SME
Remote Sensing
Change detection
Use this for “Before a fire a pixel reflects 0.4 in near infrared and 0.15 in shortwave infrared; after it, 0.2 and 0.25. How severe was the burn?”
Diagram
NBR
before
=
NIR − SWIR
NIR + SWIR
=
0.4 − 0.15
0.4 + 0.15
=
0.455.
NBR
after
=
0.2 − 0.25
0.2 + 0.25
=
−0.111.
dNBR
=
0.455
−
(−0.111)
=
0.566.
Filled dots: before the fire; open squares: after.
The curves are typical shapes, drawn for reference.
This is an example. Type your own values to start a new problem.
NIR₁
Near infrared before
example
SWIR₁
Shortwave infrared before
example
NIR₂
Near infrared after
example
SWIR₂
Shortwave infrared after
example
NBR₁
Burn ratio before
calculated
NBR₂
Burn ratio after
calculated
dNBR
Change in burn ratio
calculated
S
Burn severity
worked out (not typed)
Formulas
NBR₁
=
NIR₁ − SWIR₁
NIR₁ + SWIR₁
0.4545
=
0.4 − 0.15
0.4 + 0.15
NBR₂
=
NIR₂ − SWIR₂
NIR₂ + SWIR₂
−0.1111
=
0.2 − 0.25
0.2 + 0.25
dNBR = NBR₁ − NBR₂
0.5657 = 0.4545 − (−0.1111)
S
from
dNBR:
cuts
at
−0.1,
0.1,
0.27,
0.44
and
0.66
0.5657 < 0.66, so moderate-high
Assumptions
•
Burning strips leaves and leaves char and ash: near-infrared reflectance drops and shortwave-infrared reflectance rises, so the burn ratio falls.
•
Both images are surface reflectances from 0 to 1, taken at the same time of year so seasonal growth does not change NBR.
•
Severity uses the usual dNBR thresholds: below −0.1 regrowth, under 0.1 unburned, then low to 0.27, moderate-low to 0.44, moderate-high to 0.66, high above.
Step-by-step
WE KNOW
NIR₁ = 0.4, SWIR₁ = 0.15, NIR₂ = 0.2, SWIR₂ = 0.25
FIND
burn ratio before (NBR₁), burn ratio after (NBR₂), change in burn ratio (dNBR), burn severity (S)
Step 1 · Find burn ratio before (NBR₁)
Use NBR₁ = (NIR₁ − SWIR₁) ÷ (NIR₁ + SWIR₁)
The gap between the two reflectances before the fire, over their sum: high for green plants, low or negative for char.
NBR₁
=
0.4 − 0.15
0.4 + 0.15
0.4 − 0.15 = 0.25
0.4 + 0.15 = 0.55
NBR₁ = 0.45455
Step 2 · Find burn ratio after (NBR₂)
Use NBR₂ = (NIR₂ − SWIR₂) ÷ (NIR₂ + SWIR₂)
The gap between the two reflectances after the fire, over their sum: high for green plants, low or negative for char.
NBR₂
=
0.2 − 0.25
0.2 + 0.25
0.2 − 0.25 = −0.05
0.2 + 0.25 = 0.45
NBR₂ = −0.11111
Step 3 · Find change in burn ratio (dNBR)
Use dNBR = NBR₁ − NBR₂
How far the burn ratio fell: the bigger the drop, the more of the plants burned.
dNBR = 0.45455 − (−0.11111)
dNBR = 0.5657
Step 4 · Find burn severity (S)
Use S from dNBR: cuts at −0.1, 0.1, 0.27, 0.44 and 0.66
A big drop: most of the canopy is scorched or gone, so the burn was severe.
0.5657 < 0.66: moderate-high
S
=
the
band
of
dNBR
S
=
the
band
of
0.5657
S
=
moderate-high
Check
Put the numbers back in. Both sides should match.
0.45455
=
0.4 − 0.15
0.4 + 0.15
✓
−0.11111
=
0.2 − 0.25
0.2 + 0.25
✓
0.5657 = 0.45455 − (−0.11111) ✓
0.5657 < 0.66, so moderate-high ✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.