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

Hello Ed,

Basing requirements on what we observe on the web today could be risky. I
hope that with the Best Practices we can lay the foundations of a better,
more interoperable and more robust web in the future. Some things I think
we should consider in this case are:

   - I believe the lack of a generally usable CRS impedes publication of
   professional geographical data. And there are other reasons why data
   providers need (our) best practices before publication of spatial data on
   the web becomes a viable option. So I think the 99% is a skewed proportion:
   Relatively simple, amateuristic data are easy to publish and use on the
   web nowadays. That could be the reason for seeing so much of it. For
   professional, serious data it is another matter.
   - Current provisioning of data in many cases may be on the web but is
   still monolithic in design: one database for one application. The idea of
   putting data on the web for everyone to use as they seem fit requires a
   higher level of data quality, among which is using a CRS that is not
   only usable for a specficic application.
   - Current provisioning of data on the web often does not consider
   durability of data. With an ever increasing coordinate error, using CRS84
   does not fit well with the idea of providing long lasting data.
   - A good generally usable CRS does not necessarily have to be something
   more complex than WGS84/CRS84. It could be something simpler.

Regards,
Frans

2016-02-17 15:06 GMT+01:00 Ed Parsons <eparsons@google.com>:

> 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
>