User Manual - pancroma

12. File I/O________________________________________________________________PANCROMA TM can input and output files in several formats. The most commoninput formats are TIFF (GeoTiff), and raw binary (.L1G suffix) format - mostLandsat and SPOT image data is archived in these formats. ASTER data isoften archived in compressed HDF format, which PANCROMA TM will also read.PANCROMA TM can also import data in JPEG , BMP and PNG format. Althoughthese are not typical satellite data archival formats, it is often convenient to usethese as output formats, particularly JPEG and PNG as they are compressedand therefore take up a lot less storage space. PANCROMA can also readDTED and write elevation index files and read/write and IEEE FP32 TIFF DEMdata.The GeoTiff reader has certain limitations. It cannot read compressed GeoTiffformat at the present time. Compressed GeoTiff is indicated by a Compressioncode other than ‘1’. (PANCROMA reports the GeoTiff tag codes to the dialogscreen as it opens data files.) The file reader will recognize WGS84, WGS 72,NAD 83 and NAD 27 geodetic datum codes only. Although the reader isgenerally robust, you may be able to cause a fault by discovering and illegal filecondition that the programmer did not consider. Please file a bug report if youencounter such a condition.Standard Landsat images contain 8 bits of grayscale information per raster pixel.The number of bits per pixel is called the image quantization or dynamic range.The analog incident light is digitized into 2 8 =256 levels per channel.PANCROMA TM can also read Digital Globe WorldView®, Landsat Reflectance,GeoEye® EO-1 ALI and other data with greater dynamic range. This data isformally 16-bit format. Often the useful dynamic range is only 11 bits however.These grayscale and RGB images are archived as either TIFFPlanarConfiguration=1 or PlanarConfiguration=2 files.All common computer grayscale and color display models are based on 8-bitquantization. The reason is that 8 bits are more than sufficient to producevisually pleasing grayscale or color images, and the human eye cannotdiscriminate finer color detail anyway. This means that PANCROMA TM mustconvert or rationalize 11 or 16 bits into 8 bit quantization in order to usefullydisplay it, using one of two methods. The default method scales the 11-bitinteger by a ratio of 255 divided by the maximum pixel value. Some DigitalGlobe data files have a small percentage of pixels displaying the maximum pixelvalue of 2047 while the vast majority are clustered much closer to 255. Theresult can be a very dark image when scaling from the maximum value.PANCROMA TM offers an alternative algorithm: it will read all 11 bits into aSHORT, shift right by ‘shiftBits=1’ bits, and truncates if greater than 255. If theresult is too bright, you can successively bit shift by 0 (-1 from the default) 122