# GRASS Help System

## BIL, BSQ, MAP PROJECTIONS, RECTIFICATIONS, ETC.

When ordering digital data, the user must specify the format of the tape and data he wishes to receive. Issues are briefly reviewed here. Refer to company data ordering instructions for specific information.

Data Corrections, Map Projections, and Data Resampling: Satellite data can be geometrically and radiometrically corrected and resampled, and put in a standard, distortion-free map projection. Radiometric corrections compensate for variations in satellite sensor gain (affecting image brightness). Geometric corrections are made to compensate for the earth's rotation, spacecraft altitude and sensor variations. Image data take into account ground control points and map projections variations. A map projection describes the manner in which the spherical surface of the earth is represented on a two- dimensional surface and attempts to minimize distortion in area, shape, distance, and direction. Read Map Coordinate Conversions and Map Projections and Geometric Rectification under the Image Processing menu for an examination of GRASS commands relevant to map projections, image rectification, and coordinate conversion. Data can also be resampled to correct the brightness value of image pixels.

Area of Search and Resolution: Companies making available digital imagery and other data supply free indexes showing areas of the earth for which data are available. The user must balance data acquisition and storage costs against the extent and resolution of the data desired. Users can request that companies shift standard scenes to include their data, generally for an added fee. - LANDSAT 7-band TM data have roughly a 30 meter resolution. - LANDSAT 4-band MSS data have roughly an 80 meter resolution. - SPOT 1-band panchromatic data have roughly a 10 meter resolution. - SPOT 3-band data have roughly a 20 meter resolution. - NHAP data have roughly a 5-10 meter resolution.

Maximum Available Cloud Cover: Generally, it is advisable to order data with no more than 10&percent; cloud cover. Scenes can generally be reviewed before ordering to ensure that clouds do not obscure vital portions of your image.

Time of Year: In most regions, the season of the year at which imagery is flown will drastically impact landcover. Consider the application for which your imagery will be used when selecting a date for imagery. If you wish to detect landscape changes occurring over multiple years, attempt to order satellite data from the same time (month) of each year.

Image Bands: Different satellites detect different spectral bands. Different image bands are useful for different applications. See data ordering forms for specific spectral band ranges collected by different satellites. Panchromatic images are black/white images with 1 image band. Multi-spectral images are color images with multiple image bands.

- SPOT panchromatic data contain 1 image band.
- SPOT multi-spectral data have 3 image bands.
- LANDSAT panchromatic images contain 1 image band.
- LANDSAT Thematic Mapper (TM) data have 7 image bands.
- LANDSAT Multi-Spectral Satellite (MSS) data have 4 image bands.
- NHAP data are available in color infrared (CIR) and black/white.

Tape Formats: Multi-band data can be ordered in either band sequential (BSQ) or band-interleaved (BIL) formats, at either 1600 bpi or 6250 bpi. In BIL format, the different image bands are stored together (interwoven), for each pixel in the image. In BSQ format, band data are separated -- data for all image pixels in one image band are stored together, followed by all data pertaining to a second band, etc. GRASS can handle either BIL or BSQ data. See the "GRASS Imagery Tutorial" or the programs listed under the Image Processing menu for further information.

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Automatically created on: Sat Jan 8 06:50:26 2005