autoKrige: no visible global function definition for 'as.formula'
autofitVariogram: no visible global function definition for
  'modifyList'
autofitVariogram: no visible global function definition for 'is'
autofitVariogram: no visible global function definition for 'median'
autokrige.vgm.panel: no visible global function definition for 'grey'
compare.cv: no visible global function definition for 'quantile'
create_new_data: no visible global function definition for 'chull'
cv.compare.ggplot: no visible global function definition for 'ggplot'
cv.compare.ggplot: no visible global function definition for
  'aes_string'
cv.compare.ggplot: no visible global function definition for
  'facet_wrap'
cv.compare.ggplot: no visible global function definition for
  'scale_x_continuous'
cv.compare.ggplot: no visible global function definition for
  'scale_y_continuous'
cv.compare.ggplot: no visible global function definition for
  'coord_equal'
cv.compare.ggplot: no visible global function definition for
  'scale_color_gradient2'
cv.compare.ggplot: no visible global function definition for
  'scale_size_continuous'
cv.compare.ggplot: no visible global function definition for 'fortify'
cv.compare.ggplot: no visible global function definition for
  'geom_path'
cv.compare.ggplot: no visible global function definition for
  'geom_point'
posPredictionInterval: no visible global function definition for
  'median'
posPredictionInterval: no visible global function definition for
  'qnorm'
summary.autoKrige.cv: no visible global function definition for 'cor'
summary.autoKrige.cv: no visible global function definition for 'sd'
summary.autoKrige.cv: no visible global function definition for 'IQR'
Undefined global functions or variables:
  IQR aes_string as.formula chull coord_equal cor facet_wrap fortify
  geom_path geom_point ggplot grey is median modifyList qnorm quantile
  scale_color_gradient2 scale_size_continuous scale_x_continuous
  scale_y_continuous sd
Consider adding
  importFrom("grDevices", "chull", "grey")
  importFrom("methods", "is")
  importFrom("stats", "IQR", "as.formula", "cor", "median", "qnorm",
             "quantile", "sd")
  importFrom("utils", "modifyList")
to your NAMESPACE file (and ensure that your DESCRIPTION Imports field
contains 'methods').