# Web Mapping for Farm Management Information Systems: A Review and Australian Orchard Case Study
## 5 Discussion  
Imagery is useful for the operationalization of FMIS, for the location of farm features, e.g., orchard boundaries, and for the visualization of spatial data. The use of publicly available web mapping resources, such as GE, is driven by the convenience of the availability of these resources. Given this convenience, the use of such resources is expected to increase. However, these resources are compromised in applications that require sub-meter spatial resolutions. In the case study of the current study, the average RMSE on GCP positions was 5.930 and 3.432 m for within-farm and between-farm data, respectively; for the most recent (2023) GE web imagery, 4.334 m across historical (post-2015) GE web images of one farm and 3.036 m between the most recent images from four web imagery service providers.  
There are several sources of error in the records of location data to FMIS: (a) survey measurement, (b) tectonic shift, and (c) mapping. Survey error depends on the methodology and technology used, e.g., standalone, dGNSS, or RTK. Errors are also introduced due to the use of the WGS84 ensemble dynamic datum, which ignores the tectonic shift. The misfit vector due to the tectonic shift (at approx. 6.5 cm/year for the Australian plate) can be ignored for measurements spanning a few years but not for longer periods in applications requiring sub-meter level accuracy. This misfit will vary with location but is well modeled and thus can be corrected.  
Map errors include problems with image registration and rectification, as well as errors involved in the compositing/mosaicking of images, i.e., 'georeferencing (horizontal distortion)' and 'orthorectification (vertical distortion)', image processing and mapping, and the earth model employed. For example, GNSS data are collected using an ellipsoidal earth model, but data are converted on the fly to the web Mercator spheroid earth model for display on web maps. Another type of map error is associated with the spatial resolution of the image. The positional errors or misfit vectors were calculated based on the availability. The resolution of publicly available web imageries assessed in this study varied spatially and temporally. For example, GE web imagery from Maxar Technologies in Central Queensland (2016) has a pixel dimension of 0.15 m, while CNES/Airbus in Far North Queensland (2021) was 0.30 m. Higher image spatial resolutions are associated with higher location accuracies of GCPs and thus image registrations.  
The publicly available web map services are thus impressive for their global coverage and image currency but suffer significant positional accuracy issues in the context of farm management applications.  
Figure 8: Example of GE web imagery for a mango orchard (latitude: \(-\)12.7380033, longitude: 131.1731937). Left panel doa 5 July 2023; right panel doa 8 July 2023. In both cases, the imagery has two data sources, with the right side of both images sourced from Maxar, July 2022, and the left side of the images from CNES/Airbus, April 2019 (**left panel**), and Airbus, May 2022 (**right panel**). Red dots on the right panel represent the position of the crown center of the end trees of each row in GE (CNES/Airbus) imagery from April 2019.