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

No I disagree... For the vast majority of the current uses of spatial data
on the web WGS84 is used without problem, as the GIS/Geospatial community
we have still not accepted that we are the minority, the people with very
specific use cases.. For 99% of the potential uses of spatial data on the
web today WGS84 is fine.

Ed


On Wed, 17 Feb 2016 at 13:18 Frans Knibbe <frans.knibbe@geodan.nl> wrote:

> Hello,
>
> About the phrase " For the majority of applications a common global CRS
> (WGS84) is fine": If we say it like that, it seems we are saying that it
> is OK to use WGS84 as a default CRS. I think that would be the wrong
> message. I think it should be only used in some very specific cases: in
> which spatial resolution is higher than meter level and data are usable for
> a limited time. For other cases, WGS84 or CRS84 should not be recommended.
> In my mind, a best practice would be to *not* use WGS84/CRS84 *unless*
> you are certain the data have a sufficiently low spatial resolution and
> temporal validity.
>
> I think a general good practice for data publishing is not to make too
> many assumptions on how data will be used (applications). Some applications
> will not be hurt by wrongfully using WGS84, but you can not be sure that
> other types of data usage will not suffer.
>
> In sessions last week, I have seen examples of geomerty encodings that
> were all wrong. In combination with CRS84 extremely high coordinate
> precisions were used and no temporal metadata were present. Lots of
> antipatterns!
>
> Regards,
> Frans
>
>
>
> 2016-01-14 18:55 GMT+01:00 Jeremy Tandy <jeremy.tandy@gmail.com>:
>
>> 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.
>>>
>>>
>>>
>>>
>>>
>>
> --

*Ed Parsons*
Geospatial Technologist, Google

Google Voice +44 (0)20 7881 4501
www.edparsons.com @edparsons