[Proj] Using libproj with array data
jhrg at mac.com
Thu Jul 6 11:28:19 EDT 2006
On Jul 5, 2006, at 9:24 PM, Anthony Dunk wrote:
> What you're trying to do sounds like re-projecting a grid. (A grid
> being a two dimensional array of values). I have written code to do
> this previously.
I think what I need is simpler, but maybe not. What I'm doing is
writing a function that can be run by a data server that will return
a region of interest. The ROI, suplied by a client, will specified by
long/lat upper-left and lower-right corner points. What I need to get
are the actual array indexes that correspond to those long/lat
locations. For some of the data, the coverage is the 'global' (which
means 80N to 80S and 180W to 180E) while for the rest it a smaller
Metadata associated with these data files are spotty and often
somewhat odd. For example, one data set lists longitude as running
from 20 to 380 degrees. Before I look more at libproj, I should ask
if it will handle such oddities? I assume so, but ...
> To project a grid you basically have to perform a forward
> projection of
> some points around the edge of the input grid and use these to compute
> the bounding rectangle in the new coordinate system. (Remember some
> projections will distort the shape of the grid).
> You then create an output grid in the new coordinate system with the
> appropriate dimensions.
> Finally you have to go through each cell in the output grid and use
> reverse projection to find the corresponding cell in the unprojected
> grid. Use grid interpolation to pick up the value you need to put in
> the output grid cell.
> There's probably already code in the public domain which uses PROJ to
> do this.
> --- James Gallagher <jhrg at mac.com> wrote:
>> Hi, I hope this is not a FAQ or covered in the existing docs; I've
>> checked and apologize if I missed something obvious.
>> I have a data server which is used to access a number of files of
>> georeferenced data stored in two dimentional arrays. For each array,
>> I know the longitude and latitude values at the corners as well as
>> the projection and datum. Is there a way I can use libproj to map
>> long/lat locations to indices within the arrays?
>> James Gallagher jgallagher at opendap.org
>> OPeNDAP, Inc 406.723.8663
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>> Proj at lists.maptools.org
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James Gallagher jgallagher at opendap.org
OPeNDAP, Inc 406.723.8663
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