--- Start of Idrisi List message Hi Guillaume Larocque, Chris Huddart, Tim Shields and others CONVERTING AVHRR 8x8km DATA TO IDRISI For those of us who can't use C programming, here is the method for plebs (it took me three whole days to figure it out!!! There may still be a better way - if you know of one, let me know.): To convert AVHRR data (Goode Homolosine projection) to Idrisi, I used a free programme from the FAO, called WinDisp4, downloadable at: http://www.fao.org/WAICENT/faoinfo/economic/giews/english/windisp/dl.htm Since both WinDisp and Idrisi have a batch / macro facility, the conversions of many files are actually quite quick. In short, I imported the AVHRR data into WinDisp, converted it into Lambert Azimuthal projection, since this is recognised by Idrisi, is also an equal-area projection, and is versatile in that the origin, or centre, of a regional projection can be placed anywhere on the globe. This file could then be exported to Idrisi, where I created a custom Lambert projection suitable for my data. Once the images were correctly geo-referenced in Idrisi, they could be reprojected into lat/long or any other projection within Idrisi. Here are step-by-step instructions: I will use as example a custom window of the 8x8km NDVI data which I downloaded for southern Africa from http://daac.gsfc.nasa.gov/ >Search & Order > Data Sets > AVHRR > Data Products > FTP products > FTP user subset > Africa > custom window I also downloaded a global image (8x8km NDVI) as reference image for my subset. The global image is 5004 pixels wide and 2168 lines high. The centre of this reference image (0�N x 0�E), is at pixel 5004 / 2 = 2502, and line 2168 / 2 = 1084. To import the global image into WinDisp4: (just for interest�s sake) Process > Import > Binary Image Height: 2168 Width: 5004 Process > Header > Edit Image Type: 0, Generic Projection: 9, Goodes Homolosine Height: 2168 Width: 5004 Latitude Centre: 0 (centre of the projection) Longitude Centre: 0 (centre of the projection) X Centre: 2502 (half of width for reference image) Y Centre: 1084 (half of height for reference image) DX: 8000 (width of pixel in projection units, ie m/pixel) DY: 8000 (height of pixel in projection units, ie m/pixel) By the way, note that the X-item in the header file does not consistently come before the Y-item! So: X Y Long Lat W-E N-S Width Height Columns Rows Pixels Lines Now to the subset download. This is the Readme information on the southern African subset: Data Format: unsigned char (NDVI), unsigned integer(other parameters) Headers, trailers and delimiters none Missing data =0, water= 1, Interrupted space =2, Land data >=10 Subseted image size: 327 x 202 Goode's coordinates for subset area: min_line = 1363 max_line = 1564 min_pix = 2663 max_pix = 2989 lat,long corner values for goode's subset box area lower left -34.524973, 9.833143 (lat,long) lower right -34.524973, 38.301293 (lat,long) upper right -20.063874, 36.052255 (lat,long) upper left -20.063874, 11.082543 (lat,long) IN WINDISP4: Step 1 - Process > Import > Binary Image Width and Height as given in download documentation �subseted image size: 327 (width) x 202 (height)� Step 2 - Process > Header > Edit Set the header of the imported image. According to the WinDisp help file, the Lat Centre and Long Centre are the centre of the projection, ie 0�,0�. The X Centre and Y Centre �define the centre of the image relative to the reference projection image. For the reference image (in this case the global image), these values are half the height and width. For sub-images (here the SA data), the sub-image offsets are subtracted from the reference X Centre and Y Centre.� In relation to the global NDVI image, the southern African subset starts with pixel 2663 (�min_pixel�), so the X-offset is 2663. It starts in line 1363 (�min_line�), so the Y-offset is 1363. The centre of the global reference image (0�,0�) is at pixel 2502, line 1084, so, according to the WinDisp4 help file, you subtract the subset offset from the reference image centre, ie: the X-centre of the SA subset image is 2502 - 2663 = -161 and the Y-centre is 1084 - 1363 = -279 Image Type: 0, Generic Projection: 9, Goode Homolosine Height: 202 Width: 327 Latitude Centre: 0 Longitude Centre: 0 X Centre: -161 Y Centre: -279 DX: 8000 DY: 8000 Step 3 - Find centre of subset image To find the exact latitude and longitude of the subset centre, which you will need for the conversion to Lambert, zoom into the centre of your image, until the pixels are at least a cm wide on your screen (for maximum accuracy). The Long (X) and Lat (Y) are displayed in WinDisp4 in the third box from the left, in the status bar, and in that order. Centre image pixel = width/2 = 163.5 With cursor on the border between pixel 163 and 164 (in line 101), read off the X-coordinate = 23.77� Centre line = 101 With cursor in the middle of line 101 (at pixel 163) read off the Y-coordinate = -27.33� Long Centre: 23.77 Lat Centre: -27.33 X Centre: 163.5 Y Centre: 101 Step 4 - Process > Image > New You need to create a new blank image with the correct projection (Lambert Azimuthal) and dimensions, in order to re-project the Goode image. Height: 202 Width: 327 Step 5 - Process > Header > Edit Set the Header of the new image. Get Lat/Long Centre values from Step 3. Image Type: 0, Generic Projection: 6, Lambert Azimuthal Height: 202 Width: 327 Latitude Centre: -27.33 Longitude Centre: 23.77 X Centre: 163.5 Y Centre: 101 DX: 8000 DY: 8000 Step 6 - Process > Reproject Image to reproject: Imported Goode Homolosine image Reference image...: New Lambert Azimuthal image Image to create: Output filename Step 7 - Process > Export > Idrisi Image IN IDRISI Step 8 - Create a new Idrisi projection reference file The Origin of the new Lambert Azimuthal projection for your data area is simply the Lat and Long of the centre of your subset image (ie 23.77, -27.33 for my SA data). Use EDIT to open lazea.ref (Lambert Azimuthal Equal Area, set up for the US). Save under a new file name, eg lazea_sa.ref , and make following changes (datum NAD27 and NAD83 are for the US only. Lambert Azimuthal is for projection on a sphere.): ref. system : USGS Lambert Azimuthal Equal Area projection : Lambert Oblique Azimuthal Equal Area datum : none delta WGS84 : 0 0 0 ellipsoid : Mean Sphere major s-ax : 6370997.000 minor s-ax : 6370997.000 origin long : 23.77 origin lat : -27.33 origin X : 0 origin Y : 0 scale fac : na units : m parameters : 0 Step 9 - Modify the raster documentation file: Change the raster documentation information as follows: Calculate the minimum X and Y (which have to be in m): min. X : - (width of image / 2 * resolution) max. X : width / 2 * resolution min. Y : - (height / 2 * resolution) max. Y : height / 2 * resolution The resolution of the 8km x 8km NDVI surfaces is obviously 8000m, so the X-bounds of my SA subset are -/+ (327 / 2) * 8000, and the Y-bounds are -/+ (202 / 2) * 8000. The origin of the projection will be at 0,0 m, which is also the centre of the image, and which is set in the lazea_sa.ref file. data type : byte file type : binary columns : 327 rows : 202 ref. system : lazea_sa ref. units : m unit dist. : 1 min. X : -1308000 max. X : 1308000 min. Y : -808000 max. Y : 808000 pos'n error : unknown resolution : 8000 min. value : 0 max. value : 252 display min : 0 display max : 252 There you have it. Once the image is properly georeferenced in Idrisi, it can be projected into any of the other Idrisi-recognised projections. -------------------------------------------- Marlies Craig Medical Research Council 771 Umbilo Road / PO Box 17120 Congella 4013 DURBAN South Africa Tel: +27-31-2043600 (direct: 3653) Fax: +27-31-2043601/2051498 Email: [EMAIL PROTECTED] -------------------------------------------- --- End of Idrisi List message IDRISI-L is an unmoderated list. As a service to list users, please post a summary of useful responses to your questions. 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