# Web Mapping for Farm Management Information Systems: A Review and Australian Orchard Case Study
## 2 Background
### A Primer on GNSS Datums  
Location data collected across a site over time could be referenced to a local reference point or datum. Such data would be internally consistent, avoiding the need to consider continental drift, but would not allow for the import of external datasets based on other data. The different GNSS positioning systems use different 'datums', i.e., GPS WGS84 (World Geodetic System 1984), GLONASS PZ-90, Galileo GTRF, and the BeiDou Coordinate System (BDC). These datums provide an 'absolute' (also referred to as 'dynamic', 'time dependent', or 'earth-centric') position of any point on Earth's surface. In these datums,a given measurement will appear to shift on a map with time due to continental drift. In practice, these data do not continuously accommodate continental plate movement, but provide yearly step changes (benchmarking to ground control point positions on the 1 st of July each year). These reference frames are based on an estimate of Earth's center of mass, a reference ellipsoid used to represent the shape of the Earth, and a geoid, which is the equipotential surface of the Earth's gravity field and is estimated using an earth gravitational model. The geoid is used to determine the height/elevation of a given point on the surface of the Earth and is generally expressed as elevation relative to mean sea level (MSL). The various data differ in their accuracy of fit to Earth's surface; they are optimized in terms of representing the whole globe or a part of the globe. Transformation parameters are used for the transformation of coordinates between different reference frames. There are, however, several versions ('realizations') of each of these reference frames, and attention to the realization in use is important for sub-meter spatial measurements. An international consortium has maintained the International Terrestrial Reference System (ITRS) since 1991. The ITRS consists of procedures for creating reference frames, such as a series of implementations of this system, known as the International Terrestrial Reference Frames (ITRFs), the latest of which is ITRF2020. Navigation systems, as used by the various GNSS, are generally referenced to an ITRF solution. For example, the latest update of the GLONASS reference system was in December 2013 (PZ-90.11) [18]. The transformation from PZ-90.11 to ITRF2008 involves only a shift, without rotation or scale. The Galileo navigation system utilizes the Galileo Terrestrial Reference Frame, which is aligned to new ITRF realizations, with (2\(\sigma\)) differences of less than 3 cm [19]. The China Terrestrial Reference Frame (CGCS) 2000 is referred to as ITRF97 with the epoch of 2000.0. As the foundation system, the WGS84 coordinate system is widely used. WGS84 was implemented in 1987 with six successive refinements, each using more accurate coordinates of the reference stations. However, [PERSON] et al. [20] note the changes in WGS4 are 'not well known in the geospatial community', with many users failing to record which realization data have been captured in WGS84 (G1762), introduced in October 2013, which was reported to have an accuracy (1\(\sigma\)) of 0.5 cm relative to ITRF2008. The latest WGS84 realization, G2139 (released on 3 January 2021), aligned with ITRF2020. The difference between WGS84 and ITRF realizations is mainly due to the use of a different set of base stations by the two systems [20; 21]. If the WGS84 realization used in data capture is not recorded, the data are said to be captured in the 'WGS ensemble'. If the date of data collection is not recorded, no correction can be made for tectonic motion, which is required for a comparison of the measurements made in other years. Geosciences Australia reports the accuracy of the WGS84 ensemble to be between 2-5 m in Australia [22], with uncertainty to increase with time given tectonic movement.