| How Precise are Parallel Swathing Systems
By: Roz Buick In a toss-up between conventional agrichemical application guidance systems and GPS guidance systems, which comes out on top?
Trimble Navigation's Precision Agricultural Systems (PAS) group has been studying application or swathing efficiency in an attempt to better understand and quantify it, as well as to find ways of optimizing swathing efficiency for farmers and custom applicators. Global positioning systems (GPS), geographic information systems (GIS) and variable rate treatments are just some of the technologies coming onto the scene to allow farmers and custom applicators to achieve precise application rates or planting densities to different parts of a field. Ultimately, the goal is to maximize crop production and economic returns from a field while ideally also reducing environmental impacts. This is done primarily by developing an improved "picture" of field variability in order to better manage it. However, variable rate management is only one aspect to consider in maximizing crop production and farm efficiencies.
If farmers are investing in such variable precision management, they must be guaranteed optimal application coverage for every application. Not only the appropriate rate changes must be made in the right places, but efficient application coverage of the field is expected. In other words, agricultural applications should minimize the occurrence of double applications (overlaps) and missed applications (skips), maximizing the overall swathing efficiency for every field treatment. Guidance Systems on Trial
An initial trial was conducted in an intensive cropping region of New Zealand, on the 1000 acre intensive cropping farm of Roger and David West, West Brothers Ltd. Comparisons were made using a number of guidance techniques including a traditional foam marker system and two types of GPS-based guidance. Efforts focused on evaluating variables that could influence overall swathing or application efficiency.
The West brothers donated the use of their large spray vehicle mounted with a 78 feet (ft) or 24 meters (m) spray boom and spray equipment (AirTech Spray Systems) and an in-cab spray application controller (RDS). One guidance technique employed a foam marker system, which was installed onto the spray truck. The two other guidance techniques used GPS and a lightbar device for guidance; one technique interfaced the lightbar to a sub-meter accuracy Differential GPS receiver and the other interfaced the lightbar to a centimeter accuracy RTK (Real Time Kinematic) GPS receiver. With all three guidance techniques an RTK centimeter accuracy GPS receiver was used to track the actual path that the spray vehicle traveled (i.e., this represented the "truth" about where the vehicle drove in each guidance treatment). A computer on-board the vehicle was interfaced to the RTK GPS receiver to collect data at one second intervals for key variables; such as: distance offline, vehicle ground speed, and actual vehicle position. David West drove the spray vehicle since he was accustomed to both foam marker guidance (ten years of experience) and lightbar guidance (over twelve months' experience). Conditions during the testing were ideal for foam, a sunny day with no wind and the field was very flat with a bean crop residue left on the ground which made the foam marker blobs highly visible. Foam Marker Guidance vs. GPS Guidance
Foam marker guidance involved the driver positioning the end of the spray boom and foam cone blobber so that foam blobs dropped onto the trail of foam blobs made during the previous swath. GPS guidance involved defining two points; an "A" and a "B" point approximately 1640 ft. (500 m)apart in a field, from which an A-B line was drawn. Based on the spray boom width programmed into the GPS guidance systems, parallel lines were created from the A-B line. When exactly on-line (center of swath line), the center three LEDs lit up on the lightbar. As the operator drove off-line (i.e., to the left or right of swath center), the illumination band moved out from the center LEDs. The operator had to steer to bring the illuminated LEDs back to center.
The lightbar was situated so as to be seen in peripheral vision by the operator. Approximately ten swaths were sprayed for each of the three guidance treatments. The centimeter GPS guidance used two different settings on the lightbar guidance. Each LED spacing on the lightbar could be adjusted. This spacing influenced the sensitivity of the lightbar to movements off-line. In this study the centimeter GPS guidance method was used with the first LED spacing of 6 inches or 16 centimeters between each LED. A second setting of 3 in. (8cm) was used to see if lightbar sensitivity influenced guidance and swathing efficiency at all.
A number of techniques were explored to analyze the recorded data. Application overlap and skip were calculated for each pair of adjacent swaths and expressed as a percentage swath area. Overlap and skip for the whole field application were also calculated by summing up all areas of overlap or skip and expressing it as a percentage of swath area. The Results
Both foam marker guidance and centimeter GPS guidance showed vehicle ground speeds which were slower on initial swaths driven. By later swaths in the field application, the ground speed of all three treatments were similar and within approximately 6 - 8.5 mph (10-14 km per hour) speed range setting on the RDS spray controller. It seemed to take a few swaths before the operator became accustomed to the foam marker and the centimeter GPS guidance techniques. This was not the case with the sub-meter GPS guidance method that had a relatively constant ground speed across all swaths. Sub-meter GPS guidance was less prone to lights moving off-center on the lightbar allowing the driver to maintain a faster average ground speed. Centimeter GPS guidance was more difficult to steer manually, particularly on the first few swaths when higher accuracy and a smaller spacing between each LED was used.
No significant differences were found in actual chemical applied per swath within each guidance treatment nor between each guidance treatment, as measured by the RDS spray controller at the end of each swath.
Average distance offline (cross-track error) was recorded from the set of swaths which were pre-generated from the original A-B line. Across the entire field application (nine swaths plus the original A-B line) the foam marker showed an average distance offline of 5.3 feet (1.61 m), while the sub-meter GPS guidance system displayed an average of 1.9 ft. (0.57 m) offline distance. Average distance offline was lowest with the centimeter GPS guidance method at 0.8 ft. (0.23 m). With foam, the offline distance from pre-generated swaths continued to increase due to the higher overlap which took place. It is important to mention that given the high degree of overlap occurring with foam, the real cross-track error on each swath should have been measured from the previous swath driven. When measurements of offline distance are made from swaths generated from the original A-B line, by the time you get to the higher swaths, this offline distance is not a true measure of offline distance on each swath driven. Initial offline distances measured on the earlier swaths is one way to estimate actual offline distance to compare between guidance treatments. Foam offline distance on the first swath was comparable to that of meter GPS guidance (1.5 ft. for foam and 1.3 ft for sub-meter GPS). Average offline distance for centimeter GPS guidance along the first swath was the lowest at 0.7m.
By the time the 9th swath was reached, the foam application was being made at 10.5 ft. (3.2 m) offline from the ninth pre-generated swath. This means that in a large field using a 78 ft. spray boom and a foam marker, an entire extra swath would be made due to this overlap after each 67th swath (i.e., 78 ft. multiplied by 9 swaths, divided by 10.5 ft. = 69 swaths) assuming constant driver performance. After many hours of driving with a foam marker, the driver performance is unlikely to improve. Furthermore, these findings were carried out using close to optimal conditions for foam and less than optimal conditions for sub-meter GPS; the reasons for this are discussed further below.
When it came to overlap occurrence or overlap percentage between each pair of adjacent swaths in the field application, the foam marker system registered a higher degree of overlap than the sub-meter GPS guidance technique. The overlap percentage of the foam marker system was also markedly higher than that of the centimeter GPS guidance technique. Average field overlap percentage and skip percentage (i.e., across all pairs of adjacent swaths) for all three guidance treatments resulted in a 95 % confidence limit. Foam exhibited the highest percentage of overlap (2.04%) while centimeter GPS guidance exhibited the lowest percentage of overlap (0.59%). Sub-meter GPS guidance resulted in overlap values between the other two treatments (1.00%). Based on these findings, an economic perspective can be derived. If one had 1000 acres growing crops which required five agrichemical applications which cost an average price of $100 per acre for each agrichemical application, the savings in reduced application overlap with sub-meter GPS guidance would be $10,400 that year. An equivalent comparison between foam and the centimeter GPS guidance system would save the user $14,500 in a year.
Overlap is only one part of swathing efficiency. The equally important and related variable to overlap is the occurrence of skip or missed areas. Across all treatments the differences in skip were not significantly different in the statistical analyses. However, the foam marker technique showed a relatively low average field skip (0.35%) which naturally complements the high overlap seen with foam (2.04%). In other words, if you are overlapping more on each swath then you would expect to see a lower occurrence of skip. The sub-meter GPS guidance method showed an average percentage skip of 1.46%, which although higher than foam was statistically the same as the foam skip occurrence. Centimeter GPS guidance showed a skip percentage of 0.77%.
Missed application areas or skips are much more difficult to provide an economic perspective on. In fact potential losses in crop production due to application skips depend on each crop and chemical combination. Some chemicals may cause significant damage if double applications are made to crop plants (e.g., some herbicides) while double applications of other chemicals will not show obvious bad effects on crop plants (e.g., fertilizers). It is believed that operators and farmers can benefit from greater flexibility in the way spreading and spraying applications can be made to suit each situation. By being able to adjust the swath width setting in the GPS guidance system (e.g., to be slightly smaller than the actual boom or spreader spinner bout width size), the operator can aim to minimize skip and consequently avoid "striping" or misses in the crop while accepting a slightly higher overlap. GPS guidance systems provide this ability to consistently control and drive applications to suit the type of application being carried out. It is much more difficult to achieve this consistently with a foam marker throughout an entire field application.
High variability was found in some of the data sets in this study. Some explanations for these large variations within the sub-meter GPS treatment are believed to be due to a relatively lower accuracy signal for satellite differential GPS that has been recorded in New Zealand compared to North America and Australia. This contrasts with the very "tight" data (small error bars and low variability) where the centimeter RTK GPS receiver obtained its real-time corrections from an in-field base station that was set up. The relatively high variability in the foam marker overlap percentages is believed to be related to observations seen on the ground where approximately a 3-6 ft. (1-2m) wide band in which the foam blobs were often dropped onto existing foam blobs from the previous swath. This was under conditions that were considered optimal for a foam marker. For example, the day was sunny without any wind, the field was very flat and without undulations that would increase boom bounce and misplacement of foam blobs, and the field had a low-lying bean crop residue which made it very easy to see the foam marker blobs. Conclusion
The West brothers have made a number of observations based on their use of sub-meter GPS guidance (the Trimble AgGPS132 Parallel Swathing System) after 10 years of experience with foam markers. "We've noticed how foam markers accentuate any errors made during the application which are propagated across the rest of the field. We like the way we are brought back onto correct lines each swath with GPS lightbar guidance", says David West. The Wests believe that it is human nature to be conservative when using a foam marker, over-dropping the foam onto existing foam, so it did not surprise him that the occurrence of overlap was higher with the foam. However, there are other reasons why the Wests prefer GPS-lightbar guidance. "We no longer need to add dyes to foam marker tanks to improve visibility in the wide variety of specialty crops that we grow. We also like being able to finish the job in the evenings when the sun disappears or start earlier in the mornings. Foam marker blobs often fall between plants in many of our taller crops making the foam difficult to see. With some of our specialty seed crops we must avoid spraying chemicals when beneficial pollinating insects such as bees are working - now we can spray at night!"
The study found sub-meter GPS guidance to be at least as effective as the foam marker in terms of swathing efficiency expressed as application overlap and skip. This was observed under conditions close to optimal for foam markers and under reduced accuracy conditions for the sub-meter GPS. Centimeter GPS guidance showed higher swathing efficiency than foam marker and sub-meter GPS guidance. Having established a series of techniques to analyze swathing efficiency, future research is being planned to repeat similar application efficiency studies under a variety of agricultural spraying and spreading equipment and conditions. About the author:
Roz Buick works for Trimble Navigation New Zealand Limited where she is a product marketing manager. Back |
