converting projections

[Gerald I. Evenden]

>From: qi at cdrgsun.UCSD.EDU (ye qi)
>Subject: Re:  converting projections
>Status: R
>
>Dear Dr. Evenden:
      Mr. will do. :-), or, since I'm not the formal type, Jerry will do.
>
>I am so glad I have finally got the expert.  In fact, I have just read your 63 
>page user mannual on porj but still have no idea how to do it.
>
>Here is my problem.  I have a series of data of the global vegetation index
>propared by Kevin Gallo at NOAA/NESDIS.  The actual data coverage is within
>75N-55S and 180W-180E and they are in Mercator projection.  I want to change
>them to Lat/Lon, in order to match our other data sets.  So it is not just a 
>matter of translating one type of coordinates to another, the values (a 
>function of coordinates) must be changed as well.
>
>My raster data in Mercator projection has 1038 rows and 2048 colums (for the 
>whole earth) needs to be changed to another raster file with 1080 rows and 2160
>colums, i.e. 10' x 10' lat/lon.  Each grid cell in the new file has to be 
>assigned a value based on its area and the value(s) of its coresponding cell(s).

First, for clarity: your source information does not cover the whole
world (it can't with a Mercator Projection as the poles are at infinity)
but I presume the raster range mentioned (1038 rows) spans 55S-75N.

Your problem is interpolating a new "square" raster grid from data in
a "rectagular" grid with grid size varying N-S.  In the longitude axis
the problem is relatively simple and the correction is based on a linear
ratio of 2048 to 2160 cells.  The terms "square" and "rectangular" are
used when latitude and longitude are treated, speciously, like
cartesian coordinates (this is why someone apparently suggest Plate
Caree in your previous mail).

The latitude correction is messy because the conversion depends upon
the inverse coordinates determined by the Mercator Projection.
But, it is solvable with a little programming effort.

>Several email message I received from the grassu-list suggested to use IDRISI
>and arc/info.  I don't have arc/info.  I do have idrisi here but I prefer to 
>work on my workstation because of the size of the data sets.  Any advice will
>be greatly appreciated.
>
>Thank you very much for your time.
>
>Best regards,
>
>===================================
>Ye Qi
>Scripps Institution of Oceanography
>Email: qi at cdrgsun.ucsd.edu
>===================================

Before preceeding, two points: do you have PROJ.4 installed on your
work station (presumably a Unix system)?
Also, I presume that you do not have GRASS installed on your work
station, since there may be a GRASS procedure that can also handle
this problem?

Gerald (Jerry) I. Evenden   Internet: gie at charon.er.usgs.gov
voice: (508)563-6766          Postal: P.O. Box 1027
  fax: (508)457-2310                  N.Falmouth, MA 02556-1027