I've been fighting with this problem for weeks and have finally given
up on solving it myself. I hope that someone with more experience
manipulating maps in ENVI can offer me some advice and guidance.
I have an image file (actually many such files) that is written as an
hdf. The data in the file have been geo-referenced and projected.
Some of the projection parameters (Mercator, limit, central azimuth)
are included in the file as attributes of the scientific data set, but
there are no explicit latitude and longitude values corresponding to
the pixels in the image in the file. I know the size of the image,
the datum that was used in the projection, and I _think_ I have the
affine transform matrix for the map. My goal is to convert this image
from its native Mercator projection to a data file that is in
geographic coordinates with a different datum.
I can use ENVI_PROJ_CREATE to create a map projection structure for
the input file - for Mercator projection the parameter array includes
the semi-major and semi-minor axes (obtained from knowledge of the
original datum/ellipsoid), the latitude at the origin (assumed to be
the equator), the longitude of the central meridian (included in the
sds attribute information), and the false easting and false northing.
I've used the e and f elements of the affine transform matrix for
these. I also can use ENVI_PROJ_CREATE to define the output
projection as geographic with the different datum.
I thought the way to approach this was to read the hdf file and then
create an ENVI file in memory using ENVI_WRITE_ENVI_FILE. I thought I
could simply use ENVI_CONVERT_FILE_MAP_PROJECTION to create the new
file I wanted. To do, however, I needed to use ENVI_MAP_INFO_CREATE
to develop a map information structure to associate with the data that
I would write to the temporary ENVI file. This is where I became very
confused. In addition to the input map projection,
ENVI_MAP_INFO_CREATE requires a tie point (MC) array and pixel size
(PS) array. I think I have enough information to fill the MC array
because I know the limits of the original image from the hdf metadata,
though these values are given as latitudes and longitudes rather than
as meters. I'm not sure about the PS array. Because the original
image was generated as ISOTROPIC, I am assuming that pixel size should
be the same in each direction. Is it to be specified in angular units
or linear units? It can't be angular because scale along a meridian
varies with latitude in a Mercator projection. If it is linear, how
do I determine the values? Are these the same as the b and c
elements of the affine matrix?
Pushing on, I made some assumptions and went to the
ENVI_CONVERT_FILE_MAP_PROJECTION step. I originally thought that I
could write the output file into memory, but apparently, the /
IN_MEMORY keyword is not allowed in this routine (seems strange..), so
I use the OUT_NAME keyword. The process fails at this step - when I
try to query the file I tried to write, the ENVI_QUERY_FILE routine
Here is part of the code and the output:
ENVI_WRITE_ENVI_FILE, data, DATA_TYPE=data_type, /NO_OPEN, $
MAP_INFO=in_map, NB=1, NL=1024, NS=1024, /IN_MEMORY, R_FID=rfid
foo = ENVI_GET_PROJECTION(FID=rfid)
ENVI_CONVERT_FILE_MAP_PROJECTION, FID=rfid, OUT_NAME=oname, POS=,
R_FID=ofid, DIMS=dims, O_PROJ=OUT_proj, WARP_METHOD=3
ENVI_OPEN_FILE, oname, R_FID=ofid
I've been going back and forth on this with no success. Any
suggestions would be most welcome.