Documentation of the BYU-MERS "SIR" image format The BYU-MERS "sir" image format was developed by the Brigham Young University (BYU) Microwave Earth Remote Sensing (MERS) research group to store images of the earth along with the information required to earth-locate the image pixels. A "sir" file consists of one or more 512 byte headers containing all the information required read the remainder of the file and the map projection information required to map pixels to lat/lon on the Earth surface. Pixel values are generally stored as 2 byte (high order byte first) integers though can be stored as bytes or IEEE floating point. The latter is not portable to all machines and so is not recommended. Scale factors to convert the integer or byte pixel values to native floating point units are stored in the file header. The origin of the images are in the lower left corner. The earth locaion of a pixels is identified with its lower-left corner. The standard sir format supports a variety of image projections including: 0. Rectangular array (no projection) 1. A rectangular lat/lon array 2. Two different types of Lambert equal-area projections which can be use in both non-polar and polar projections 3. Polar stereographic projections 4. EASE grid polar projection with various resolutions 5. EASE global projection with various resolutions In general, *.sir image data files have been generated using the scatterometer image reconstruction (SIR) resolution enhancement algorithm or one of its variants for radiometer processing. The multivariate SIR algorithm is a non-linear resolution enhancement algorithm based on modified algebraic reconstruction and maximum entropy techniques. The algorithm is described in Long, D.G., P.J. Hardin, and P.T. Whiting, "Resolution Enhancement of Spaceborne Scatterometer Data," IEEE Trans. Geoscience Remote Sens., Vol. 31, No. 3, pp. 700-715, May 1993 and D.S. Early and D.G. Long, "Image Reconstruction and Enhanced Resolution Imaging from Irregular Samples," IEEE Trans. Geoscience Remote Sens., Vol. 39, No. 2, pp. 291-302, 2001 The singlevariate SIR algorithm developed originally for radiometers but also used for SeaWinds is described in Long, D.G., and D.L. Daum, "Spatial Resolution Enhancement of SSM/I Data," IEEE Transactions on Geoscience and Remote Sensing, Vol. 35, No. 2, pp. 407-417, 1998. The SIR w/filtering (SIRF) algorithm has been successfully applied to SASS and NSCAT measurements to study tropical vegetation and glacial ice. Variants of SIR has been successfully applied to ERS-1/2 scatterometer and various radiometers (SSM/I and SMMR). (e.g. the modified median filter is not used with ERS-1/2 data and QuikSCAT/SeaWinds egg data.) For fanbeam scatterometers, the multivariate form of the SIR algorithm models the dependence of sigma-0 on incidence angle as sigma-0 (in dB) = A + B * (Inc Ang - 40 deg) over the incidence angle range of 15 to 60 deg. The output of the SIR algorithm is images of the A and B coefficients. A represents the "incidence angle normalized sigma-0" (effectively the sigma-0 value at 40 deg incidence angle). The units of A are dB. Typically, +2 < A < -45 dB. However, in the SIR images A is typically clipped to a minimum -32 dB with values of A < -32 used to indicate no data. The B coefficient describes the incidence angle dependence of sigma-0 an has the units of dB/deg. At Ku-band global average of B is approximately -0.13 dB/deg. Typically, -0.2 < B < -0.1. B is clipped to a minimum value of -3 dB/deg. This value is used to denote no data as well. Single variable SIR or SIRF algorithms are used for radiometers and produce only an A (in this case, the brightness temperature) image. Typically, this can range from 165 to 320. Be sure to use binary ftp to transfer *.sir files! Subdirectories: c/ Code to read sir files using C. Utilities to convert SIR images to other file types (e.g., bmp, gif) f/ Code to read sir files using fortran77. Utilities to convert SIR images to other file types (e.g., byte) idl/ Code to load sir file images into IDL or PVWAVE, save to file and do forward/inverse transforms in IDL. see idl/readme.txt for information. matlab/ Code to load sir fileimages into matlab, save file, and to do forward/inverse transforms in matlab. Also includes some display routines. see matlab/readme.txt for information. sirsamples/ Example SIR images green.sir Sample A image generated from NSCAT data. Image is lambert projection 331x530. Lambert center point is -42.5 lon, 71.75 lat, scales 0.2181977 km/pix origin (lower-left corner) at -769, -1220. Old style v1 header. greenb.sir Byte format version of green.sir with v2 header. greenf.sir Float format version of green.sir with v2 header. greeni.sir Integer format version of green.sir with v2 header. greeni.sir_lat pixel latitude image of greeni.sir greeni.sir_lon pixel longitude image of greeni.sir greeni.sir.gz gzip'ed greeni.sir. sass_a_worldv.sir.Z Sample A image generated from SASS data. Image is global rectangular projection, 8 pixels/deg, 2880x1440 pixels (360 deg lon x 180 deg lat). File is compressed using unix compress utility. xv_byu/ Source code for a modified xv program which can read and display BYU .SIR files. See xv_byu/xv_byu_readme.txt for more details. Note: This code may be copied and modified so long as (1) original or modified code is not redistributed for profit and (2) acknowledgement is made that the original code was obtained courtesy of David G. Long at the Microwave Earth Remote Sensing Laboratory at Brigham Young University, Provo, UT. If you have any questions, please contact me. David Long ============================================================================== Dr. David G. Long long@ee.byu.edu Professor, MERS Lab Head voice: (801) 422-4383 Electrical and Computer Engineering Department fax: (801) 422-0201 459 Clyde Building Brigham Young University Provo, Utah 84602 BYU Electrical and Computer Engineering home page: http://www.ee.byu.edu/ BYU Microwave Earth Remote Sensing (MERS) Laboratory home page: http://www.ee.byu.edu/ee/mers/ ============================================================================== Last revised: 11 Aug. 2003 (c) 1999, 2000, 2001, 2003 BYU MERS