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GEOTIFF generation from BYU-MERS "SIR" image format


This document briefly describes how to convert a BYU .SIR file to a GEOTIFF file. It is a bit tricky to make GEOTIFF files from arbitrary images and projections.

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 easily earth-locate the image pixels. The binary SIR file includes a header with all the information to geolocate each pixel in the image. The origin of the .SIR file image is in the lower left corner. The earth location of a pixel is identified with its lower-left corner. Most sir files are in either polar stereographic or Lambert equal-area projections, though some EASE grid images are regularly produced.

The GDAL library and its utility program gdal_translate is used to create geotiff files. This programs can be run on Unix/Linux platforms or Windows.GDAL: http://trac.osgeo.org/gdal/wiki/DownloadingGdalBinaries
This page allows for the download of the GDAL software for various platforms. Download and install as per instructions to allow the execution of GDAL commands as part of the PATH variable.

Once the GDAL utility programs are installed one can execute a set of commands in the command shell to convert SIR file images to the geotiff formats. The gdal_translate program inserts coordinate reference details into the header of capable files (e.g. GeoTIFF).

A utility program, sir2geotiff.c, when compiled and run locally, automates the following process. Instructions for compiling and running this utility program are in the README.txt file.

To manually create a geotiff image, first create a full scale GIF or BMP image from the SIR file. Use a one-to-one pixel version. This can be done with sir2gif, sir2bmp, or sir_util. Source code for sir2gif, sir2bmp, and sir_util can be compiled for most platforms. (sir_util.exe is available as a precompiled windows executable.)

Then, run gdal_translate to create a .tif file that has the geotiff header. You will have to input the the corner coordinates (in meters) of the SIR image. The required gdal_translate projection parameters and corners can be determined using the sir_util tool.

Consider conversion of a QuickSAT slice Arctic image (qush-a-ArcYY-ddd-ddd.sir). Examinging the .SIR file header, .SIR image is in polar stereographic form with reference latitude 70 deg and vertical longitude -45 deg. The image corner coordinates are at -3400000 3406275 3406275 -3400000 km. (While all q*s*-*-Arc* have the same header, images of other areas will have different corners and projections). In this example a full scale .gif image is first created using sir2gif or sir_util, then gdal_translate is run with the image file as input, i.e.

gdal_translate -a_srs "+datum=wgs84 +proj=stere +lat_ts=70.0 +lat_0=90 +lon_0=-45" -a_ullr -3400000 3406275 3406275 -3400000 -of GTiff -co compress=LZW INPUT.gif OUTPUT.tif where
-a_srs (assigned Spatial Reference System, followed by proj4 params)
+datum (wgs84)
+proj (projection stereographic method)
+lat_ts (latitude of true scale 1:1)
+lat_0 (the latitude of origin)
+lon_0 (the longitude of the origin)
-a ullr (the projected coordinates of the upper left and lower right)
-of (output format)
-co (conversion options relevant to output format)

The Marine Geospatial Ecology Tools (MGET) project (also known as GeoECho Python) has developed an open source geoprocessing tool box that enables conversion of BYU SIR files into ArcGIS raster format files.