Identify grid shift use in CoordinateOperation

[jechterhoff]

Hello,

Is there any reliable and generic - i.e., not implementation specific - way to identify if a coordinate operation actually makes use of a grid shift file (NTv2 and otherwise)?

I would like to programmatically inspect this using a CoordinateOperation object (or any other means provided by GeoAPI), but am not sure how to do it in a generic way. One idea could be to inspect the WKT of the operation, e.g. if PARAMETERFILE occurred in there, but a test transforming NAD 27 to NAD 83 with the NTv2-file from NRCAN with GeoAPI 3.0.2 and Apache SIS results in the WKT not containing that keyword.

Any idea? Any help would be greatly appreciated.

Best regards,
Johannes

[desruisseaux]

Hello Johannes! Yes it is possible. Given a CoordinateOperation, first there is a choice:

• If the operation is an instance of ConcatenatedOperation, invoke getOperations() and apply the following checks on all elements.
• If the operation is an instance of PassThroughOperation, invoke getOperation() and apply the following check on the return value. This is relevant mostly for 3- or 4-dimensional CRS. This check can be ignored if only two-dimensional operations are expected.
• If the operation is an instance of SingleOperation, then apply the following checks.

On each SingleOperation, we can do one of the following checks, at your choice. A first possibility is to invoke getMethod(), then invoke getIdentifiers() on the returned object. It will return a Set<ReferenceIdentifier>. We can loop over these identifiers in search for the identifiers defined by EPSG, then check if the code is one of the EPSG operations for datum shift based on a grid. For example, the EPSG code for the "NTv2" operation is 9615 (note that this is the code of an operation method, not a CRS. See this definition on epsg.org):

boolean isNTv2(SingleOperation operation) {
    for (ReferenceIdentifier identifier : operation.getMethod().getIdentifiers()) {
        if ("EPSG".equalsIgnoreCase(identifier.getCodeSpace())) {
            int code = Integer.parseInt(identifier.getCode());
            if (code == 9615) {    // NTv2
                return true;
            }
        }
    }
    return false;
}

An inconvenient of this approach is that it recognizes only operations defined by EPSG. An alternative is to iterate over the parameters and search for a parameter which is a file. For example:

boolean useUriParameter(SingleOperation operation) {
    for (GeneralParameterValue parameter : operation.getParameterValues().values()) {
        if (parameter instanceof ParameterValue<?> pv) {
            Object value = pv.getValue();
            if (value instanceof URI) {
                return true;
            }
        }
    }
    return false;
}

An inconvenient of this approach is that it will return true for any operation having a URI as a parameter, with no certainty that this operation is a datum shift. If being dependent of EPSG definitions is acceptable, the approach based on EPSG code is probably more reliable. Otherwise a variant of this approach is to look at the operation method name instead of the identifiers.

[desruisseaux]

I forgot to said, just for clarity: looking at the EPSG code of the NTv2 operation method (not to be confused with the code of the coordinate operation) works for all coordinate operations defined by EPSG which uses a NTv2 file. So this is only one code to check for all. There are a few other operation methods based on datum shift grids, for example there is another method code for all operations based on NADCON grids. But in all, I think that there are maybe about 5 codes to check for covering all operations.

[jechterhoff]

Hello Martin. Thank you for the detailed explanation! With it, I can identify grid shift use now. I prefer the approach with explicit identification via coordinate operation method code from the EPSG database.