User Manual - pancroma

• image resampling• geometric model precision correction using ground control• terrain correctionThe terrain (T) and geometric (G) corrections are as described athttp://landsathandbook.gsfc.nasa.gov/data_prod/prog_sect11_3.html. Thisreference also states that during 1G product rendering image pixels areconverted to units of absolute radiance using 32 bit floating pointcalculations. Pixel values are then scaled to byte values prior to mediaoutput. The following equation is used to convert DN's in a 1G product backto radiance units:Lλ = Grescale * QCAL + BrescalePublished Landsat data is corrected using algorithms 1-8, listed above. Theterrain correction using a DEM is only applied for L1T, and not L1G. It is possibleto convert from DNs to radiances using the band gain and bias parameters inthe Landsat metadata. (DNs or Digital Numbers are sensed radiation quantizedto discrete levels, for example 0-255 for Landsat data.)Conducting analyses using Landsat data (for example NDVI analysis) may ormay not require further processing. NDVI or other image parameters can becomputed using uncorrected DNs from L1T data and the index can besuccessfully used to highlight vegetative areas in an L1T color composite image.However, problems could be introduced when comparing derived imagecharacteristics or statistics using DNs between one Landsat scene and another,or between two Landsat scenes acquired at different dates.The problem is that the parameter that you are interested will probably be relatedto ground reflectance, i.e. the light reflected off the earth’s surface if you werestanding next to the plant and aiming a spectrophotometer at it. You couldcollect data during different seasons or during different years and drawconclusions from the readings if you took into account the differences in theincident solar radiation as a function of solar zenith angle, atmosphericcharacteristics and other factors.Comparing DNs acquired by a satellite in a similar fashion can lead to problemsbecause even though the Landsat DNs have been calibrated and processed,they have been acquired under different lighting and atmospheric conditions.This can be partially corrected by working with Top of Atmosphere (TOA)reflectance’s rather than DNs. There are of course a great many other factorsthat could influence the validity of such comparisons. The USGS tutorial postedat http://www.pancroma.com/data that you can download has a lot moreinformation on atmospheric physics and corrections. Also see the paper entitled“Measuring Urban Air Quality Using Worldview2 Multiangle Multispectral BandData” published at http://www.pancromap.com/whitePapers.html.180