Burn Severity Map of the 2009 Station Wildfire in Southern California

Irene Nester
ES 775 Remote Sensing
Emporia State University
10 December 2009

The Station Wildfire

The Station Fire was the largest wildfire in Los Angeles County history. It burned an area of approximately 250 square miles (approx. 648 km2) in the Angeles National Forest north of Los Angeles, California, in late August through late September 2009. The fire started near a ranger station on 26 August and was almost completely extinguished by the fourth week of September, although 100 percent containment was not declared until 15 October 2009. The fire was caused by arson. Two fire fighters were killed and over eighty homes were destroyed by the fire (The 818 Now, 2009). The map to the left shows the location of the burned area (maroon-brown) relative to Los Angeles and the surrounding area.

In order to estimate the severity of burning in different areas within the burned zone, Landsat 5 Thematic Mapper images acquired before the fire began (6 August 2009) and after the fire was extinguished (23 September 2009) were used to generate normalized burn ratio (NBR) images for each date. The difference between the pre-fire and post-fire NBR images, or dNBR, is an indication of burn severity (Howard et al. 2002). The map is presented below.

Station Fire Location Map
Image processing and map by Irene Nester. Base map is a false-color composite of Landsat 5 Thematic Mapper Bands 3, 4 and 5. Image acquired on September 7, 2009.

Station Fire Burn Severity Map
Image processing and map composition by Irene Nester.

Relative Burn Severity Level and Corresponding dNBR Range
(Based on Howard et al. 2002)

0.00 - Anomalous (unrelated to burning): -1000 to -500

0.50 - Enhanced Regrowth: -500 to -100

1.00 - Unburned: -100 to 100

2.00 - Low Severity: 100 to 270

3.00 - Moderate-low Severity: 270 to 440

4.00 - Moderate-high Severity: 440 to 660

5.00 - High Severity: 660 to 1300


Image Processing Steps

The initial assessment fire severity map was created using the following process. All image processing and calculations were performed using IDRISI Andes software.

1)  The Station Fire began burning on 26 August 2009 (USDA 2009) and was extinguished by 9 September 2009. Landsat 5 Thematic Mapper (TM) images that were acquired before the fire started (6 August 2009) and after the fire was extinguished (23 September 2009) were obtained. The images are terrain corrected (reference system Utm-11n).

2)  The values in calibrated digital numbers (DNs) for each pixel of Bands 4 and 7, which ranged from 0 to 255, were converted to Radiance (L) values using the following equation (Chander et al. 2007):

L = ((LMAX - LMIN) / (QCALMAX - QCALMIN)) × (QCAL - QCALMIN) + LMIN

Radiance represents the intensity of electromagnetic energy in each band at the aperture of the TM sensor, and is determined by accounting for the appropriate values of sensor-specific gain and bias. "QCALMAX" and "QCALMIN" are the maximum and minimum calibrated DN value, respectively, in the scene for the band in question. "(LMAX - LMIN) / (QCAL - QCALMIN)" represents the "gain factor" of the sensor and "LMIN" represents the "bias factor". "LMAX" and "LMIN" are the maximum and minimum spectral radiance value in units of W/(m2 × sr × µm). These values were obtained from the metadata for each scene.

3)  No haze correction was deemed necessary because the minimum DN of Bands 4 and 7 of both images is 0.

4)  In order to make a valid comparison of the solar energy reflected and measured by the TM on the two different dates, the Radiance values were then converted to Reflectance (R), or planetary albedo, using the following equation (Chander et al. 2009):

R = (L × π × d2) / (ESUN × cos(θ))

The term "d" is the distance between the Earth and the Sun, in astronomical units, on the date each image was acquired (Chander et al. 2009, Table 6). ESUN is the band-specific mean exoatmospheric solar irradiance obtained from published look-up tables (Chander et al. 2009, Table 3). "θ" is the solar zenith angle in degrees. The cosine of the solar zenith angle is equal to the sine of the solar elevation angle, which was obtained from the metadata for each scene.

5)  A normalized burn ratio (NBR) image was created for each scene, pre- and post-fire, according to the following formula (Howard et al. 2002):

NBR = (RBand 4 - RBand 7) / (RBand 4 + RBand 7)

6)  The two images were registered and the post-fire NBR was subtracted from the pre-fire NBR to obtain the differenced NBR, i.e., dNBR = pre-fire NBR - post-fire NBR (Howard et al. 2002). The resulting image was displayed using the IDRISI default quantitative palette.


Ground Views

All photographs below were taken along the Angeles Forest Highway on 6 December 2009 by Irene Nester.

Unburned vegetation outside fire zone. Severely burned hillsides near the edge of the burned area. Area where the fire burned the understory and only the bases of taller trees. Severely burned hillside with shrubs and succulent plants.
Adjacent severely burned (left) and partially burned (right) areas on a hillside. Burned pine and deciduous trees. Burned valley and hillsides. Vegetation regrowth about ten weeks after the fire. Plants in the foreground are Agave.


References

Chander, G., Markham, B. L., and Barsi, J. A. 2007. Revised Landsat-5 Thematic Mapper Radiometric Calibration. IEEE Geoscience and Remote Sensing Letters, Vol. 4, No. 3, July. http://landsat.usgs.gov/documents/L5TM_postcal.pdf

Chander, G., Markham, B. L., and Helder, D. L. 2009. Summary of current radiometric calibration coefficients for Landsat MSS, TM, ETM+, and EO-1 ALI sensors. Remote Sensing of Environment 113: 893-903. http://landsathandbook.gsfc.nasa.gov/handbook/pdfs/Landsat_Calibration_Summary_RSE.pdf.

Howard, S.M., Ohlen, D.O., McKinley, R.A., Zhu, Z. Kitchen, J. 2002. Historical fire severity mapping from Landsat data. Pecora 15/Land Satellite Information IV/ ISPRS Commission I Symposium, Proceedings. American Society for Photogrammetry and Remote Sensing. http://www.isprs.org/commission1/proceedings02/paper/00041.pdf

The 818 Now (From the Associated Press). 2009. Day 52: Station Fire completely contained. October 20. http://the818now.com/2009/10/20/day-52-station-fire-completely-contained/

USDA Forest Service Remote Sensing Applications Center, MODIS Active Fire Mapping Program, http://activefiremaps.fs.fed.us/activefiremaps.php?op=archive&rCode=cgb.