# Web Mapping for Farm Management Information Systems: A Review and Australian Orchard Case Study
## 2 Background
### Positional Accuracy of Web Imagery  
Web mapping thus provides an easy-to-use display of geographic information but not a precise geographic information system (GIS). Data collected using the standard WGS84 ellipsoid model is converted on the fly to the Web Mercator spherical model. There is no error at the equator in this conversion, but error increases at higher latitudes, and points can be offset by up to 43 km near the poles. This 'georeferencing' error is significant for high-resolution applications, even in near-equatorial positions. Therefore, Web Mercator is not recommended for use in navigation or relative positioning in official use by United States government agencies [50]. Another source of misalignment between WGS84 coordinates of GE imagery and GNSS survey lies in the registration of remotely sensed images to ground control points (GCP) and the compositing/mosaicking of these images, i.e., 'georeferencing (horizontal distortion)' and 'orthorectification (vertical distortion)' errors. Web tile images are an ortho-mosaic of images from different data sources and spatiotemporal resolutions. Web map providers rely on the image registration undertaken by the image providers. Ideally, this process involves stretching and warping of the image to achieve registration to ground control points (GCPs), e.g., if the camera view is oblique to the ground. However, the satellite imagery consumed by web map providers, such as GE, will not have been registered to GCPs. For example, the positional accuracy (RMSE) of GeoEye-1 images was documented to be 6.0 m on average, ranging from 2 to 9 m for panchromatic images of seven image sets [51]. Further, the mosaicking of images to produce a single image involves automated routines using feature matching, with the translating, stretching, and rotation of images used to match features. The process is not perfect, with positional errors remaining, particularly at high resolutions. The web map providers do not use mosaic images from different sources but rather 'composite' them (without feature matching). The merger of images is often visibly noticeable in web imagery. Positional accuracies (horizontal and vertical) and spatial resolution of web map tile images will, therefore, vary temporally (given image updating) with geographic location [52]. Web imagery providers do not provide information on the photogrammetric accuracy of their maps. Various researchers have reported that the positional accuracy of GE imagery varies by location [52; 53; 54; 55]. In these studies, the position of a number of 'ground control points' (GCPs) of known (WGS84) geolocations is compared to the location given for that point on Google Maps. For example, a horizontal positional accuracy (RMSE) of 1.73 m was documented in Khartoum across 16 checkpoints [53], while a horizontal positional accuracy (95% confidence level interval) of close to 1 m was documented in Rome from GE imagery across 41 checkpoints [54], and a horizontal positional accuracy (mean absolute error) of 0.13 m in the south and 2.3 m in the northeast of Montreal, Canada, with an overall RMSE of 1.08 m, was estimated using 10 checkpoints [52]. In Addis Ababa, Ethiopia, [PERSON] et al. [55] reported horizontal positional accuracy (RMSE) of GE imagery at 4.58 m with an error range of between 0.0125 and 5.0 m between GCPs. The RMSE on checkpoint coordinates from Google Maps and corresponding points on Orthophotos (1:4000 scale) in Thailand was reported as 3.3 m (a minimum error of 0.0 m and a maximum error of 28.6 m) [56]. In these examples, a higher error is not associated with a higher latitude, indicating that the Web Mercator projection is not the primary issue in these cases. It is likely that the results relate to the resolution of the available Google Map and the ability of the user to find a checkpoint on the Google Map. For example, as of 13 April 2023, Google Maps provides satellite imagery with maximum resolution that is similar to both Montreal and Rome but lower for Khartoum.