[x3d-public] geoSystem UTM case - into mantis now...

Sun Apr 2 11:27:50 PDT 2023

[cc: x3d-public for review]

Nicholas, thanks for clarifications on geosystem prose.  Makes better sense now.  Have also applied some style=’code’ markup so that enumerations are better distinguishable from regular prose (in web browser at least).  Mantis record:

*	Mantis 1437, geoSystem example improvement
*	https://www.web3d.org/member-only/mantis/view.php?id=1437
*	from Nicholas: the better, more-correct example then is ["UTM", "Z10", "S", "easting_first"]
*	and points out that "easting_first" is documented in 25.2.4 Specifying geospatial coordinates.

Here are resulting changes

*	https://www.web3d.org/specifications/X3Dv4Draft/ISO-IEC19775-1v4-IS.review/Part01/components/geospatial.html#SpecifyingSpatialReferenceFrame
*	https://www.web3d.org/specifications/X3Dv4Draft/ISO-IEC19775-1v4-IS.proof/Part01/components/geospatial.html#SpecifyingSpatialReferenceFrame

Hope that the following excerpts all looks good.  Thanks for insights by you and Andreas,  thanks for all review.

25.2.3 Specifying a spatial reference frame

All the X3D nodes that allow inclusion of geographic coordinates support a field called geoSystem. This field is used to specify the particular spatial reference frame that will be used for the geospatial coordinates in that node. This is an MFString field that can include a number of arguments to fully designate the spatial reference frame. Each argument appears in a separate string within the MFString array. Argument matching is case sensitive. Optional arguments may appear in any order. The following values are supported. 

*	" GD" - Geodetic spatial reference frame (latitude/longitude). An optional argument may be used to specify the ellipsoid using one of the ellipsoid codes that are defined in Table 25.3 <https://www.web3d.org/specifications/X3Dv4Draft/ISO-IEC19775-1v4-IS.review/Part01/components/geospatial.html#t-earthellipsoids> . If no ellipsoid is specified, then "WE" is assumed (i.e., the WGS84 ellipsoid). An optional "WGS84" string can be specified if you wish all elevations to relative to the WGS84 geoid (i.e., mean sea level) (see Table 25.4 <https://www.web3d.org/specifications/X3Dv4Draft/ISO-IEC19775-1v4-IS.review/Part01/components/geospatial.html#t-earthgeoids> ); otherwise, all elevations will be relative to the ellipsoid. An example spatial reference frame definition of this format is [ "GD", "WD" ], for a geodetic spatial reference frame based upon the WGS72 ellipsoid with all elevations being relative to that ellipsoid.

*	" UTM" - Universal Transverse Mercator. One further required argument shall must be supplied for UTM in order to specify the zone number (1..60). This is given in the form "Z<n>", where <n> is the zone number. An optional argument of "S" may be supplied in order to specify that the coordinates are in the southern hemisphere (otherwise, northern hemisphere will be assumed). A further optional argument may be used to specify the ellipsoid using one of the ellipsoid codes that are defined in Table 25.3 <https://www.web3d.org/specifications/X3Dv4Draft/ISO-IEC19775-1v4-IS.review/Part01/components/geospatial.html#t-earthellipsoids> . If no ellipsoid is specified, then "WE" is assumed (i.e., the WGS84 ellipsoid). An optional "WGS84" string can be specified if you wish all elevations to relative to the WGS84 geoid (i.e., mean sea level (see Table 25.4 <https://www.web3d.org/specifications/X3Dv4Draft/ISO-IEC19775-1v4-IS.review/Part01/components/geospatial.html#t-earthgeoids> )); otherwise, all elevations will be relative to the ellipsoid. An example spatial reference frame definition of this format is [ "UTM", "Z10", "S", "easting_first" ] [ "UTM", "Z10", "S", "WGS84" ], for a southern hemisphere UTM spatial reference frame in zone 10 with all elevations being with respect to mean sea level.

*	" GC" - Earth-fixed Geocentric with respect to the WGS84 ellipsoid. No additional arguments are supported. An example spatial reference frame definition of this format is [ "GC" ].

*	" WM" - Web Mercator projection used for all web mapping (slippy maps). An example spatial reference frame definition of this format is [ "WM" ].

If no geoSystem field is specified, the default value is [ GD", "WE" ].

25.2.4 Specifying geospatial coordinates

Once the spatial reference frame has been defined, a single geographic coordinate is specified as an SFVec3d. Lists of geographic coordinates are encoded as an MFVec3d. The meaning of each component value depends upon the particular spatial reference frame that was defined via the geoSystem field in the same node. Given the following geoSystem definitions, the meaning of each component is defined as follows. 

*	GD: (<latitude>, <longitude>, <elevation>) or (<longitude>, <latitude>, <elevation>). The order of latitude and longitude is controlled by the geoSystem field. 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. Elevation is always specified third. 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). Longitudinal values are relative to the Greenwich Prime Meridian. Elevation is given in units of length base units above the ellipsoid (the default) or above the WGS84 geoid (if you supplied the "WGS84" parameter in the geoSystem field).

EXAMPLE  (37.4506, −122.1834, 0) is the latitude/longitude coordinate for Menlo Park, California, USA. 

*	UTM: (<northing>, <easting>, <elevation>) or (<easting>, <northing>, <elevation>). The order of northing and easting is controlled by the geoSystem field. If "northing_first" is specified, the order is northing then easting. If "easting_first" is specified, the order is easting then northing. If neither is specified, "northing_first" is the default. Elevation is always specified third. Northings, eastings, and elevation are all given in units of length base units. The zone of the coordinate, and whether it is in the southern hemisphere, are defined in the geoSystem string. Elevation is given with reference to the ellipsoid (the default) or the WGS84 geoid (if the "WGS84" parameter is specified in the geoSystem field).

EXAMPLE  (4145173, 572227, 0) is the zone 10 northern hemisphere UTM coordinate for Menlo Park, California, USA.

*	GC: (<x>, <y>, <z>). These values are all given in units of metres. The coordinate represents an offset from the center of the planet, based upon the WGS84 ellipsoid. The z-axis passes through the poles while the x-axis cuts through the latitude/longitude coordinate (0,0) degrees.

EXAMPLE  (−2700301, −4290762, 3857213) is the geocentric coordinate for Menlo Park, California, USA.

*	WM: (<x>, <y>, <elevation>). These values are all given in units of metres. The x and y values represent Web Mercator coordinates with an origin at 0 degrees latitude and longitude, based upon the WGS84 ellipsoid. The elevation is also with respect to the WGS84 ellipsoid.

EXAMPLE  (-13601393.87, 4502102.12, 0) is the Web Mercator coordinate for Menlo Park, California, USA.

Have fun with geospatial X3D4!   8)

v/r Don

From: Nicholas Polys <npolys at vt.edu> 
Sent: Friday, March 31, 2023 2:57 PM
To: Richard F. Puk <puk at igraphics.com>; Brutzman, Donald (Don) (CIV) <brutzman at nps.edu>
Subject: geoSystem UTM case - into mantis now...

25.2.3 in https://www.web3d.org/documents/specifications/19775-1/V4.0/Part01/components/geospatial.html#Spatialreferenceframes

has

"An example spatial reference frame definition of this format is [ "UTM", "Z10", "S", "GD" ], for a southern hemisphere UTM spatial reference frame in zone 10 with all elevations being with respect to mean sea level."

This is confusing. The example should read [ "UTM", "Z10", "S", "WGS84" ] . "GD" and "UTM" should not occur together.

Perhaps there should also be an explanation that providing the geoid "WGS84" rather than an ellipsoid such as "WE" is really only useful if a scene uses multiple geoSystems with mixed geoid/ellipsoid use. x3dom does not support geoids since they require storing and querying huge grids.

-- 

Nicholas F. Polys, Ph.D.

Director of Visual Computing 
Virginia Tech Research Computing 

Affiliate Professor
Virginia Tech Department of Computer Science

https://people.cs.vt.edu/~npolys/ <https://nam10.safelinks.protection.outlook.com/?url=https%3A%2F%2Fpeople.cs.vt.edu%2F~npolys%2F&data=05%7C01%7Cbrutzman%40nps.edu%7C60aa21cd242e4fd68d0b08db3232d0bc%7C6d936231a51740ea9199f7578963378e%7C0%7C0%7C638158966100042711%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=5wVh%2BXB8qSQByeAQWMtbwpcAYJbNBW8AsHSB%2BCbeVVs%3D&reserved=0>  

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