# Web Mapping for Farm Management Information Systems: A Review and Australian Orchard Case Study
## 2 Background
### A Primer on GNSS Datums
For applications requiring higher accuracy, it is critical that the WGS84 realization epoch and date of data capture ('coordinate epoch') be recorded as metadata (Appendix B) to allow for correct transformation to other coordinate systems. The metadata for the transformed dataset should also record the transformation method and parameters used. This ensures other users of the data are aware of the accuracy and lineage of the data. Epochs are recorded as year and decimal year, e.g., 1 January 2020 is 2020.000. Unfortunately, recording formats such as XML or JSON do not provide for such metadata. ISO19115, which defines the schema required for an enhanced description of the acquisition and processing of geographic information, including images [23; 24], should be updated to accommodate such a requirement. The geodetic datum used in China (GCJ-02) is based on WGS-84 but with the use of an obfuscation algorithm that adds random offsets to both the latitude and longitude. A GCJ-02 map will correctly display the location of a point with GCJ-02 coordinates, but a WGS-84 marker will be randomly offset between 100 and 700 m from the expected location on a GCJ-02 map. As required by Chinese law, there is no official API for conversion between GCJ-02 and WGS-84 [25]. As an alternative to the use of a global reference frame, such as WGS84 or ITRF, a local reference frame can be used to provide a better model of the shape of Earth's surface in a portion of the globe. For example, Australia has implemented the Australian Terrestrial Reference Frame 2014 (ATRF2014) as a dynamic local datum. National data are often mandated for use in activities, such as national mapping and cadastral surveying. However,'static' objects are changing in global position due to continental drift. Australia is the world's fastest-moving continent, drifting to the northeast at approximately 7 cm per year, with a much smaller intercontinental movement [26]. Thus, the position of a feature such as an orchard boundary will have shifted by approximately 2 m in 30 years. For operational convenience, location data can be reported in terms of position at a set date using a'static' (also known as 'time independent' or 'plate centric') datum. For example, Australia operated on the Geocentric Datum of Australia 1994, or GDA94 (with epoch 1999.000), until 2017, when GDA2020 became available [27]. GDA2020 represents locations on the Australian continent as of 2020.000, using the ITRF 2014 at epoch 2020.000. Thus, high-resolution GNSS measurements are made in terms of a global reference frame, such as WGS84 (G1762), but the data can be presented in a product, e.g., for use in FMIS, in the national geodetic datum. In Australia, both the static datum of GDA2020 and the dynamic datum of ATRF are recognized by the Australian National Measurement Act as standards for measurement of position [28]. Australia supports both dynamic and static data (ATRF and GDA2020, respectively) to cater to the needs of both plate- and global-centric users. A number of countries for which continent drift is not as great an issue have implemented only a local and not a static datum [21]. The parameters for transformation between a given pair of reference frames, e.g., realizations of GDA94, GDA2020, ARTF2014, and WGS84, are described by an 'EPSG' number [29] (Figure 1). There are over 6000 EPSG parameter sets, reflecting the myriad of reference frames used globally [30]. The use of an inappropriate transformation in moving between reference frames will introduce spatial error. For example, if a position reported in a static datum, such as GDA94, is transformed to a dynamic datum, such as WGS84, the point will be represented by its position in the dynamic datum as of 1994.000, unless an additional correction for the date of data collection and continental shift is made (as shown in Figure 1 for the transformation from GDA2020 to ITRF2014).