Image Enhancement Objective: better visualization of remotely sensed images visual interpretation remains to be the most powerful image interpretation even though we have unprecedented computing ability. Two types of enhancement: Spectral enhancement Spatial enhancement Note: Image enhancement leads to loss of spectral characteristics of targets and statistical integrity of the original data.

Techniques to Enhance Visual Effects 1. Minimum and Maximum Linear Stretch Very few pixels in the lower and upper end of the histogram take a large dynamic range and majority of the pixels are squeezed into a some range of DN values. It is a waste of resources for optimal visual effects. Usually we can allow a total of five percent saturation at the lower and the upper end of the histogram and stretch the remaining 95% of DNs into the full dynamic range. min max 0 255

Piecewise Linear Stretch Piecewise linear stretch applies a different linear stretch for different sections of the DN values. DN freq Output DN Input DN Can stretch one part of the histogram and compress other parts of the histogram.

Nonlinear Stretch- Histogram Equalization DN freq Transform the histogram of any shape to which has the same frequency along the whole range of digital number. This is realized by equally partitioning the cumulative histogram of the original image into 255 pieces (8-bit image). Due to the fact that image histograms are often bell shaped, histogram equalization often reduce contrast at lower and higher end of DNs and expand the middle DNs to the low and high end of histogram. Often make image look harsh, but it is easy to apply.

Spatial Enhancement Spatial enhancement improve the spatial patterns in the image, including texture, shapes, size, etc., thus help visual interpretation of the image. It is implemented based on neighborhood operations (convolution) using a kernel. A kernel is a small window of often 3x3 pixels in size (can be 5x5 or 7x7). The convolution will multiply each DN numbers by the kernel weight and add them up and put it in the center of the pixel. The commonly used kernels are: Mean kernel: 3 1/9 1/9 1/9 The mean kernel is often used to remove noise. What the kernel would be if you want to do a mean over 5x5 or 7x7 window?

Vertical line enhancement: Horizontal line enhancement: –1 –1 – –1 –1 –1 Edge detection: –1 –1 –1 –1 8 –1 –1 –1 –1 Edge enhancement original image: –1 –1 –1 –1 9 –1 –1 –1 –1 Diagonal line enhancement: –1 0 diagonal line enhancement: –1 –

What is GPS? GPS stands for Global Positioning System which measures 3-D locations on Earth surface with the aid of satellites Created and Maintained by the US Dept. of Defense and the US Air Force System as a whole consists of three components satellites (space segment) receivers (user segment) ground stations (control segment) Note: Russia and a European consortium are implementing similar systems.

Satellites (space segment) 24 NAVSTAR satellites (21 operational and 3 spares) orbit the Earth every 12 hours ~11,000 miles altitude positioned in 6 orbital planes orbital period/planes designed to keep 4-6 above the horizon at any time controlled by five ground stations around the globe

Ground-based devices can read and interpret the radio signal from several of the NAVSTAR satellites at once. Use timing of radio signals to calculate position on the Earth's surface Calculations result in varying degrees of accuracy -- depending on: Receivers (User Segment)

Five control stations master station at Falcon (Schriever) AFB, Colorado monitor satellite orbits & clocks broadcast orbital data and clock corrections to satellites Map from P. Dana, The Geographer's Craft Project, Dept. of Geography, U. Texas-Austin. Ground Stations (control segment)

Error Breakdown (typical case): satellite clock: satellite orbit: ionosphere/troposphere: multipath distortion: receiver errors: 1.5 meters 2.5 meters 5.5 meters 0.6 meters 0.3 meters

Differential Correction Any errors in a GPS signal are likely to be the same among all receivers within 300 miles of each other. Note: differential correction Can be applied in "real time" or after the fact (post-processing) GPS Reference Receiver sits over Precisely surveyed point Error signals