Your measure column in your data data.frame currently contains a charcter vector. The c() function will convert your numerics to characters, so:
measure <- c(10,"AB",41,"CGHa",100)
is the same as:
measure <- c("10","AB","41","CGHa","100")
This is called coercion and R does it a lot whenever values are of different types.
You can exploit this by using as.numeric() which will return NA when it cannot convert a character to a numeric.
data$measure <- as.numeric(data$measure)
It will give you a Warning that some values could not be converted, but if you are sure that you don't want the character values, then this Warning can be ignored.