You don't mention whether these data are georeferenced, or whether you've figured out hot to read them in ENVI yet. In general, you should be able to read them into ENVI using File > Open External File > Generic Formats > HDF. If it is georeferenced already, then you could read it out to GeoTIFF by choosing File > Save File As > TIFF/GeoTIFF. If it's not already georeferenced, or not in the right projection/datum, then you would need to georeference it or change it's map projection in ENVI before writing it out. Let me know if you need information about how to do that. - Peg

Dear Peg, 1. Thank you very much for youur reply. Yes, I managed to open the HDF file in ENVI. The details is as follows: Dims: 8192 x 4096 x 1 [BSQ] Size: [Unsigned Int] 14,460,890 bytes. File Type : HDF Scientific Data Sensor Type: Unknown Byte Order : Host (Intel) Projection : None Wavelength : None Upper Left Corner: 1,1 Description: HDF File Imported into ENVI [Tue Feb 08 10:34:49 2011] 2. It is not georeferenced and I need to project it to WGS1984 with 4 km pixel size(0.04 degree). GCS: 4326/WGS 84 Datum: 6326/World Geodetic System 1984 Ellipsoid: 7030/WGS 84 (6378137.00,6356752.31) Prime Meridian: 8901/Greenwich (0.000000/ 0d 0' 0.00"E) Projection Linear Units: 9001/metre (1.000000m) Corner Coordinates: Upper Left (180d 0' 0.00"W, 90d 0' 0.00"N) Lower Left (180d 0' 0.00"W, 90d 0' 0.00"S) Upper Right (180d 0' 0.00"E, 90d 0' 0.00"N) Lower Right (180d 0' 0.00"E, 90d 0' 0.00"S) Center ( 0d 0' 0.00"E, 0d 0' 0.00"N) Could you kindly help me on this ? Thanks in advance. Idham Dims: 8192 x 4096 x 1 [BSQ] Size: [Unsigned Int] 14,460,890 bytes. File Type : HDF Scientific Data Sensor Type: Unknown Byte Order : Host (Intel) Projection : None Wavelength : None Upper Left Corner: 1,1 Description: HDF File Imported into ENVI [Tue Feb 08 10:34:49 2011] Dims: 8192 x 4096 x 1 [BSQ] Size: [Unsigned Int] 14,460,890 bytes. File Type : HDF Scientific Data Sensor Type: Unknown Byte Order : Host (Intel) Projection : None Wavelength : None Upper Left Corner: 1,1 Description: HDF File Imported into ENVI [Tue Feb 08 10:34:49 2011] Dims: 8192 x 4096 x 1 [BSQ] Size: [Unsigned Int] 14,460,890 bytes. File Type : HDF Scientific Data Sensor Type: Unknown Byte Order : Host (Intel) Projection : None Wavelength : None Upper Left Corner: 1,1 Description: HDF File Imported into ENVI [Tue Feb 08 10:34:49 2011]

It sounds like you will have to register the image, using some kind of map information from another source. You will probably want to use the tools under Map > Registration > Select GCPs: Image to Map. You can find the documentation for those tool in the ENVI online help (from the main ENVI menu, under Help > Start ENVI Help), under Using ENVI > Map Tools > Registration > Image-to-Map Ground Control Points. Unless these data happen to come with images of latitude and longitude for each pixel. Or unless it happens to already be in a known coordinate system, and you just have to tell ENVI what it is. Do you know if either of these is true for these data? It looks like the tricky part may be setting up your projection correctly in ENVI. I'm not sure from what you say exactly what you want. It looks like this will be a "Geographic" projection, which means that the data will actually not be projected, but will use lat/long as the map coordinates. That's fine, except that you then provide a projection unit of meters, which doesn't make sense for an unprojected dataset. Since this is a global dataset, it's not simple to convert from degrees to meters. The conversion is very different at different latitudes. This might be a little more to figure out than can easily be handled via this forum. I recommend that you contact ITT VIS Technical Support or your local ENVI distributor for more detailed help figuring out how to set up the projection you need in ENVI. - Peg

It sounds like you will have to register the image, using some kind of map information from another source. You will probably want to use the tools under Map > Registration > Select GCPs: Image to Map. You can find the documentation for those tool in the ENVI online help (from the main ENVI menu, under Help > Start ENVI Help), under Using ENVI > Map Tools > Registration > Image-to-Map Ground Control Points. Unless these data happen to come with images of latitude and longitude for each pixel. Or unless it happens to already be in a known coordinate system, and you just have to tell ENVI what it is. Do you know if either of these is true for these data? It looks like the tricky part may be setting up your projection correctly in ENVI. I'm not sure from what you say exactly what you want. It looks like this will be a "Geographic" projection, which means that the data will actually not be projected, but will use lat/long as the map coordinates. That's fine, except that you then provide a projection unit of meters, which doesn't make sense for an unprojected dataset. Since this is a global dataset, it's not simple to convert from degrees to meters. The conversion is very different at different latitudes. This might be a little more to figure out than can easily be handled via this forum. I recommend that you contact ITT VIS Technical Support or your local ENVI distributor for more detailed help figuring out how to set up the projection you need in ENVI. - Peg

Hi Peg, Thanks again for your reply. Basically...I will have to analyse time series of AVHRR data. You are right on "geographic" projection. I only need the data to be in WGS1984 lat long. Converting from equal area to wgs1984 seems a problem for me. Found something from pathfinder avhrr site but not sure how to proceed. To calculate the geographic location of a specific pixel use the following equations. longitude = (-180. + (x * dx)) + (dx/2) latitude = (90. - (y * dy)) - (dy/2) for the version 5.0 4 km resolution: x = grid point in x-direction (0-8191) dx = grid x-direction spacing (360 deg / 8192.) y = grid point in y-direction (0-4095) dy = grid y-direction spacing (180 deg / 4096.) Note that the upper left hand corner of first element of the data array is located at 180 West and 90 North, and that adding dx/2 and -dy/2 moves the geographic coordinates to the center of a grid box.

Hmm. It looks like your image is already in a Geographic projection, then. So, you would just need to set up the Map Info for the image once you've read it into ENVI. You can do that by selecting the image in ENVI, then right clicking and choosing Edit Header. In the Header Info dialog, click Edit Attributes > Map Info. In the next dialog, you will see that the default projection is Geographic Lat/Lon, so keep that there. But click the button with the up/down arrows to switch the coordinates to lat/long (from easting/northing). Now you can enter the image coordinates (at the top) and the latitude and longitude for any location in the image. Enter the pixel size, then click OK. Now your image should be georeferenced.

Dear Peg, Yo have been so helpful! I can't thank you enough. Your steps really work. Its really helps. Cheers. Idham