In an open field, how close can a person get to a Niftar/Nifteret before he contracts Tum’ah?
If the person is not a kohen he will not become tameh until he either walks or puts his hand above the niftar or touches the niftar.
Regarding a kohen, he may not go within 4 amos of a buried person, unless there is a fence hat is 10 tefacim high seprating between them, and even then he may only go up to 4 tefachim away. If the meis is lying in a feild and this is not his permenent rsting place, i.e. by the funeral, according to some poskim the kohen has to stay 4 amos away, and according to some poskim it is only 4 tefachim away. The minhag however is that kohanim make sure to stay 4 amos away from the niftar.
Y:D 371-5, Shach ibig 18, Aruch Hashulchan ibid 27.