Then I had a look into autoKrige code and found these lines:
>>
>>     if (missing(new_data))
>>         new_data = create_new_data(input_data)
>>     krige_result = krige(formula, input_data, new_data,
>> variogram_object$var_model,
>>         block = block, ...)
>>
>> My guess is that create_new_data() creates non-projected grid, and
>> krige() calls predict.gstat() with input_ and new_data, checks if they
>> have the same projection and in this case fails.
>>
>> In v.autokrige.py code, Mathieu Grelier workarounded this autogeneration
>> of new_data creating a correctly projected grid and passing it as
>> parameter.
>>
>> Shall I do the same or is it possible to get create_new_data() to keep
>> input_data's projection?
>>
>> many thanks in advance,
>> Anne
>>
>> [0] http://grass.osgeo.org/wiki/V.autokrige_GSoC_2009
>> [1] http://casoilresource.lawr.ucdavis.edu/drupal/node/438
> 

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