| Field Boundary Mapping and Rock Marketing
Innovation helps expedite a farmer's return-on-investment
By: Art Dalton and Ron Bell In this time of increasing input costs and decreasing commodity prices, farmers are looking for new ways to increase efficiency and cut costs. Precision farming equipment such as yield monitors and variable rate controllers is expensive and, if a producer decides to make the investment, the equipment must be able to be used in as many ways as possible.
Most precision farming experts recommend 3-years of monitoring before starting full-scale variable rate application. This means that the pay-back period for the yield monitor and GPS will be at least 3-years... unless producers find additional ways to make their precision farming equipment pay for itself. Two such applications -- field acreage mapping and rock marking/removal were proven to provide immediate benefits. Field Acreage Mapping
The exact size and dimensions of a field are important for farmers and their fertilizer/chemical dealers for several reasons, including input cost estimates, rental payment calculations, ordering fencing supplies, determining chemical and fertilizer requirements, planning land clearing and drainage work, yield calculations, etc. Having an actual 'picture' of a field which can be photocopied and given out to custom applicators and bush clearing contractors can eliminate confusion regarding which portion of a field to spray or clear.
Since 1995, Prairie Geomatics, in Birtle, Manitoba, Canada, has been helping producers and agricultural dealers acquire and efficiently use geomatics tools such as GPS, computer mapping programs, air photos and satellite images. "What is the exact size of this field?" is a common question asked by dealers and producers. A good estimate of field size can be made by using georeferenced air photos on a computer. Using a commercially available computer mapping program, the computer mouse is used to trace or outline the field boundary. The computer then uses standard geometric calculations to compute the physical area within the boundary or polygon. This method works well for many fields, however there are two problems: One is the lack of recent photography which is needed if any new land clearing has taken place, and the other is that sometimes the nature of the ground cover at the time of the photo makes it difficult to determine the exact edge of the cultivated area. To solve this problem, Prairie Geomatics uses a method referred to as real-time GPS mapping.
In simple terms, real-time GPS mapping involves driving or walking the perimeter of a field with a GPS system which records map coordinates (latitude, longitude or UTM easting/northing). The resulting coordinates are downloaded to a computer back at the office and brought into a computer mapping program which calculates the exact acreage, as well as other dimensions such as perimeter length. Field Procedure
The GPS data collection interval is set to record a point every one or two seconds; this creates a variable resolution outline as the operator slows down along the curved boundary and speeds up along the straight sections. GPS mapping is turned off and on again as the operator drives to the interior sloughs and/or bluffs that characterize most fields on the prairies. The operator uses the on-the-go mapping feature of the GPS unit to display the outline map on the screen while driving or walking - this ensures that no bluffs or sloughs are missed, and that the data is collected right back to the starting point on the field boundary. For practical reasons (such as saving one's truck exterior from barbed wire, rocks and tree branches!) it is not advisable to drive the actual field boundary, therefore a set offset is chosen (6 feet or 2 meters) and the roof-mounted GPS antenna is kept at this standardized distance just inside of the actual edge of the field.
Office Procedure
The GPS unit is removed from the quick-release vehicle bracket and brought into the office. A data cable is attached between the GPS and the serial port of the computer, and a simple download program is used to copy the boundary points to a file on the computer and save the data in a standard 'vector' file format.
A computer mapping program, in this case, MapInfo Professional, can be used to create the maps and calculate acreage. There are several steps required, including moving the last points in each boundary to create closed polygons (since you can't measure the area of a line!), expanding the polygon by the offset distance, and finally removing the bluffs and sloughs 'islands' so these are not included in the acreage value for the cultivated part of the field.
The final map can be enhanced with a scale bar, field names or any other desired information and then printed using any standard computer printer. The computer file is available for use with precision farming, crop record keeping or word processing programs, etc. Accuracy
The GPS coordinates in this case were accurate to approximately 2 meters(m), since we used a differential correction receiver and a local GPS base station. This accuracy was verified in several ways including driving back over a previous track while viewing the map display on the GPS, occupying known benchmarks, and by overlaying the field maps onto independently georeferenced digital orthophotos. A close match between the separately georeferenced GPS track and the orthophoto base map indicates very good GPS position accuracy.
Other methods of measuring field acreage are subject to various inherent errors and, as a result, will not be as accurate as the GPS mapping method described above. Rock Marking and Removal
Ron Bell, a producer in the Isabella/Birtle area, has used a yield monitor for the past three harvest seasons. Yield maps and load summaries are the main data collected - the yield data will be used in the future for planning variable rate inputs, while the load summaries and moisture levels are currently used to evaluate test plots, and for several other on-farm management tasks.
The yield monitor does have another function as a field marker which allows four different features to be marked by pressing one of four buttons on the monitor during harvesting. This season, two of the markers were dedicated to marking rocks; one for surface rocks and one for buried rocks which would require more than just a rock picker to remove. Rocks are a recurring hazard which can cause expensive delays during harvest and it was felt that any improvement in the ability to easily mark and remove rocks would be beneficial in terms of time and money.
A 12 channel, hand-held GPS navigator was purchased and installed in a Bell Farms tractor. The differential receiver, normally used in the combine with the yield monitor/GPS system, was removed and installed in the tractor to give the GPS unit the required increase in accuracy. A new GPS unit was acquired since it has a graphical display screen with navigation features that are required for the task of real-time navigation to the rock locations. The GPS unit purchased 3-years earlier for the yield monitor system in the combine is a 'black-box' type with no display screen. The price of a 12 parallel channel, differential-ready GPS units with built-in map and navigation screens has come down dramatically and was not a big factor compared to the total investment in precision farming equipment. The older yield monitor GPS has a separate differential receiver that made it easy to remove and use in another vehicle-in this case the tractor with the new display-screen GPS. Field Procedure
Rocks, both surface and buried, were marked during harvest using an AgLeader 2000 monitor with the optional field marker. The rock locations were stored along with all of the yield and moisture information on small PC (PCMCIA) cards which were removed from the monitor and downloaded onto the office computer every few days during the harvest season.
The new GPS unit was installed in the tractor during post harvest anhydrous application, and was used to collect additional rock locations on-the-go using the 'mark' button. Office Procedure
The Ag Leader Precision Map 2000 program was used to plot maps for each field showing the two types of rocks. The latitude/longitude coordinates were read off of the computer screen and manually transferred to the GPS unit as a set of 'way-points'. This task was time consuming, but can likely be accomplished in a more automated manner once the correct procedure is worked out. Rock Picking
The GPS unit, with rock locations stored internally as 'way-points' or specific point locations with a label, was used in the tractor with the rock picker (in this case a Degelman). The 'goto' button was used to select the rocks (which were simply numbered sequentially) and bring up the navigation screen which showed the current direction of travel, current distance to the rock and a large arrow pointing to the rock location. The tractor was turned while traveling so the arrow pointed straight ahead, then the operator monitored the displayed distance to the rock and slowed down as the distance got below 100 feet. The rock typically came into view sometime in the last 50 ft. and was then picked up by the rock picker. Rock way-points were deleted as they were removed. The map display on the GPS was used to view the field with the remaining rock locations and the next rock slated for removal was selected as the next navigation target. The GPS made it easy and efficient to get to each rock in the field, and ensured that no problem rocks were missed. Summary
Acreage measurement and rock marking/retrieval are new uses for precision farming GPS systems. They add value and decrease the pay-back time for this investment in new technology. The two examples were developed by a producer and a precision farming consultant, and illustrate the benefits of collaboration which is necessary to take advantage of new technologies. About the authors:
Art Dalton owns and operates Prairie Geomatics Ltd. in Birtle, Manitoba, Canada. He has worked with producers and agricultural dealers since 1995, helping them acquire and efficiently use GPS, computer mapping programs, and remote imagery in crop monitoring. Ron Bell farms in the Isabella/Birtle area in Manitoba, Canada. Back |
