[R-sig-Geo] error occured when combining st_sample() with raster::extract()

[Bede-Fazekas Ákos]

Dear Xiang,

I guess that the reason for the error is that zion_points is an sf 
object, while sampled_sites is an sfc object. You should create a 
0-column sf object from the sfc using function st_as_sf().
Please try the following code:
extract(srtm, st_as_sf(sampled_sites))

HTH,
Ákos
__________
Ákos Bede-Fazekas
Centre for Ecological Research, Hungary


2024. 06. 23. 19:24 keltezéssel, Xiang Ye via R-sig-Geo írta:
> Dear community,
>
> I am exploring the functionality of raster package by following the online textbook Geocomputation with R, 1st ed. (I know the newer version of the textbook is online, I just want to finish what I left). The problem I encountered is associated with the example from Section 5.4.2, raster extraction:
> https://bookdown.org/robinlovelace/geocompr/geometric-operations.html#raster-extraction
> When I want to combine it with st_sample() from Section 5.2.3, i.e. extract the pixel values from say 15 randomly chosen locations, I met an error.
>
> Here comes the script:
>
> library(sf)
> library(raster)
>
> filename=system.file('raster/srtm.tif', package='spDataLarge')
> srtm=raster(filename)
> filename=system.file('vector/zion.gpkg', package='spDataLarge')
> zion=st_read(filename)
> zion=st_transform(zion, projection(srtm))
>
> data('zion_points', package='spDataLarge')
> extract(srtm, zion_points) # textbook example, work as expected
>
> sampled_sites=st_sample(zion, 15)
> extract(srtm, sampled_sites) # error occurred.
>
> The error message says "����: ������extract����ǩ��x = "RasterLayer", y = "sfc_POINT"���Ҳ����̳з���"
> There is Chinese text embedded because my OS is in Chinese. Translated into English, it reads "ERROR: function 'extract' labelling ��x = "RasterLayer", y = "sfc_POINT"�� cannot find the inherited methods".
>
> My question: what's the reason of this problem, and how to fix it?
>
> Thank you in advance!
>
> Ҷ�� YE, Xiang
> THINKING SPATIALLY<http://www.linkedin.com/in/spatialyexiang>.
> Ph.D. in Spatial Statistics
>
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>
>
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