[R-sig-Geo] rast() in terra doesn't show cell values

[Noah Charney]

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

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