Hi Glen

A standard GPS will only get you 4+ m accuracy. We tend to use differential gps while on ground to get sub-meter measures, though they are not cheap to purchase. You should be able to hire a system for the period of your field trip which is more cost effective.. Another method would be to set up a known base station and post process your GPS data.

Thanks. Your tip for establishing a base station and post-processing is a good one. I'll do that.