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

Wed Apr 13 10:41:23 PDT 2016

I have no memory of why the "UNIT sentence" was introduced in v3.3.

Was there discussion captured somewhere regarding this? (I do remember using our members-only wiki for other geospatial component discussions.)

-Mike

On Apr 13, 2016, at 10:04 AM, Andreas Plesch <andreasplesch at gmail.com> wrote:

> 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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