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19990618: Image calculations and raster files (cont.)

>From: Tye Parzybok <address@hidden>
>Organization: Oregon State
>Keywords: 199906171817.MAA28605 McIDAS

Tye,

>WELL IN ORDER TO MAKE THE AXFORM ASCII FILE READABLE BY A GIS IT
>NEEDS A HEADER LIKE SUCH:
>
>north: 50.020833333333
>south: 24.062500000000
>east: -64.979166666667
>west: -126.020833333333
>rows: 623
>cols: 1465
>
>I CAN MANUALLY ADD THIS HEADER TO THE AXFORM ASCII FILE, BUT MY
>DIFFICULTY IS DETERMINING THE CORRECT LAT/LON/ROWS/COLS VALUES.

OK, now I understand what you are trying to do.

>I WAS THINKING THAT "RECTILINEAR" WAS ANOTHER WAY OF SAYING
>GEOGRAPHIC, WHICH IS THE "PROJECTION" I AM WORKING WITH IN MY GIS.
>I GOT THIS IDEA FROM THE E-MAIL SUPPORT MENTIONED ABOVE. A GEOGRAPHIC
>PROJECTION IS A TERM USED TO DESCRIBE A LAT/LON GRID IN WHICH ALL LAT/LON
>INTERSECTIONS FORM RIGHT ANGLES.

A rectilinear projection is indeed one in which lat,lon intersections are
right angles.  The "measure" (distance between points in the horizontal
or vertical direction) may be different between horizontal and vertical,
but it is fixed for both in an image.

re: display the image and interrogate it
>I WORRY THIS WILL NOT BE ACCURATE ENOUGH.

You could produce files of LAT,LON using AXFORM.

   NAV=YES inc | produce .LAT/.LON data files with an element
                 increment; these files contain LAT/LON stored as
                 degrees*100 (2 bytes per element); the increment is the
                 the number of data elements associated with each LAT/LON
                 element (def=NO 1)

You would then have to pull out the first and last values from each of
the .LAT and .LON files and then invert the longitude sign to take
into account McIDAS' use of positive west longitued.

re: read AREA file header and compute the LAT,LONs
>THERE MUST BE AN EASIER WAY...AT LEAST I HOPE SO. :-|

Since you are already using AXFORM, you might as well use the NAV=YES
option and get an output of the LATs and LONs.

re: take a look at GEWCOMP.MCB

>SUPER...I'LL GIVE THIS A TRY.
>
>THANKS AGAIN TOM.

You are welcome.

Tom