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

Thanks Chris - I've incorporated your suggestions in the Editors Draft and
added your comments to the respective issues.

Jeremy

On Thu, 14 Jan 2016 at 16:20 Little, Chris <chris.little@metoffice.gov.uk>
wrote:

> 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 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.
>
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>