Raster crosstab output: -1 values?

Hi everyone,

I'm computing a very large crosstab analysis (using the raster 
package) for around 700 tiles of 25m resolution vegetation raster 
data. Essentially, I want to figure out how much forest is contained 
within each land use, land tenure and jurisdictional boundary (eight 
Australian States) over a 39 year time period (20 years with data).

The land tenure, land use and State layers have all been cut up into 
the same extent, resolution and projection as the forest cover data. I 
then create a raster stack of the forest cover, State and land 
tenure-use (the latter was combined into one raster layer) tiles, and 
subsequently a crosstab. This process is repeated for each tile and 
year (~700 loops, taking a rather long time). The forest cover data 
(in this particular year -
1972) has values of only 0 and 1 (non-forest/forest). However, I'm 
finding in some cases the crosstab output contains data in a "-1" 
category, despite this value not being part of the original forest cover

I'm at a loss to explain this; all datasets have the same number of 
cells, resoluton, and no missing data. Would really appreciate any

Cheers, Megan

Data details below:

Forest Cover (ttile):
class       : RasterLayer
dimensions  : 16000, 28360, 453760000  (nrow, ncol, ncell) resolution  
: 0.00025, 0.00025  (x, y)
extent      : 112.91, 120, -28, -24  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=longlat +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 
+no_defs data source :
D:/Analysis/R/Data/forestextent_reg/forest_v8_72_11/sg50/forst72.grd
names       : layer
values      : 0, 1  (min, max)
Raster Attribute Table
     fields : ID     COUNT
        min :  0  30065009
        max :  1 423694991

State (st_tex):
class       : RasterLayer
dimensions  : 16000, 28360, 453760000  (nrow, ncol, ncell) resolution  
: 0.00025, 0.00025  (x, y)
extent      : 112.91, 120, -28, -24  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=longlat +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 
+no_defs data source :

names       : states_p
values      : 8, 8  (min, max)

Land Use/Tenure (lu_ten_tex):
class       : RasterLayer
dimensions  : 16000, 28360, 453760000  (nrow, ncol, ncell) resolution  
: 0.00025, 0.00025  (x, y)
extent      : 112.91, 120, -28, -24  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=longlat +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 
+no_defs data source :

names       : lu_ten
values      : 1, 73  (min, max)
Raster Attribute Table
     fields : ID       COUNT
        min :  1 -1816924096
        max : 73  1681761600

Relevant code:
# Create raster stack between forestextent, lu and state
        x <-stack(ttile, lu_ten_tex, st_tex)

        #Crosstab will calculate the frequency of each level 
(0=non-forest, 1=primforest, 2=regforest) in each land use/tenure/state
        vegfreq <-crosstab(x, digits=0, long=FALSE, progress="text")

vegfreq

, , states_p = 8

     lu_ten
layer         0         1         3        11        13        61

33
   0     400975   2059222   2355223  28336430  13144612   2565663

42855
   1      15814      1098     19824   6497248   3958995    761517

345
   -1        11      1280       153      3522       393        20

0
     lu_ten
layer        43        73        23        53        71        51

22
   0       8447     16777     64401     50205     19375     17463

664199
   1       1153      7223     66799       995     31825       137

126201
   -1         0         0         0         0         0         0

0
     lu_ten
layer        62         2        32
   0     310439      1586     13062
   1     201561        14      2938
   -1         0         0         0




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