Hello Adrian
Adrian Custer a écrit :
> You're going to have to walk me through this step by step. I've hit my
> head against the wall too much and I'm ready to give up.
I'm sorry for that... I realize that I wrote way too few documentation for the
referencing module.
Please have no hesitation to ask me instead of consuming too much time on
issues.
> 1) Make a ParameterValueGroup using a magic_string:
> pg = mathTransformFactory.getDefaultParameters("Mercator_1SP");
>
> A? Do we get a list of all the possible magic_strings for the
> ParameterValue Groups from the main() method in
> DefaultMathTransformFactory with parameter '-all'?
Yes. Option "-all" will list the magic strings for every math transforms. If
you want only the magic
strings (parameter names) for one specific math transform, e.g. "Mercator_1SP",
give that specific
math transform name instead.
Note: the magic strings are not Geotools specific. One of our main source is
the following site
(giving directly the URL to the "Mercator_1SP" projection, but other pages like
this one exist for
most projections):
http://www.remotesensing.org/geotiff/proj_list/mercator_1sp.html
Note that more than one "magic string" may exist for the same thing. For
example "Mercator_1SP" is
the OGC name, but GeoTIFF calls the same thing as "CT_Mercator". Geotools will
understand both. I
mean, "Mercator_1SP" and "CT_Mercator" are synonymous (aliases) for Geotools.
Same applies to
projection parameters listed in the above-cited page: "NatOriginLat" and
"latitude_of_origin" are
synonymous for Geotools.
Note (just for information, most users can ignore that): a name can optionally
be prefixed by its
name space, which will be verified. "Mercator_1SP", "OGC:Mercator_1SP",
"CT_Mercator" and
"GeoTIFF:CT_Mercator" are all legal names. But "OGC:CT_Mercator" and
"GeoTIFF:Mercator_1SP" are
illegal and will be rejected by Geotools. This mean that users can prefix a
name by "OGC:" (for
example) if they want to be sure that a name is an OGC name, not a GeoTIFF one.
The following command
java org.geotools.referencing.operation.DefaultTransformFactory Mercator_1SP
should list magic strings understood by Geotools for the "Mercator_1SP"
projection. Every aliases
should be listed as well. Actually, the output of the above-cited command
should be pretty similar
to the table in the above-cited internet link.
Magic strings are expected by three WKT elements: PROJECTION["Mercator_1SP"] or
PARAM_MT["Mercator_1SP"] (where "Mercator_1SP" is just an example) and
PARAMETER["latitude_of_origin", 0] (where "latitude_of_origin" is just an
example). No WKT element
other than PROJECTION, PARAM_MT and PARAMETER come to my mind right now. For
all other WKT elements,
the name is just an arbitrary name and can be whatever the user which.
We can see PROJECTION as a special way to describe a PARAM_MT (MathTransform)
inside a PROJCS
(ProjectedCRS) definition.
Lets also keep in mind that in the special case of DATUM, Geotools will
consider two Datum object as
equivalent only if they share the same name (or at least one alias). For all
other objects, Geotools
will inspect the Object properties instead. For example for a
CoordinateReferenceSystem, Geotools
will compare the CoordinateSystem and all axis - the name is considered as a
metadata and ignored by
CRS.compareIgnoreMetadata(...). But for the special case of Datum, the name is
the only property we
can use in order to determine if two Datum are equivalent or not. For this
reason, Datum name are
considered as a fundamental property, not a metadata we can ignore. This is why
Datum names are more
important than the name of other referencing object, and why Geotools maintains
a list of Datum aliases:
http://svn.geotools.org/geotools/trunk/gt/module/referencing/src/org/geotools/referencing/factory/DatumAliasesTable.txt
Why it is important to decide if two Datum object are equal or not? Because if
a CoordinateOperation
is requested between two CRS, and if Datum of those two CRS are considered
different, then Geotools
will attempt a datum shift, which requires Bursa-Wolf parameters (the TOWGS84
element in WKT).
Note that Datum names are important only when comparing two Datum objects. At
the opposite of
MathTransform names, this is not a "magic string" selecting the algorithm to be
used for some
processing.
Just for reference: WKT specification is there:
http://geoapi.sourceforge.net/snapshot/javadoc/org/opengis/referencing/doc-files/WKT.html
> B? The values are being obtained from the EPSG database?
Yes, exactly. Or specified by PARAMETER elements in WKT format, or by calls to
group.parameter("magic_name").setValue(someValue);
> C? How do we know what parameters are going to be in the group returned?
> Again from the list in the main() method?
Yes. Or from the above-cited link (copying the link there, pointing to all
projections). Note
however that Geotools do not yet implements all projections listed there; the
main() method lists
the projections actually implemented.
http://www.remotesensing.org/geotiff/proj_list/
Users can also get this list programmatically. Example:
pg = mathTransformFactory.getDefaultParameters("Mercator_1SP");
ParameterDescriptorGroup dg = pg.getDescriptor();
for (GeneralParameterDescriptor descriptor : dg.descriptors()) {
System.out.println(descriptor.getName().getCode());
}
> 2) If needed, change the parameters in the ParameterValueGroup
> pg.parameter("falseEasting").setValue(...);
>
> C? How do we know what the valid values are for each parameter?
The DefaultMathTransformFactory main() method should list the minimum and
maximum values. User can
also get more information programmatically as below:
ParameterDescriptorGroup dg = pg.getDescriptor();
ParameterDescriptor descriptor = (ParameterDescriptor)
pg.descriptor("false_easting");
or (may be more convenient and avoid a cast):
Parameter p = pg.parameter("false_easting");
ParameterDescriptor descriptor = pg.getDescriptor();
then:
if (descriptor.getMinimumOccurs() == 0) {
// This parameter is optional
} else {
// This parameter is mandatory
}
Set<Object> values = descriptor.getValidValues();
if (values != null) {
// This parameter is restricted to a set of valid values...
if (!values.contains(myValue)) {
// ... and my value is not one of those valid values.
}
}
Comparable min = descriptor.getMinimumValue();
if (min != null) {
// This parameter is restricted to a minimal value...
if (min.compareTo(myValue) > 0) {
// ... and my value is greater than the minimal value.
}
}
// etc.
> 3) Make a DefiningConversion which requires:
> String name :
> ParameterValueGroup: generated above
Yes.
> D? Is the name arbitrary or from some list somewhere?
Arbitrary. The only place where the name is significant is:
- MathTransform (or ParameterGroup for a MathTransform) and its Parameters;
The MathTransform name is a magic string which determine the algorithm
being
used. Parameter names are magic string as well.
- Datum (significant only when comparing two datums for determining
if a "datum shift" is required or not during a coordinate operation).
> 5) Make a Map for the projectedCRS's properties.
>
> E? What's supposed to be in this Map? A name presumably, but is there
> other stuff? Is the name arbitrary?
Yes. The Map must constain at least the name. All other properties are
optional. The Map keys are
listed as IdentifiedObject constants:
http://geoapi.sourceforge.net/snapshot/javadoc/org/opengis/referencing/IdentifiedObject.html#field_summary
More specific keys are defined in some subinterfaces, for example:
http://geoapi.sourceforge.net/snapshot/javadoc/org/opengis/referencing/ReferenceSystem.html#field_summary
Example:
Map properties = new HashMap();
properties.put(IdentifiedObject.NAME_KEY, "My arbitrary name"); //
Mandatory
properties.put(ReferenceSystem.VALID_AREA_KEY, myValidArea); // Optional
As a rule of thumb, the Map contains all metadata. Everything specified in this
map, no matter which
"createFoo(...)" method is invoked (except Datum and subtype) is ignored by
CRS.equalsIgnoreMetadata(...). This include the name, valid area, scope, etc.
Everything not
specified in this map is taken in account by CRS.equalsIgnoreMetadata(...).
This includes the
coordinate system axis, etc. Let remind that Datum name and aliases are the
only exception to the
above-cited rule.
Optional note: some properties can be localized, e.g.:
properties.put(IdentifiedObject.REMARKS_KEY, "My remarks"); //
Default language
properties.put(IdentifiedObject.REMARKS_KEY + "_en", "My remarks"); //
English
properties.put(IdentifiedObject.REMARKS_KEY + "_fr", "Mes remarques"); //
French
The above is actually just a convenient way to create an InternationalString.
Geotools will assemble
the above lines in a single InternationalString for us.
> 6) Create the ProjectedCRS using the FactoryGroup's convenience method
> and all the pieces above.
>
> is that right?
Right :)
Martin.
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