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

Wed Apr 13 08:09:28 PDT 2016

Hi Roy,

it would be nice to get input from other members of the geospatial working
group.
If I would remain the only voice in the geospatial working group to have an
opinion on the angle units question, should the proposed change then be
made ? At first glance, it breaks backward compatibility but in effect the
text change would maintain it, as de facto use is units of degrees even in
3.3.

I agree that  the specification of the *_first options should be moved to
section 25.2.3 as they relate to the geoSystem field. The choice of
ellipsoid is completely independent.

What about this ?

Section 25.2.3 second paragraph:

Add after Geodetic spatial reference frame (latitude/longitude):

The order of latitude and longitude in geographic coordinates is controlled
by an optional argument. If "latitude_first" is specified, the order is
latitude then longitude. If "longitude_first" is specified, the order is
longitude then latitude. If neither is specified, "latitude_first" is the
default.

Section 25.2.4 second paragraph:

remove:
"If "latitude_first" is specified, the order is latitude then longitude. If
"longitude_first" is specified, the order is longitude then latitude. If
neither is specified, "latitude_first" is the default."
Perhaps link the word geoSystem in first sentence to Section 25.2.3.

and the exact equivalent for UTM (third paragraphs).

Andreas

On Wed, Apr 13, 2016 at 10:33 AM, Roy Walmsley <roy.walmsley at ntlworld.com>
wrote:

> 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>
> 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
> E-Mail:   holger.seelig at create3000.de
> Web:      http://titania.create3000.de
>
> Future to the fantasy ★ ★
>
>
>
>
> --
>
> Andreas Plesch
> 39 Barbara Rd.
> Waltham, MA 02453
>

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