6. METHODOLOGY
6.4 Field Data Processing and Analysis
The GNSS field survey from the Trimble® GNSS receiver were processed and corrected to enhance positional accuracy of the surveyed positions using Pathfinder Office® software and TrigNet GNSS base station data. The data output from Pathfinder Office® consisted of the corrected positions of the manually surveyed lines. These lines were used to indicate the position of field edges, intermittent crop rows and vehicle wheel positions. This information was exported into ESRI® ArcView® GIS software and multiple GIS shape file layers for each data type created for further processing and analysis in the Quantum® GIS software package.
Interim processing and generation of CAD related data were conducted on the AutoCAD® Civil3D® software package. This interim CAD processing included the generation of lines to represent the centreline position of all crop rows for the entire field and all infield traffic movements. This consisted of generating and distributing line entities evenly between the intermittently surveyed crop rows to match and represent each row within the field. Surveyed
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lines representing the traffic of a single wheel through the field also needed to be offset for the corresponding alternative wheel. The wheelbase of the equipment was used as the distance offset to create the corresponding parallel track. Such functions are easily conducted using CAD drafting software. The set of completed CAD files consisting of line entities representing field, crop and separate vehicle traffic layers were then exported for analysis in the GIS software.
Within GIS, a polyline enclosing an area is able to be converted into a polygon area. This conversion was required where a polyline was used to represent, for example, a field edge or boundary and then be converted into a polygon area to represent the field area. This polygon area is required for field based queries and analyses to be conducted. The importing of field boundary lines and generation of field areas would typically be the first step in the GIS process.
The next step would typically consist of importing the line entities representing field and crop attributes and vehicle movements. These imported polylines from CAD, however, would need to be widened to account for the width of the rows and width of wheels for the range of equipment represented. A GIS processing technique termed “Buffering” allows for the propagation of areas surrounding drawing entities. This buffering process was used to generate an area of set distance (half of the width of a crop row or wheel width) from the polylines within an entire layer. For the purpose of this study, a width of 0.4 m for a typical sugarcane crop row was assumed to contain the majority of cane stools, although this may vary in practice depending on crop age, plant populations and row spacing configurations.
Figure 6.8 gives an indication of the width of cane rows typically found infield. The lines representing crop row centrelines were thus buffered by 0.2 m to represent a crop row width of 0.4 m, for example.
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Figure 6.8 Determining the typical width of cane rows. The variance in crop row width is visible along the cane row (left) and can be as wide as 600 mm in places (right). A crop row width of 400 mm was assumed for analysis purposes To create the crop inter-row areas, the row areas were subtracted from the polygon area representing the entire field. The entire field area was thus separated into two sub areas consisting of the rows and inter-rows, thereby allowing for row and inter-row area queries and analyses to be conducted.
The position of infield traffic movements had also been represented as line entities. In a similar way, these lines representing the position of wheel tracks and infield traffic also needed to be buffered to correspond to the width of the equipment tyres. The width of the tyre, as displayed in Figure 6.9, is required to determine the areas affected by traffic and further analyses to be conducted.
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Figure 6.9 Excerpt from a map of infield traffic areas generated by the buffering of surveyed polylines to match the tyre widths of equipment used infield
The traffic layer needed to be further separated by position of either row or inter-row traffic components. In GIS there are data processing tools that allow for the intersection of vector based layers. By intersecting the traffic layer with the crop row layer, a new layer was created that consists only of traffic that occurred over the rows in the field. Similarly, the traffic layer was intersected with the crop inter-row layer. This allowed for the entire field to be categorised into areas where row traffic, inter-row traffic or no traffic had occurred. Various integration and intersections of layers were required to finally produce a map as shown in Figure 6.10 that distinguishes between areas of row traffic, inter-row traffic or where no traffic had occurred for the range of equipment used in each system.
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Figure 6.10 Traffic positions for a single stack self-loading trailer system further classified into row or inter-row traffic components, where the darker colours represent row traffic and the lighter colours represent inter-row traffic areas within a field
GIS software allows each layer to be analysed separately. The total area of all polygons that exist in a particular layer can be determined. In this way, each layer representing vehicle traffic within a particular system was analysed to determine the total area of row traffic, inter- row traffic and where traffic did not occur within the field. Having determined the total row and inter-row area of a field, the proportion of rows where traffic had occurred and the proportion of inter-rows that had been trafficked were determined. An estimated field based yield loss for each vehicle in the system was determined by multiplying the ‘point of impact yield losses’, as determined from the synthesis of literature in Chapter 5, by the proportion of row and inter-row trafficked within the field.