[R-sig-eco] How to extract values from contiguous raster cells that ate not touched by SpatialLines?

[Bede-Fazekas Ákos]

Hi Andre,
I don't know the answer, but I think you should post your question to 
R-sig-Geo mailing list (R-sig-Geo at r-project.org), where several authors 
of the spatial R-packages and other GIS experts are members and will 
read your post.
HTH,
Ákos Bede-Fazekas
Hungarian Academy of Sciences

2017.08.18. 17:09 keltezéssel, Andre Rovai írta:
> Hi all,
>
> I've been trying to extract values from a single attribute raster (area, in
> m2) that overlaps with lines (that is, a .shp SpatialLines).
>
> The problem is that, along these lines, my raster sometimes goes from one
> to several contiguous cells in all directions.  Using the
> extract function only values from cells that are touched by the lines are
> extracted.  Thus, when I add up the extracted values from all lines a
> significant amount of area (m2) is lost due to cells that were not touched
> by the line and therefore values were not extracted.
>
> I tried to work it around by:
>
> Step 1 - first aggregating my raster to a lower resolution (i.e. increasing
> the fact argument) and then
> Step 2 - rasterizing the lines using this aggregated raster (created in
> step 1) as a mold to make sure the rasterized lines would get thick enough
> to cover the horizontal spread of cells in my original resolution raster.
> Step 3 - Then I resample the rasterized lines (created in step 2) back to
> the original resolution I started with.
> Step 4 - Finally, extracted the values from the resampled rasterized lines
> (created in step 3).
>
> However, it didn't quite work as now the total area (m2) varies according
> to the fact="" value I use when first aggregating the raster (in step 1).
>
> I really appreciate if anyone has already dealt with a similar problem and
> can help me out here.  Here are the codes I've been running to try to get
> it to work:
>
>
> # input raster file
>
> g.025 <- raster("ras.asc")
>
> g.1 <- aggregate(g.025, fact=2, fun=sum)
>
>
>
> # input SpatialLines
>
> Spline1 <- readOGR("/Users/xxxxx.shp")
>
> Spline2 <- readOGR("/Users/xxxxx.shp")
>
> Spline3 <- readOGR("/Users/xxxxx.shp")
>
>
>
> # rasterizing using low resolution raster (aggregated)
>
> c1 <- rasterize(Spline1, g.1, field=Spline1$type, fun=sum)
>
> c2 <- rasterize(Spline2, g.1, field=Spline2$type, fun=sum)
>
> c3 <- rasterize(Spline3, g.1, field=Spline3$type, fun=sum)
>
>
>
> # resampling back to higher resolution
>
> c1 <- resample(c1, g.025)
>
> c2 <- resample(c2, g.025)
>
> c3 <- resample(c3, g.025)
>
>
>
> # preparing to extract area (m2) values from raster “g.025”
>
> c1tab <- as.data.frame(c1, xy=T)
>
> c2tab <- as.data.frame(c2, xy=T)
>
> c3tab <- as.data.frame(c3, xy=T)
>
> c1tab <- c1tab[which(is.na(c1tab$layer)!=T),]
>
> c2tab <- c2tab[which(is.na(c2tab$layer)!=T),]
>
> c3tab <- c3tab[which(is.na(c3tab$layer)!=T),]
>
>
>
> # extracting area (m2) values from raster “g.025”
>
> c1tab[,4] <- extract(g.025, c1tab[,1:2])
>
> c2tab[,4] <- extract(g.025, c2tab[,1:2])
>
> c3tab[,4] <- extract(g.025, c3tab[,1:2])
>
> names(c1tab)[4] <- "area_m2"
>
> names(c2tab)[4] <- "area_m2"
>
> names(c3tab)[4] <- "area_m2"
>
>
>
> # sum total area (m2)
>
> c1_area <- sum(c1tab$area_m2)
>
> c2_area <- sum(c2tab$area_m2)
>
> c3_area <- sum(c3tab$area_m2)
>
> tot_area <- sum(c1_area, c2_area, c3_area)
>
>
> Thanks!
>
> Andre Rovai
> Department of Oceanography and Coastal Sciences
> Louisiana State University
>
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