[R-sig-Geo] Confused by overlay(x = "SpatialPixelsDataFrame", y = "SpatialPoints")

[David Wahlund]

On Thu, Jan 15, 2009 at 12:07, Edzer Pebesma
<edzer.pebesma at uni-muenster.de> wrote:
>
>
> David Wahlund wrote:
>>
>> Hi,
>> trying to extract by mask using sp package.
>> Was surprised that I couldn't find 'overlay(x =
>> "SpatialPixelsDataFrame", y = "SpatialPolygon")' to extract pixels
>> within a polygon.
>>
>
> There is one except that it is y="SpatialPolygons" rather than
> "SpatialPolygon":
>
>> showMethods(overlay)
> Function: overlay (package sp)
> x="SpatialGridDataFrame", y="SpatialPoints"
> x="SpatialGrid", y="SpatialPoints"
> x="SpatialPixelsDataFrame", y="SpatialPoints"
> x="SpatialPixelsDataFrame", y="SpatialPolygons"
>   (inherited from: x="SpatialPointsDataFrame", y="SpatialPolygons")
> x="SpatialPixels", y="SpatialPoints"
> x="SpatialPointsDataFrame", y="SpatialPolygons"
> x="SpatialPoints", y="SpatialPolygons"
> x="SpatialPolygons", y="SpatialPoints"
>
> (note that it's only visible this way only after you've used it)

Might be a good idea to include something about this in the docs .
Seems a bit hidden now.

>>
>> So instead I'm using
>> ascPoints <- SpatialPoints(coordinates(asc))
>> ascPointsHR <- ascPoints[!is.na(overlay(ascPoints,hr))]
>>
>> Now when I try to use overlay using 'ascPointsHR' on 'asc' I get a
>> strange return. sp docs says it 'returns grid values x at spatial
>> point locations
>> y' which I interpret as me getting the raster values.
>
> This is wrong; where exactly does it say so? Documentation is a bit
> scattered sometimes.

It's in 'sp.pdf' created 2009-01-04 15:55, page 56, under 'overlay-methods'.

>
>> Instead I seem
>> to be getting the index values of the pixels or something. The raster
>> is float, as seen from the summary below. But the return from
>> overlay() is ordered integer.
>>
>> Am I missing something?
>>
>
> Do you manage to get the pixels you want from the indexes returned?

I'll try your suggestions. I figured it was the indices of the pixels
but since it contradicted the documentation I thought I was doing
something wrong. I'll get back to you with results.

> --
> Edzer
>>
>> Thank you
>> David Wahlund
>>
>>
>>>
>>> summary(asc)
>>>
>>
>> Object of class SpatialPixelsDataFrame
>> Coordinates:
>>      min     max
>> x 1592482 1622382
>> y 7121568 7144668
>> Is projected: NA
>> proj4string : [NA]
>> Number of points: 1105104
>> Grid attributes:
>>  cellcentre.offset cellsize cells.dim
>> x           1592494       25      1196
>> y           7121580       25       924
>> Data attributes:
>>     Min.   1st Qu.    Median      Mean   3rd Qu.      Max.
>> 0.000e+00 0.000e+00 0.000e+00 1.448e-09 4.750e-11 4.954e-08
>>
>>>
>>> summary(ascPointsHR)
>>>
>>
>> Object of class SpatialPoints
>> Coordinates:
>>      min     max
>> x 1599144 1617094
>> y 7127130 7138680
>> Is projected: NA
>> proj4string : [NA]
>> Number of points: 198320
>>
>>>
>>> ascHR <- overlay(asc,ascPointsHR)
>>> summary(ascHR)
>>>
>>
>>   Min. 1st Qu.  Median    Mean 3rd Qu.    Max.
>>  266300  464700  556800  552900  646600  818600
>>
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>>
>
> --
> Edzer Pebesma
> Institute for Geoinformatics (ifgi), University of Münster
> Weseler Straße 253, 48151 Münster, Germany. Phone: +49 251
> 8333081, Fax: +49 251 8339763 http://ifgi.uni-muenster.de/
> http://www.springer.com/978-0-387-78170-9 e.pebesma at wwu.de
>
>