# Difference between revisions of "IKONOS"

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[[Category: Reflectance]] | [[Category: Reflectance]] | ||

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## Revision as of 05:18, 27 July 2013

## Contents

**[This page is under construction]**

IKONOS is a commercial earth observation satellite. Details about the sensor are provided at Digital Globe's IKONOS Data Sheet

## Availability (Sample Data)

- Search for commercial satellite image providers in the internet.
- The Global Land Cover Facility (GLCF) provides four openly available IKONOS scenes of western Sichuan.
- ISPRS provides a small IKONOS data set, fragments from a Panchromatic image as well as from a Stereo product.

## Pre-Processing Overview

Typically, multispectral satellite data are converted into physical quantities such as *Radiance* or *Reflectance* before they are subjected in multispectral analysis techniques (image interpretation, band arithmetic, vegetation indices, matrix transformations, etc.). The latter can be differentiated in *Top of Atmosphere Reflectance* (ToAR) which does not account for atmospheric effects (absorption or scattering) and in *Top of Canopy Reflectance* (ToCR) which introduces a "correction" for atmospheric effects.

In order to derive Reflectance values, likewise as with remotely sensed data acquired by other sensors, IKONOS raw image digital numbers (DNs) need to be converted to *at-sensor spectral Radiance* values. At-sensor spectral Radiance values are an important input for the equation to derive Reflectance values. Note, *Spectal Radiance* is the measure of the quantity of radiation that hits the sensor and typically expressed in , that is *watts* per *unit source area*, per *unit solid angle*, and per *unit wavelength*.

Converting DNs to at-sensor Radiance can be done by using the following equation:

Converting to Top of Atmosphere Reflectance, also referred to as Planetary Reflectance, can be done by using the following equation:

where:

- - Unitless Planetary Reflectance
- - mathematical constant (3.14159265358)
- spectral Radiance at the sensor's aperture, from equation...
**ToADD** - - Earth-Sun distance in astronomical units, interpolated values
- - Mean solar exoatmospheric irradiance(s) (W/m2/μm), interpolated values
- - Solar_zenith_angle from the image acquisition's metadata

## Modules overview

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## File Formats & Metadata

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## Pre-Processing

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### Importing data

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### Deriving Physical Quantities

Converting Digital Numbers to Radiance/Reflectance requires knowledge about the sensor's specific spectral band parameters. Those are, as extracted from the document *IKONOS Planetary Reflectance and Mean Solar Exoatmospheric Irradiance*, by Martin Taylor (see references):

```
Pan_CalCoef=161 ; Pan_Width=403 ; Pan_Esun=1375.8
Blue_CalCoef=728 ; Blue_Width=71.3 ; Blue_Esun=1930.9
Green_CalCoef=720 ; Green_Width=88.6 ; Green_Esun=1854.8
Red_CalCoef=949 ; Red_Width=65.8 ; Red_Esun=1556.5
NIR_CalCoef=843 ; NIR_Width=95.4 ; NIR_Esun=1156.9
```

The following examples exemplify the conversion of raw Blue band digital numbers into Radiance and Reflectance.

#### Spectral Radiance

Converting a Blue Band (Digital Numbers) in to Spectral at-sensor Radiance in the *correct* units to be further used for the conversion in to unitless Reflectance:

`r.mapcalc ``"Blue_Radiance = ( (10000 * IKONOS_Blue_Band_DN) / (728 * 71.3) )"`

#### Planetary Reflectance

The equation to derive Reflectance values incorporates in addition:

- The mathematical constant Pi
`PI=3.14159265358`

. - The Earth-Sun distance in astronomical units which depends on the acquisition's day of year (DOY -- also referred to as Julian day, Ordinal_date) and can be retrieved from the following spreadsheet <http://landsathandbook.gsfc.nasa.gov/excel_docs/d.xls>.
- The mean solar exoatmospheric irradiance in . See 3rd column of (interplated) values given above.
- The cosine of the
*Solar Zenith Angle*(SZA) at the time of the acquisition. The SZA can be calculated from its complementary*Solar Elevation Angle*(SEA) given in the image acquisition's metadata.

In the following example we accept as the acquisition's `DOY=166`

and `SEA=52.78880`

. Hence, we get the Earth-Sun distance `ESD=1.0157675`

and `SZA = 37.21120 deg`

.

Converting the in-Blue spectral band at-sensor Radiance in to Planerary Reflectance:

`PI=3.14159265358; ESD=1.0157675; BAND_Esun=1930.9; SZA=37.21120`

r.mapcalc`"Blue_Reflectance = ( ${PI} * Blue_Radiance * ${ESD}^2 ) / ( ${BAND_Esun} * cos(${SZA}) )"`

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### Atmospheric correction

## Post-Processing

Having beforehand satellite image data expressed in physical quantities (radiance or reflectance) is preferred to follow-up with digital image analysis techniques.

### Color composites

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### Pan Sharpening

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### Vegetation Indices

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## IKONOS Image classification

## References / Sources

- http://www.apollomapping.com/wp-content/user_uploads/2011/09/IKONOS_Esun_Calculations.pdf
- Ikonos DN Value Conversion to Planetary Reflectance Values, by David Fleming
- Landsat7 Science Data Users Handbook, Chapter 11, Section 3
- Some short presentation about the DN to Reflectance conversion: Calibrated Landsat Digital Number (DN) to Top of Atmosphere (TOA) Reflectance Conversion, by Richard Irish

## See also

- GRASS-Wiki page about Image Processing

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