[Gdal-dev] Dealing with RPC offsets and parameters in GDAL

[Frank Warmerdam]

Ozy Sjahputera wrote:
> my colleagues and I were given a set of NITF images.  We found that the
> pan and multispectral image of the same scene do not always
> co-registered correctly.  When this happens, we found that some of the
> RPC parameters in the two images, such as RPC_LONG_OFF, RPC_LONG_SCALE,
> RPC_LAT_OFF, RPC_LAT_SCALE, RPC_HEIGHT_OFF, and RPC_HEIGHT_SCALE, hold
> different values. 
> 
> We thought that we can correct the co-registration problem by *somehow*
> matching the values of these RPC parameters in both pan and MS. 
> Changing these parameters would be done by applying some RPC-based
> transformation, correct?
> 
> Do you have any information or examples on how to deal with RPC-based
> transformation in GDAL?
> 
> I have found some RPC related functions in GDAL, such as
> GDALCreateRPCTransformer().  I am still tracing how to get the
> GDALRPCInfo object needed by GDALCreateRPCTransformer().  I have found a
> function called GDALExtractRPCInfo() in gdal_misc.cpp that takes a char
> ** , extracts the RPC parameters from the string array, and saves them
> in a GDALRPCInfo object.  But I am still unclear how to obtain the
> string array containing the RPC parameters from the image metadata of an
> NITF image.
> 
> Any info on RPC related functions in GDAL you can share with me will be
> very helpful.

Ozy,

I imagine you are on the right track, but I don't know how practical it
will be to establish correspondance.

The char ** argument to GDALExtractRPCInfo() is the metadata list containing
the parameters.  That is, the list you would get back from a call to
GDALGetMetadata() on the dataset.

Note that the GDAL RPC Transformer does *not* use a variable elevation
model, and should be considered suitable only for gross geolocation, not
the precision geolocation for which RPCs were intended.

Loosely speaking, GDAL does a good job of reading RPC parameters, but
it does not offer good tools to exploit them.  They were really implemented
for applications that had their own higher level RPC algorithms.

Best regards,
-- 
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