For most practical purposes in wilderness navigation, a error of this magnitude is usually rather insignificant, say in climbing to a 10,000 foot high pass, or a 13,000 high mountain.

The geoid must correspond to the datum in my neck of the woods. I recall reaching a benchmark - holding the GPS receiver I noted that it was reading three feet high - then I realized that I was holding it at waist level, about three feet above the brass cap marking the benchmark.