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Reporter: rbarnes@… | Owner: jmpolk@…
Type: defect | Status: closed
Priority: major | Milestone: draft-ietf-geopriv-3825bis
Component: rfc3825bis | Version: 2.0
Severity: Active WG Document | Resolution: fixed
Keywords: |
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Changes (by bernard_aboba@…):
* status: new => closed
* version: => 2.0
* resolution: => fixed
Comment:
Since we haven't seen any text to address the remaining issue with
rfc3825bis, here is some text that defines a mapping to GML. Hope this
helps,
--Richard
-----BEGIN-----
Section X. GML Mapping for Resolution
This section describes how the location value encoded in DHCP format for
geodetic location can be expressed in GML. This mapping is valid for
versions 1 and 2, and for the currently-defined datum values (1, 2, and
3). Further version or datum definitions should provide similar mappings.
The DHCP format for location described above logically describes a
geodetic prism, rectangle, or point, depending on whether altitude and
resolution values are provided. These shapes can be mapped to GML by
first computing the bounds that are described using the coordinate and
resolution fields, then encoding the result in a GML Polygon or Prism
shape.
Section X.1. GML Templates
If altitude is provided in meters (altitude type 1) and the datum value is
WGS84 (value 1), then the proper GML shape is a Prism, with the following
form (where $value$ indicates a value computed from the DHCP option as
described below):
<gs:Prism srsName="urn:ogc:def:crs:EPSG::4979"
xmlns:gs="http://www.opengis.net/pidflo/1.0"
xmlns:gml="http://www.opengis.net/gml">
<gs:base>
<gml:Polygon>
<gml:exterior>
<gml:LinearRing>
<gml:posList>
$lowlatitude$ $lowlongitude$ $lowaltitude$
$lowlatitude$ $highlongitude$ $lowaltitude$
$highlatitude$ $highlongitude$ $lowaltitude$
$highlatitude$ $lowlongitude$ $lowaltitude$
$lowlatitude$ $lowlongitude$ $lowaltitude$
</gml:posList>
</gml:LinearRing>
</gml:exterior>
</gml:Polygon>
</gs:base>
<gs:height uom="urn:ogc:def:uom:EPSG::9001">
$highaltitude - lowaltitude$
</gs:height>
</gs:Prism>
The Polygon shape is used if altitude is omitted or specified in floors,
or if either NAD83 datum is used (value 2 or 3). The corresponding GML
Polygon has the following form:
<gml:Polygon srsName="$2D-CRS-URN$"
xmlns:gml="http://www.opengis.net/gml">>
<gml:exterior>
<gml:LinearRing>
<gml:posList>
$lowlatitude$ $lowlongitude$
$lowlatitude$ $highlongitude$
$highlatitude$ $highlongitude$
$highlatitude$ $lowlongitude$
$lowlatitude$ $lowlongitude$
</gml:posList>
</gml:LinearRing>
</gml:exterior>
</gml:Polygon>
The value "2D-CRS-URN" is defined by the datum value: If the datum is
WGS84 (value 1), then the 2D-CRS-URN is "urn:ogc:def:crs:EPSG::4326". If
the datum is NAD83 (value 2 or 3), then the 2D-CRS-URN is
"urn:ogc:def:crs:EPSG::4269".
A Polygon shape with the WGS84 three-dimensional CRS is used if the datum
is WGS84 (value 1) and the altitude is specified in meters (altitude type
1), but no altitude resolution is specified (that is, AltRes is 0). In
this case, the value of the altitude field is added after each of the
points above, and the srsName attribute is set to the three-dimentional
WGS84 CRS, namely "urn:ogc:def:crs:EPSG::4979".
A simple Point shape is used if either latitude or longitude resolution is
not specified. With altitude, this uses a three-dimensional CRS;
otherwise, it uses a two-dimensional CRS.
<gml:Point srsName="$CRS-URN$"
xmlns:gml="http://www.opengis.net/gml">
<gml:pos>$latitude$ $longitude$ $[altitude]$</gml:pos>
</gml:Point>
Section X.2. Finding Low and High Values using Resolution Fields
The resolution fields indicate the bounds of the location region described
by a DHCP location object. For version 0, the resolution fields indicate
how many bits of a value contain information. Any bits beyond those
indicated can be either zero or one. For version 1, the resolution fields
indicate uncertainty distances.
The two sections below describe how to compute the latitude, longitude,
and altitude bounds (e.g., $lowlatitide$, $highaltitude$) in the templates
above. The first section describes how these bounds are computed in the
"resolution encoding" (version 0), while the second section addresses the
"uncertainty encodign" (version 1).
Section X.2.1. Resolution encoding
Given a number of resolution bits (i.e., the value of a resolution field),
if all bits beyond those bits are set to zero, this gives the lowest
possible value. The highest possible value can be found setting all bits
to one.
If the encoded value of latitude and resolution are treated as 34-bit
unsigned integers, the following can be used (where ">>" is a bitwise
right shift, "&" is a bitwise AND, "~" is a bitwise negation, and "|" is a
bitwise OR).
mask = 0x3ffffffff >> resolution
lowvalue = value & ~mask
highvalue = value | mask
Once these values are determined, the corresponding floating point numbers
can be computed by dividing the values by 2^25 (since there are 25 bits of
fraction in the fixed-point representation).
Alternatively, the lowest possible value can be found by using resolution
to determine the size of the range. This method has the advantage that it
operates on the decoded floating point values. It is equivalent to the
first mechanism, up to floating-point errors.
scale = 2 ^ ( 9 - resolution )
lowvalue = floor( value / scale ) * scale
highvalue = lowvalue + scale
Altitude resolution uses the same process with different constants. There
are 22 whole bits in the altitude encoding (instead of 9) and 30 bits in
total (instead of 34).
Section X.2.2. Uncertainty encoding
In the uncertainty encoding, the uncertainty fields directly represent the
logarithms of uncertainty distances. So the low and high bounds are
computed by first computing the uncertainty distances, then adding and
subtracting these from the value provided. If "uncertainty" is the
unsigned integer value of the uncertainty field and "value" is the value
of the coordinate field:
distance = 2 ^ (8 - uncertainty)
lowvalue = value - distance
highvalue = value + distance
Altitude resolution uses the same process with different constants:
distance = 2 ^ (21 - uncertainty)
lowvalue = value - distance
highvalue = value + distance
-----END-----
--
Ticket URL: <http://wiki.tools.ietf.org/wg/geopriv/trac/ticket/3#comment:9>
geopriv <http://tools.ietf.org/geopriv/>
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