Ratios
• Relationship between two numbers of a similar kind, e.g.
The Mullet Ratio:“business at the front, party at the back”
RS Ratios
• compare the reflectance in two bands
• if you know the shape of the spectral signature of target features, ratios can help distinguish features
• not (usually) a method used in isolation – part of process • convergence of evidence
Other ways of looking …
• Marked differences in reflectance across bands can be revealing – e.g. the Red Edge
Th
e R
ed E
dge
• the region of rapid change in reflectance of vegetation between the red and the near infrared ranges of electromagnetic spectrum
• reflectance from the leaf surface is low in the visible spectrum, and
• NIR is reflected strongly due the arrangement of cells in the spongy mesophyll, causing an increase in NIR reflection
Veg
etat
ion
in
dic
es • Reflectance Ratio Vegetation Index
IR/R
• Reflectance Difference Vegetation IndexIR-R
• Normalised Difference Vegetation Index
• Transformed Normalised Difference Vegetation Index
)(
)(
RNIR
RNIRNDVI
5.0)(
)(
RIR
RIRTNDVI
• Different ways of expressing the relationship that the Red Edge illustrates
• Magnitude of the resultant values differs for each index, making some easier to interpret than others
• There are many other vegetation indices
No
rmal
ized
Dif
fere
nce
VI
• Low negative values are often associated with water
• Areas of barren rock, sand, or snow usually show very low NDVI values (for example, 0.1 or less)
• Sparse vegetation such as shrubs and grasslands or senescing crops may result in moderate NDVI values (approximately 0.2 to 0.5)
• High NDVI values (approximately 0.6 to 0.9) correspond to dense vegetation such as that found in temperate and tropical forests or crops at their peak growth stage.
Oth
er “
edges
”• Other phenomena can be detected in a similar way
• Reflection in the NIR is unaffected by amount of water in the leaf canopy, where as SWIR reflectance increases with water content in the leaf canopy – giving rise to the Moisture Stress Index
MSI = SWIR/NIR
Mo
istu
re S
tres
s In
dex
The value of this index ranges from 0 to ≈ 1 (note the values are inverted)
Normalized Difference Water Index
There are two varieties:• NDWI (Vegetation) (Gao 1996)
• NDWI (Waterbodies) (McFeeters 1996)
In LS8 B3=green, B5=NIR, and B6=SWIRIn Sentinel2 B3=green, B8 = NIR, B11= SWIR (20m)
𝐺𝑟𝑒𝑒𝑛 − 𝑁𝐼𝑅
𝐺𝑟𝑒𝑒𝑛 + 𝑁𝐼𝑅
𝑁𝐼𝑅 − 𝑆𝑊𝐼𝑅
𝑁𝐼𝑅 + 𝑆𝑊𝐼𝑅
No
rmal
ised
Dif
fere
nce
B
uilt
-up
In
dex
The value of this index ranges potentially from -1 to 1
Similar to the NDWI (vegetation), but inverts the order of SWIR and NIR
𝑆𝑊𝐼𝑅 − 𝑁𝐼𝑅
𝑆𝑊𝐼𝑅 + 𝑁𝐼𝑅
En
han
ced
Built
-up
an
d
Bar
ren
nes
s In
dex
Extends the NDBuI, to include thermal infra-red data – based on the understanding that built-up areas reflect/emit more heat than vegetated areas
𝐸𝐵𝐵𝐼 =𝑆𝑊𝐼𝑅 − 𝑁𝐼𝑅
10 𝑆𝑊𝐼𝑅 + 𝑇𝐼𝑅
Rem
emb
er… • Not definitive representations
• Use as element of ‘convergence of evidence’
• Defensible is the objective…
Imag
e dif
fere
nci
ng
• A way of assessing change in a landscape is to quantify the change in the values of RS indices on a pixel-by-pixel basis
• Can be estimated using a standard % change analysis as a point of departure, e.g. NDVI change over two images
• Implementation is a little more complex due to the range of NDVI values, but can be resolved
• This can be interpreted, using the NDVI as an example, as areas of vegetation vigour intensification or reduction
𝑁𝐷𝑉𝐼𝑡1 − 𝑁𝐷𝑉𝐼𝑡2𝑁𝐷𝑉𝐼𝑡1
Classify the result into 5 (or more) classes
First, decide on a change threshold you want to explore:
• +ve greater than the specified threshold
• +ve less than the specified threshold
• Unchanged
• -ve less than the specified threshold
• -ve greater than the specified threshold