I was being serious; this is the standard way that it is done professionally, albeit with more complex foam generating equipment.
Sorry no disrespect intended i just had this image of a tractor racing around the field with a foam party behind it!
that looks like a good idea havin the foam things at the end of our spray booms...
The foam markers are a common and standard way to prevent overspray for tractor spray rigs (as well as most truck mounted systems).
The other method farmers use is GPS but not a standard uncorrected unit. The professional units use corrections from a variety of external sources such as WAAS, Omnistar, Starfire, etc.
For greater accuracy (<2cm) you can use RTK GPS which uses corrections from a second GPS unit. This second unit may be purchased by the farmer and hosted on his property (costs an extra >$15,000). The other way is to subscribe to companies that provide a subscription service where the corrections are sent to your GPS receiver by mobile phone or radio network.
To do it on the cheap, try using a 'normal' GPS module (without corrections) and put the output through a Kalman filter and see how much deviation you get over a few days.
Thanks Lemming i was thinking about using a Kalman on the GPS and seeing what the deviation would be over different times.
Also to use the mag to filter out any wayward readings from the GPS. this could increase accuracy yes?
i looked at GLONASS looks interesting and will read some more
I think the GPS + Mag wil increase the streightness of the lines already as at the moment the tractors drive from person to person by eyeball
if i could get a reliable way for the tractor to start at the same places then it will work perfect