diff --git a/R/tiles.R b/R/tiles.R
new file mode 100644
index 0000000..eaf4e15
--- /dev/null
+++ b/R/tiles.R
@@ -0,0 +1,54 @@
+## tiles
+## we need row,col,cell converters for mosaics
+
+
+## these are internal use versions that don't detect NA out of bounds
+col_from_x_inf <- function(dimension, extent = NULL, x) {
+  extent <- extent %||% extent0(dimension)
+  .check_args(dimension, extent)
+
+  colnr <- trunc((x - x_min(dimension, extent)) / x_res(dimension, extent = extent)) + 1
+  colnr[ x == x_max(dimension, extent) ] <- n_col(dimension)
+  #colnr[ x < x_min(dimension, extent) | x > x_max(dimension, extent) ] <- NA
+  return(as.vector(colnr))
+}
+
+row_from_y_inf <- function(dimension, extent = NULL, y) {
+  extent <- extent %||% extent0(dimension)
+  .check_args(dimension, extent)
+
+  rownr <- 1 + (trunc((y_max(dimension, extent) - y) / y_res(dimension, extent = extent)))
+  rownr[y == y_min(dimension, extent) ] <- n_row(dimension)
+#  rownr[y > y_max(dimension, extent) | y < y_min(dimension, extent)] <- NA
+  return(as.vector(rownr))
+}
+
+
+## child from parent or parent from child (using _inf versions that allow out of bounds row/col numbers)
+cell2cell <- function(dimension, extent, ## that's the parent
+                      dim0, ext0, cell0 ) {    ## the chid tile) {
+
+                        ## checks
+                      orig <- origin(dimension, extent)
+                      orig0 <- origin(dim0, ext0)
+                      stopifnot(all.equal(orig, orig0))
+
+                      res <- c(x_res(dimension, extent), y_res(dimension, extent))
+                      res0 <- c(x_res(dim0, ext0), y_res(dim0, ext0))
+                        stopifnot(all.equal(res, res0))
+
+                      ## the col,row of the parent where the child starts (tr)
+                      cr <- c(col_from_x_inf(dimension, extent, ext0[1]),
+                              row_from_y_inf(dimension, extent, ext0[4]))
+
+
+
+                     # print(cr)
+                      rc0 <- rowcol_from_cell(dim0, ext0, cell0)
+
+                      cr_global <- cbind(cr[1] + rc0[,2] - 1,
+                                     cr[2] + rc0[,1] - 1)
+                      cell_from_row_col(dimension,  cr_global[,2], cr_global[,1])
+}
+
+
diff --git a/man/extent_vrt.Rd b/man/extent_vrt.Rd
index dc71a23..b54f6e2 100644
--- a/man/extent_vrt.Rd
+++ b/man/extent_vrt.Rd
@@ -13,7 +13,7 @@ extent_vrt(x)
 Get extent from index values in VRT text.
 }
 \details{
-(I can't understand XML tech for the life of me so I hack the text as lines with strsplit)
+(I can't understand XML tech  so I hack the text as lines with strsplit)
 }
 \examples{
 #src <- "https://opentopography.s3.sdsc.edu/raster/NASADEM/NASADEM_be.vrt"