diff --git a/ext/RbaseTerraExt/RbaseTerraExt.jl b/ext/RbaseTerraExt/RbaseTerraExt.jl
index f492bd3..4854739 100644
--- a/ext/RbaseTerraExt/RbaseTerraExt.jl
+++ b/ext/RbaseTerraExt/RbaseTerraExt.jl
@@ -132,9 +132,9 @@ function NetCDFTools.coverage_fraction(f, shp; union=false)
   """ |> rcopy
   
   # return a mask, true is inside
-  data = nc_read(f, ind=(:, :, 1)) # time should be in the last
-
-  mask = falses(size(data))
+  dims = length.(nc_dims(f)[1:2])
+  # data = nc_read(f, ind=(:, :, 1)) # time should be in the last
+  mask = falses(dims)
   mask[info.cell] .= true
   info, mask
 end
diff --git a/src/nc_dim.jl b/src/nc_dim.jl
index 29e7848..7786ec4 100644
--- a/src/nc_dim.jl
+++ b/src/nc_dim.jl
@@ -19,6 +19,7 @@ function Ipaper.names(dims::Vector{NcDim})
   map(x -> x.name, dims)
 end
 
+Base.length(dim::NcDim) = dim.dimlen
 
 function find_dim(dims::Vector{NcDim}, name::AbstractString)
   names = Ipaper.names(dims)