February 3 Interpretation of Digital Data Bit and Byte ASCII Binary Image Recording Media and Formats Geometric corrections Image registration Projections

Computer Basics (1)Bit and byte 1 bit bit bit One bit in computer requires one register– a switch between 0 and bit bit bit bit bit One byte has 2 =256 combinations with 8 registers. It therefore can be used to specify Letter, arabics and symbols up to a total of 256

Computer Basics 2. ASCII file (text file) (American Standard Code for Information Interchange) bytes Each digit uses one byte. One byte is required between two numbers and at the end of a line or file. Information on an image, such as date, location, level of processing, solar zenith angle, view angle, etc., are usually contained in ASCII format i.e., header of an image file.

Computer Basics 2. binary file bytes Advantage of binary files: (i) less memory requirement, (ii) fast reading and saving operations Remote sensing images are commonly stored in binary files Disadvantage of binary files: (i) content can’t be displayed as text, (ii) the information can not be retrieved without knowing the image format, i.e., # of bits/datum, pixels/line, # of lines

Packing Multispectral Data (1)Band interlived by pixel (BIP) : data are stored pixel by pixel for all bands simultaneously …… Band 1 Band 2 Band 3 Band … … ….. In a binary file: ……… Pixel (1,1) Pixel (2,1) Pixel (3,1)

Packing Multispectral Data (2) Band interlived by line (BIL): data are stored line by line for all bands simultaneously …… Band 1 Band 2 Band 3 Band … … ….. In a binary file: Band 1 line 1, band 2 line 1, band 3, line 1, band 4, line 1, band 1 line 2, band 2 line 2, ……

Packing Multispectral Data (3) Band sequential (BSQ) : data are stored one image (band) by one image …… Band 1 Band 2 Band 3 Band … … ….. In a binary file: band 1, band 2, band 3, band 4

Geometric Distortions Sources:variations in altitude and velocity of sensor platform panoramic distortion, earth curvature, atmospheric refraction, relief displacement, and nonlinearities in the swath of a sensor’s field of view (FOV) Campbell chapter 10 Lillesand-Kiefer 7.2 Geometric corrections compensate for the distortion introduced by these factors so that the corrected images will have geometric integrity.

Image distortions: nonsystematic Campbell 8.7 Earth rotationAltitude variation Pitch variation Spacecraft velocity Roll variation Yaw Variation Distorted image Restored image

Image distortions: systematic Cross-track distortionMirror velocity variationScan skew Time Mirror angle Actual velocity Nominal velocity

Image Matching (1) Overlay of two images (2) Registration of two images (or one image to a map) (3) Analytical registration of two images Campbell 10.5 Projections and re-sampling No re-sampling Ground control points and re-sampling

Registration Find the same features, Ground Control Points (GCPs), on two images or on one image and a map. GCPs can also be taken during field experiments with Global Positioning Systems (GPS). GPS is a US Military satellite-based navigation system that uses at least three satellites to calculate a position. Verbyla Chap. 5 Campbell 12.7

Choice of G round C ontrol P oints From Maps and GPS Road intersections Rivers Water bodies Campbell 10.6

Image Rectification models Verbyla Chap. 5 Linear Y = AX + B Residual Errors X Y Affine Coordinate Transformations Y’ = AX + BY + C X’ = DX + EY + F Polynomial Models Y’ = A + BX + CY + DX 2 + EY 2 + FX 3 +GY

How many GCPs? Polynomial order First Second Third Minimum Number of GCPs Required Verbyla Chap

PCI: GCPWorks

Distorted Restored Sabins, Chapter 8

Geometric Correction Output Image Original Image Campbell 10.6 Lillersand-Kiefer 7.2 In general, a pixel in the output image will not directly overlay a pixel in the original image What values will the pixels in output image have?

Nearest Neighbour Bilinear Interpolation (weighted average) Re-sampling Campbell 10.6

Cubic Convolution (weighted average) Re-sampling Campbell 10.6 Most widely used

Map Projections Campbell 10.7 Verbyla Chap. 5 Satellite Images that cover large areas need map projections because of the Earth’s spherical surface. It is impossible to project a spherical surface onto a flat sheet without distortions to the shape, area, distance, and direction of the surface features.

U niversal T ransverse M ercator Coordinates Campbell Zones of 6 degrees wide (east-west) zone 1 is at o W From 84 o N to 80 o S in latitude

Projections Polar azimuthal (planar) Regular conic Sabins, Chapter 8