Atmospheric corrections

The atmosphere is evil Clouds Humidity Gasses Pollution/haze Different sun angles

clouds Block pretty much everything in the V-MIR. Shadows. Annoying as heck. Only solution, radar data

humidity Water absorbs. The more, the “hazier” the image. Not much you can do about it.

Pollution, dust, etc Also blocks light. Also annoying. Also, not much you can do about it.

gasses Scatter – increases brightness Absorbtion Haze and dust Water Raleigh Scatter, especially in blue. Decreases with increasing wavelength MIE scatter – dust and smoke, in the red region (sunsets, etc). Water Nonselective scatter through whole range. Basically, white water! Absorbtion Water vapor and CO2. Biggest effects in the MIR.

Sun angle Different angles = different shadows and different reflection. Sensors *try* to collect as close to noon as possible to minimize these effects.

Radiometric corrections (or what to do about these issues) Physical models of the atmosphere. Requires a LOT of data, often not available. Look at reflections from known objects and compare to a library. Adjust each band according to differences from “truth”. Easy example is deep, clear water. Should have reflection of 0 in all bands. Histogram equalization – shuffle each band such that they all have a minimum value of zero. Just basic subtraction. Simple. Statistically as good as the others…. Regression – another way of comparing reflectances and then shuffling them back to zero.

When to do? Honestly, the only time it’s really worth it is if you are using data over the same area over different times and you want to compare. The histogram equalization will help minimize whatever different atmospheric conditions exist at the different times.

More info? Book. Especially starting at page 491. But there’s info scattered about. ERDAS – raster – radiometric. Bunches of stuff. Note – “histogram equalization” in erdas is NOT the radiometric correction noted here. It’s a contrast stretch. The one real radiometric correction model requires additional software.

Sensor collection errors Striping and line drop.

http://www.slideshare.net/parodign/atmospheric-correction-albuferaweb

In erdas Again, in raster – radiometric Look at. Note – all this stuff (and pretty much everything all quarter) are covered in your text. Be sure to look it up and read, especially while studying! The text is pretty clear.

Did you know?