Ha, that's me M Ljin is referring to. I built Leaftide partly because I had this exact problem, wanting to know where everything was planted without relying on memory or scrappy hand-drawn maps. It doesn't do the walk-and-sense thing Judith is describing though, you still place plants manually on a plot map. The GPS accuracy on phones is only good to about 3-5 metres which makes it pretty useless for individual plant positions unfortunately. For what you're describing, graph paper and a tape measure is honestly still hard to beat for accuracy.