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

2016-02-18 9:20 GMT+01:00 Jon Blower <j.d.blower@reading.ac.uk>:

> Hi Ed, Frans, all,
>
> [snip]
>

> One constant area of pain is the discontinuity at 180 degrees. Services
> and software are very inconsistent about how they handle data that span
> this discontinuity. Many people (particularly those interested in the
> Pacific Ocean!) have expressed a need for a CRS where the discontinuity is
> defined elsewhere (or one in which the longitude axis values are explicitly
> allowed to “wrap”).
>

This is a remark I find interesting as a UCR editor, because it looks like
a requirement for spatial data on the web (graceful handling of the 180º
discontinuity). Do you think such a requirement should be included in the
UCR document?

>
> Polar science is another area where a cylindrical CRS is not ideal.
>

But CRS84 is not cylindrical?
Could polar science have specific requirements that should be recorded in
the UCR document?

Greetings,
Frans



>
> Cheers,
> Jon
>
>
>
>
>
> On 17 Feb 2016, at 14:44, Frans Knibbe <frans.knibbe@geodan.nl> wrote:
>
> 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
>>
>
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