I have used a rope with 12 equal distance knots - in my case 1m. When you make a triangle 3x4x5 units, you get a very accurate large 90* angle, wich I found helpful, especially if you use sectional / pre-curved track. (This trick was used by the Egyptians building pyramids and stuff ;-)
I put stakes in the ground, marking the layout. Then I assembled most of the track and trackbed, before I actually raised it, using 2 reebars every 0,5m (1,5 foot).
The whole process was very rapid, and I got a track to run on in only a few days. You can se the whole process at
http://web.mac.com/anders.grassman/iWeb/Topsida/Einfach_Garten_Gleisbett.html
However, you have to use twice as thick wooden planks, than depicted, or they will sag in a few months... (I added iron angle underneath.) Also, I found electric cable straps to weak, and replaced them with the common soft iron wire normally used to join reebar. The track has remained stable and useful since.
But, after a few derailments causing the trains to plunge 2,5 feet to the ground, I've decided to use the much wider (discarded) restaurant trays as roadbed for a replacement layout. The trays are of laminated (dishwashable) wood = extremely weatherresistant and incredibly stiff.
I guesss I diverted some from the subject... ;-)