I also found issues with the raster::extract function, for some reason, I
got crazy values when I compared the resulting value with the one from
QGIS(Zonal Statistics Plugin) and ArcGIS（zonal statistics), while the value
from qgis and arcgis were close but the raster::extract has a lot of
disturbances:
#example
library(raster)
library(rgdal)
r <- raster("PRISM_ppt_stable_4kmM3_201608_bil.bil
http://prism.nacse.org/6month/";)
setwd("D:/PRISM/2005-2016/cb_2015_us_state_500k/")
psa <- readOGR(".",paste("cb_2015_us_state_500k
<http://www2.census.gov/geo/tiger/GENZ2015/shp/cb_2015_us_state_500k.zip>
",sep=''))
e <- extract(r, psa, weights=TRUE, sp=TRUE, na.rm=TRUE, df=TRUE,fun=mean)
result <- cbind(psa$monmean,e$PRISM_ppt_stable_4kmM3_201609_bil)
result
[,1] [,2]
[1,] 67.756948 59.672969
[2,] 8.163987 44.415848
[3,] 91.171480 53.295749
[4,] 74.446065 52.633753
[5,] 128.915104 56.779383
[6,] 105.266005 93.266030
[7,] 187.633359 45.947235
[8,] 106.659815 90.639262
[9,] 160.519008 46.249172
[10,] 57.266614 26.728985
[11,] 23.258635 49.885868
[12,] 132.101744 46.713378
[13,] 21.995937 51.782905
[14,] 168.313586 93.790604
[15,] NA NA
......
[56,] 76.527362 82.369481
Has anyone have looked into this issue?
Thanks,
Peter
On Tue, Oct 18, 2016 at 10:10 AM, chris english <
englishchristoph...@gmail.com> wrote:
> Mel,
>
> Looking at detail in cjg.png the northmost missing data island shows approx
> 20-25 in Southern three quarters and 0-5 the small North at tip of island.
> Directly above this 0-5 another coastal 0-5.
> If a substantial east-west scarp bisected the island it might explain. I
> would otherwise expect a very slight mismatch in projection, a thing I
> often have problems with.
>
> Though reading raster::extract it looks like you want to employ the weights
> are if your polys are relatively smaller than your cells.
>
> HTH
> Chris
>
> On Oct 18, 2016 4:35 PM, "Bacou, Melanie" <m...@mbacou.com> wrote:
>
> > Hi,
> > I'm summarizing biophysical rasters (UDEL precipitation and temperature)
> > across administrative units for countries in Africa using (pseudo code):
> >
> > raster::extract(udel, admin, fun=mean, na.rm=T, small=T)
> >
> > Out of the 756 units I need data for, extract() fails to return means for
> > a few coastal units (in red on the maps below) even though the rasters
> show
> > data at these locations.
> >
> > Is there a particular reason why this might happen? Shall I look for
> > possible geometry errors in my source shapefiles, or could there be
> another
> > reason?
> >
> > Maps here:
> > https://dl.dropboxusercontent.com/u/30925475/eclgcdmhapfplaml.png
> > https://dl.dropboxusercontent.com/u/30925475/lmkgmdoohpmhdcjg.png
> >
> > Thanks for any tip. --Mel.
> >
> > _______________________________________________
> > R-sig-Geo mailing list
> > R-sig-Geo@r-project.org
> > https://stat.ethz.ch/mailman/listinfo/r-sig-geo
> >
>
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