[x3d-public] [geospatial] Latitude/Longitude units

Wed Apr 13 10:04:36 PDT 2016

Holger, I tend to agree but I think it is an unfortunate, perhaps
unintended consequence of introducing the UNIT statement which needs to be
addressed.

There is a little bit of wiggle room. The UNIT statement and Table 4.2 talk
about angle _standard_ units while section 25.2.4 talks about angle _base_
units. So angle _base_ units may be different from angle _standard_ units.
I am not sure if such a distinction was intentional when the spec. language
was authored but it may have been and then not fully followed through.

v 3.2 of the spec. just prescribes units of degrees while Table 4.2 has
radians as standard angle unit. We should just go back to that.

Andreas

http://www.web3d.org/documents/specifications/19775-1/V3.2/Part01/components/geodata.html#Specifyinggeospatialcoords
http://www.web3d.org/documents/specifications/19775-1/V3.2/Part01/concepts.html#t-Standardunits

On Wed, Apr 13, 2016 at 10:51 AM, Holger Seelig <holger.seelig at yahoo.de>
wrote:

> The specification in 3.3 is absolutely exact at this point.  The
> assumption that the base unit is in degree refers to to the range values
> specified. If the assumption would be radians then the corresponding values
> would be given. That mean in 3.3 the default angle unit for long/lat are
> radians.
>
>
> 25.2.4 Specifying geospatial coordinates
>
> Latitude and longitude are given in units of angle base units. The
> following assumes an angle base unit of degrees. If a UNIT statement for
> angle base units has been provided, the following values for latitude
> and/or longitude should be suitable converted to that angle base units.
> Latitude is in the range −90..+90, and longitude can be in the range
> −180..+180 or 0..360 (0 deg longitude is the same point in both cases).
>
>
> Holger
>
>
>
>
>
>
>
> Am 13.04.2016 um 16:33 schrieb Roy Walmsley:
>
>> Thanks for this great discussion. I can see that text modifications are
>> likely to be required here.
>>
>> In addition, I would like to raise a related question. In 25.2.4
>> Specifying geospatial coordinates, the ‘GD’ option allows either
>> “latitude_first” or “longitude_first”, the order being controlled by the
>> /geosystem/ field. There is no other specific mention of an option
>> within the geosystem field to specify either of these. Should there be?
>> Or is it automatically determined by the choice of ellipsoid from Table
>> 25.3?
>>
>> Under the UTM option the option of “easting_first” or “northing_first”
>> is an analogous case. What is the answer for this case?
>>
>> Roy
>>
>> *From:*geospatial [mailto:geospatial-bounces at web3d.org] *On Behalf Of
>> *Andreas Plesch
>> *Sent:* 13 April 2016 14:30
>> *To:* Holger Seelig
>> *Cc:* geospatial at web3d org; X3D Graphics public mailing list
>> *Subject:* Re: [geospatial] Latitude/Longitude units
>>
>> Good point. Section 25.2.4 could be read that way as it does point out
>> that the angle unit is a choice.
>>
>> The problem with using angle standard units is that authors would want
>> to use radians for orientations and rotations but at the same time
>> degrees for latitude/longitude. In my experience radians is very rarely
>> used as a unit for geodetic coordinates. The only example example I can
>> think of is in equations.
>>
>> Some options:
>>
>> 1) no changes to the spec., modify examples
>>
>> 2) prescribe degrees as unit for geodetic coordinates in section 25.2.4
>>
>> 3) add a standard unit for angular distance in Table 4.2
>> (
>> http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/concepts.html#t-Standardunits
>> )
>> with a default of degrees and refer to that in 25.2.4
>>
>> My preference would be option 2), followed by 3).
>>
>> Therefore I propose that in section 25.2.4, second paragraph:
>>
>> " Latitude and longitude are given in units of angle base units. The
>> following assumes an angle base unit of degrees. If a UNIT statement for
>> angle base units has been provided, the following values for latitude
>> and/or longitude should be suitable converted to that angle base units."
>>
>> shall be replaced by:
>>
>> " Latitude and longitude are given in units of degrees."
>>
>>
>> for simplicity.
>>
>> Andreas
>>
>> On Wed, Apr 13, 2016 at 6:21 AM, Holger Seelig <holger.seelig at yahoo.de
>> <mailto:holger.seelig at yahoo.de>> wrote:
>>
>> My interpretation was that with version 3.3 the UNIT statement defines
>> what unit a angle value has. And therefore on default the angles of
>> geodetic coordinates should be in radians. The author has then with the
>> UNIT statement the choice to define its preferred angle unit.
>>
>> Holger
>>
>>
> Am 13.04.2016 um 06:16 schrieb Andreas Plesch:
>>
>>
>> http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/components/geodata.html#Specifyinggeospatialcoords
>>
>> defines that coordinates in the GD geoSystem are given in angular base
>> units.
>>
>>
>> http://www.web3d.org/documents/specifications/19775-1/V3.3/Part01/concepts.html#t-Standardunits
>>
>> defines the standard angle unit to be radians.
>>
>> Cobweb interpretes this to mean latitude/longitude in geoCoords are
>> given in radians which I think should not be the intention of the spec.
>> All geospatial example scenes use degrees. So I hope this is a
>> misinterpretation.
>>
>> Although section 25.2.4 as linked goes on to assume that the base
>> angular unit is degrees, it is a bit unclear what unit is really expected.
>>
>> If more precise language is needed, I suggest simply to add "angular
>> base units of degrees" to section 25.2.4 .
>>
>> Andreas
>>
>>
>>
>> --
>> Holger Seelig
>> Mediengestalter Digital – Digital Media Designer
>>
>> Scheffelstraße 31a
>> 04277 Leipzig
>> Germany
>>
>> Cellular: +49 1577 147 26 11 <tel:%2B49%201577%20147%2026%2011>
>> E-Mail: holger.seelig at create3000.de <mailto:holger.seelig at create3000.de>
>> Web: http://titania.create3000.de
>>
>> Future to the fantasy ★ ★
>>
>>
>>
>>
>> --
>>
>> Andreas Plesch
>> 39 Barbara Rd.
>> Waltham, MA 02453
>>
>>
>
> --
> Holger Seelig
> Mediengestalter Digital – Digital Media Designer
>
> Scheffelstraße 31a
> 04277 Leipzig
> Germany
>
> Cellular: +49 1577 147 26 11
> E-Mail:   holger.seelig at create3000.de
> Web:      http://titania.create3000.de
>
> Future to the fantasy ★ ★
>

-- 
Andreas Plesch
39 Barbara Rd.
Waltham, MA 02453
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