Re: lat/long as float

As Mike is suggesting, the semantics of a lat/long coordinate should be 
clearly expressed. The following description is what is being used in 
various IETF draft RFCs and is being used in OASIS for any of their 
standards that require the specification of location as lat/long. This 
description may be of use here. Some argue that it is a bit long, but one of 
the quirks of obtaining interoperability in the geospatial domain is that 
semantic clarity is mandatory.

Regards

Carl
OGC

All coordinates in this profile shall, as a default, be expressed as 
Latitude Longitude (in that order) as decimal degrees white space separated. 
Please note that if another CRS is specified, such as UTM, GML supports the 
ability to express coordinates in any CRS.



In the case of Latitude Longitude, the following clarification applies:



  a.. Values for latitude and longitude shall be expressed as decimal 
fractions of degrees.

  a.. Whole degrees of latitude shall be represented by a decimal number 
ranging from 0 through 90.  Whole degrees of longitude shall be represented 
by a decimal number ranging from 0 through 180.  When a decimal fraction of 
a degree is specified, it shall be separated from the whole number of 
degrees by a decimal point (the period character, ".").  Decimal fractions 
of a degree should be expressed to the precision available, with trailing 
zeroes being used as placeholders if required.  A decimal point is optional 
where the precision is less than one degree. Some effort should be made to 
preserve the apparent precision when converting from another datum or 
representation, for example 41 degrees 13 minutes should be represented as 
41.22 and not 41.21666, while 41 13' 11" may be represented as 41.2197.

  a.. Latitudes north of the equator MAY be specified by a plus sign (+), or 
by the absence of a minus sign (-), preceding the designating degrees. 
Latitudes south of the Equator MUST be designated by a  minus sign (-) 
preceding the digits designating degrees.  Latitudes on the Equator MUST be 
designated by a latitude value of 0.

  a.. Longitudes east of the prime meridian shall be specified by a plus 
sign (+), or by the absence of a minus sign (-), preceding the designating 
degrees.  Longitudes west of the prime meridian MUST be designated by a 
minus sign (-) preceding the digits designating degrees. Longitudes on the 
prime meridian  MUST be designated by a longitude value of 0.  A point on 
the 180th meridian shall be taken as 180 degrees West, and shall include a 
minus sign.
----- Original Message ----- 
From: "Mike Dean" <mdean@bbn.com>
To: <public-xg-geo@w3.org>
Sent: Wednesday, July 19, 2006 8:44 AM
Subject: lat/long as float


>
> [1] notes that "the string representation of lat and long should follow 
> the
> rules for XML Schema float".  I'm a bit concerned about this, since the
> precision of xsd:float is limited to 24 bits (including the hidden bit). 
> In
> the worst case at the equator
>
>  24000 miles * 5280 feet/mile / (2^24) = 7.553 feet
>
> which is achievable by current handheld GPS units using WAAS, not to 
> mention
> differential GPS.
>
> I generally represent lat/long's using xsd:double.  An alternative may be 
> to
> specify xsd:decimal and let applications worry about the precision.
>
> I'd prefer to see explicit rdfs:range's for these properties.  The 
> syntactic
> burden for datatyping individual Literal values is an unfortunate feature 
> of
> RDF.
>
> For clarity, the spec and property definitions should also note that North
> latitudes and East longitudes use positive values.
>
> Mike
>
> [1] http://www.w3.org/2003/01/geo/
>
>

Received on Thursday, 20 July 2006 14:26:19 UTC