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

From: Frans Knibbe <frans.knibbe@geodan.nl>
Date: Wed, 17 Feb 2016 14:17:00 +0100
Message-ID: <CAFVDz42kqNSJkhxySY4RXfi0eJG_yJm4hveHC4e_+pv0gwTLBw@mail.gmail.com>
To: SDW WG <public-sdw-wg@w3.org>
Cc: "Little, Chris" <chris.little@metoffice.gov.uk>, Jeremy Tandy <jeremy.tandy@gmail.com>
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.
>>
>>
>>
>>
>>
>
Received on Wednesday, 17 February 2016 13:17:33 UTC

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