Chris Little's comments on the BP - Issues 128 and 204

Jeremy,

A little late for the BPFPWD, but some text to address issues 128 and 204. In American English.

Chris

Why
The choice of CRS<file:///C:\_ChrisLittle\International\sdw_index.html%20at%20eab791e6b938979f03e20e6d554068d8c7c5d0b8%20%C2%B7%20w3c_sdw%20%C2%B7%20GitHub.htm#dfn-crs> is sensitive to the intended domain of application for the geospatial data. For the majority of applications a common global CRS (WGS84) is fine, but high precision applications (such as precision agriculture, digging holes in roads and defence) require spatial referencing to be accurate to a few meters or even centimeters.

One aspect is the confusion of precision and accuracy. Seven decimal places of a latitude degree corresponds to about one centimeter. Whatever the precision of the specified coordinates, the accuracy of positioning on the actual earth's surface using WGS84 will only approach about a metre horizontally and may have apparent errors of up to 100 metres vertically, because of assumptions about reference systems, tectonic plate movements and which definition of the earth's 'surface' is used.


Issue 128
Add explanation of why there are so many CRSs.

For example, North America and Europe are receding from each other by a couple of centimeters per year, whereas Australia is moving several centimeters per year north-eastwards. So, for better than one meter accuracy in Europe, the European Terrestrial Reference System 1989 (ETRS89) was devised and it is fixed with respect to the European tectonic plate. Consequently, coordinates in the ETRS89 system will change by a couple of centimetres per year with respect to WGS84.

Issue 204
Need to clarify when and why people use different CRS's

Even if a CRS, tied to a tectonic plate, is used, local coordinates in some areas may still change over time, if the plate is rotating with respect to the rest of the earth. Many existing useful maps pre-date GPS and WGS84 based mapping, so that location errors of tens of metres, or more, may exist when compared to the same location derived from a different technology, and these errors may vary in size across the extent of a single map.

Note
The misuse of spatial data, because of confusion about the CRS, can result in catastrophic results; e.g. both the bombing of the Chinese Embassy in Belgrade during the Balkan conflict and fatal incidents along the East Timor border are generally attributed to spatial referencing problems.
Intended Outcome
A Coordinate Reference System (CRS) sensitive to the intended domain of application (e.g. high precision applications) for the geospatial data should be chosen.