Originally when spraying a field, we used flaggers. These were people who would stand at the end of the field and wave a flag in a figure eight pattern; this gave the pilot something on which to line up, for spacing while making repeated passes down the field.
A very handy device was introduced called the "automatic flagman" which was a box that attached to the right wing. In this box were about two hundred trapezoid-shaped pieces of cardboard, with a 25' crepe paper streamer attached, folded flat. These were "flags." These would be arranged in the box such that an open hole in the front would allow ram air pressure to push them aft. The pilot used a trigger or button on the stick in the cockpit to electrically fire a small solenoid; the solenoid would push a flag out of of the box, one per push. The flag would catch the slipstream and lay out across the ground, or over a fence.
The common technique with the flag was to drop them at intervals across long fields or fields with uneven terrain, or to drape them over a fence on entry or exit from a field. These provided references when lining up for the next pass, or when approaching from the opposite end of the field. They also served as markers when one ran out of chemical, so one could find the same spot upon a return to the field with a fresh tank.
The flags were made of paper products, and thus biodegradable, but also relatively expensive. They offered speed and flexibility, and efficiency.
When GPS became available in ag operations, it really revolutionized the way spraying was done. The actual aerial application process largely remained the same, and has for several decades, but the accuracy with which a field could not only be treated, but recorded, became phenomenal.
We used to use three airplanes either in formation, or running opposite to each other in the field, to establish spacing. It made for rapid, efficient, accurate work. Today, larger airplanes have replaced the smaller ag airplanes of yesterday (the 800 gallon Air Tractor AT-802 vs. the effectively 150-gallon Pawnee, for example), and a single airplane with a Satloc system can work independently with great efficiency.
You'll note in the video that started this thread, some of the flying was being done at night; this is the case in many locations where the heat of the day is an issue, and also where beneficial insects (bees, etc) that might be harmed by the chemicals aren't present nocturnally. GPS nav in that case is especially useful for accurate chemical placement.
Variances in altitude or spacing of even a few inches in some cases can create gaps in coverage in a field. With pesticides, this is seldom an issue, but in various herbicides, it's very crucial. Gaps in a field can produce streaks of weeds or other undesirable plants. With some chemicals costing upward of a thousand dollars a gallon, the application of those chemicals in the agricultural setting puts the ag operator on a razor-thin profit margin. A farmer that sees streaks in his field and refuses to pay for the work can very nearly bankrupt an ag operator...it only takes one or two of those events to destroy an entire seasons profit or work, or even put a man out of business. Accurate application means minimizing those risks.
Drift claims are one of the biggest issues for ag operators. When chemical drifts, it can damage downwind crops, trees, etc. Claims from farmers who accuse the ag operator of damaging their crops represent a substantial threat to an ag aviation business, and the single biggest liability that an operator has. Back in the day, we'd take wind readings and record them on every job sheet. Today the ag nav systems can also record that information, as well as the point that the spray started and stopped on every pass, making for a very accurate, legal record of what was done....and what was not.
For operators that get paid to treat wildland, or christmas trees, or fertalize or re-seed a burn area, these systems also produce an accurate, legal record of exactly where the fertalizer, seed, or chemical went. As the airplane passes over washes and gulleys that may not need as much seed or any seed, the system records the dump gate or spray boom opening and shutting off, giving a precise record of what work was done. Some government agencies and some farms require this as proof that the work was done per a given contract, or as proof that they did indeed treat the land that they say they did.
For some operations such as the covert application of chemicals on drugs, these systems provide a very necessary record of what was treated, and what was not. Those who grow the drugs tend to be very vocal, surprisingly, about the collateral damage that they assert takes place by misapplication of chemical. The carriage of observers on the flights, as well as the records kept by these nav systems are meticulously recorded in order to show that the claims by the drug growers, producers, and the narcoterrorists that support them, are false. The nav systems also serve to ensure that the herbicides being applied are done so with precision and (there's that word, again) accuracy.
These systems are also very useful for other types of ag dispensing operations, from sterilized fruit flies to treatment of mosquito larvae in populated areas...not just for flying straight lines down fields that contain row crops.
Spraying row crops is relatively simple when one does a back-and-forth pattern along established rows of corn. Spraying perpendicular to the rows, or in fields that don't contain clear rows, is another matter. Spraying other patterns or multiple fields at the same time, can make spacing and evenness a very difficult proposition. With an ag nav system it's simplified and accurized. Each system comes with a number of pre-designed spray patterns built in, including traditional back-and-forth passes, race-track patterns, etc. The system is also entirely customizable to the aircraft and to the operator. An operator that wants to apply fertalizer on a mountain side by slowly doing a spiral descent around the hill can make it happen and apply the spacing by wingspan. An accurate measurement of a fire perimeter (and thus acerage) can be established. One can return to the same exact spot in a field or on a mountain when one runs out of herbicide or fertalizer, meaning no gaps in coverage. One can even pass that information on to someone else without any need for further target description, so they can return to the same spot.
Ag nav has made all the difference in the world, and really did revolutionize the ag business in many respects.