Hi Noah,
It appears to be the same problem as this: [reading rasters in r using
terra
package](https://gis.stackexchange.com/questions/398061/reading-rasters-in-r-using-terra-package).
As in that question, adding 0 forces the conversion, but loses all the
information about color table, etc.
canopy <- rast('nlcd_2016_treecanopy_2019_08_31.img', lyrs=1)
c2 <- canopy + 0
> c2
class : SpatRaster
dimensions : 104424, 161190, 1 (nrow, ncol, nlyr)
resolution : 30, 30 (x, y)
extent : -2493045, 2342655, 177285, 3310005 (xmin, xmax, ymin, ymax)
coord. ref. : Albers Conical Equal Area
source : spat_t0Rj6gEgi2CzsAZ_16778.tif
name : Histogram
min value : 0
max value : 255
I didn't try the devel version of terra, though.
Sarah
On Wed, Mar 8, 2023 at 6:17 AM Noah Charney <noah.char...@maine.edu> wrote:
>
> Hi,
> Can someone tell me what I'm missing? I can't get the rast() function in
> terra to return the actual cell values from an .img file, it just keeps
> giving strange histogram values. The plot() looks ok, except the legend
> has those same useless values. Similarly on the categorical NLCD Land
> Cover layer it gives me data associated with each cell value (e.g. land
> cover type), which is fine to work with, but still not the actual cell
> values as I'd expect. The old raster() function in raster package works
> fine (see below). Thanks for insights! -Noah
>
> > #Download Tree Canopy Layer from NLCD
> > # https://www.mrlc.gov/data/nlcd-2016-usfs-tree-canopy-cover-conus
> >
> > #terra package gives me nonsensical histogram values
> > library(terra)
> > canopy <- rast('D:/GIS/Land
> Cover/nlcd_2016_treecanopy_2019_08_31/nlcd_2016_treecanopy_2019_08_31.img')
> > canopy
> class : SpatRaster
> dimensions : 104424, 161190, 1 (nrow, ncol, nlyr)
> resolution : 30, 30 (x, y)
> extent : -2493045, 2342655, 177285, 3310005 (xmin, xmax, ymin, ymax)
> coord. ref. : Albers Conical Equal Area
> source : nlcd_2016_treecanopy_2019_08_31.img
> color table : 1
> categories : Histogram, Red, Green, Blue
> name : Histogram
> min value : 5737352684
> max value : 7854320129
> > plot(canopy)
> > canopy[50000,50000]
> Histogram
> 1 24314964
> > extract(canopy,data.frame(x=1000000,y=1000000))
> ID Histogram
> 1 1 44832754
> >
> > #raster package works fine
> > library(raster)
> > canopy <- raster('D:/GIS/Land
> Cover/nlcd_2016_treecanopy_2019_08_31/nlcd_2016_treecanopy_2019_08_31.img')
> > canopy
> class : RasterLayer
> dimensions : 104424, 161190, 16832104560 (nrow, ncol, ncell)
> resolution : 30, 30 (x, y)
> extent : -2493045, 2342655, 177285, 3310005 (xmin, xmax, ymin, ymax)
> crs : +proj=aea +lat_0=23 +lon_0=-96 +lat_1=29.5 +lat_2=45.5 +x_0=0
> +y_0=0 +datum=NAD83 +units=m +no_defs
> source : nlcd_2016_treecanopy_2019_08_31.img
> names : Histogram
> values : 0, 255 (min, max)
> attributes :
> ID Histogram Red Green Blue
> from: 0 5737352684 255 255 255
> to : 255 7854320129 255 255 255
> > canopy[50000,50000]
> 24
> > extract(canopy,data.frame(x=1000000,y=1000000))
> 94
> >
> > #Land Cover (categorical) layer gives associated categories (not
> values) in terra
> > # https://www.mrlc.gov/data/nlcd-2019-land-cover-conus
> >
> > nlcd <- rast('D:/GIS/Land
> Cover/nlcd_2019_land_cover_l48_20210604/nlcd_2019_land_cover_l48_20210604.img')
> > nlcd
> class : SpatRaster
> dimensions : 104424, 161190, 1 (nrow, ncol, nlyr)
> resolution : 30, 30 (x, y)
> extent : -2493045, 2342655, 177285, 3310005 (xmin, xmax, ymin, ymax)
> coord. ref. : Albers Conical Equal Area
> source : nlcd_2019_land_cover_l48_20210604.img
> color table : 1
> categories : NLCD Land Cover Class, Histogram, Red, Green, Blue, Opacity
> name : NLCD Land Cover Class
> min value : Unclassified
> max value : Emergent Herbaceous Wetlands
> > nlcd[50000,50000]
> NLCD Land Cover Class
> 1 Evergreen Forest
> > extract(nlcd,data.frame(x=1000000,y=1000000))
> ID NLCD Land Cover Class
> 1 1 Evergreen Forest
> >
>
>
> ~~~
> Dr. Noah Charney (he/him)
> Assistant Professor of Conservation Biology
> Department of Wildlife, Fisheries, and Conservation Biology
> University of Maine, Orono, ME
> 230 Nutting Hall
> (207) 581-1284
>
> [[alternative HTML version deleted]]
>
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--
Sarah Goslee (she/her)
http://www.numberwright.com
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