I work on a golf course that is built on old farm land. We have old clay drains from the turn of the 20th century and lots of additional drains that have been added in the meantime years. 

No maps have been produced and we are constantly discovering forgotten work. Our problem is that we cannot remember all of the lines on which they run so would like to start mapping as we are probing, rodding and digging.

Please can someone suggest a free software that we can use to produce maps?

We just need to plot lines on either maps (or preferable aerial images) of the land with some sort of way of locating them in the future. Either using datum points (trees etc) or GPS (though we will only have our mobile phones to get our co-ordinates. 

The golf course in in Wales, UK

Thank you!