Thermal & Emissivity
Thermal & Emissivity turns a thermal infrared band into an actual land surface temperature, in degrees, rather than a picture of relative warmth.
The subject matters for urban heat islands, for irrigation and crop water stress, for geothermal and volcanic monitoring, and for anything about evapotranspiration.
Why it takes three tools
Section titled “Why it takes three tools”A thermal sensor measures the radiation arriving at the satellite, which is not the temperature of the ground. Two things stand between them.
The atmosphere absorbs and re emits thermal radiation, mostly through water vapour, so the signal is attenuated on its way up.
Emissivity. Objects at the same temperature do not radiate equally. A dense forest radiates almost like a perfect emitter, dry bare soil noticeably less. Getting the emissivity wrong by a few percent shifts the retrieved temperature by a degree or more, which is often larger than the effect being studied.
Hence the workflow: estimate the emissivity, then invert the radiative transfer with one of the two retrieval methods.
Start here
Section titled “Start here”Ndvi Based Emissivity
Section titled “Ndvi Based Emissivity”Estimates emissivity from the vegetation fraction, itself derived from NDVI. Dense vegetation gets the emissivity of a canopy, bare surfaces that of soil, and mixed pixels a weighted value.
It is an approximation, and it is the standard practical method, because it needs only the red and near infrared bands you already have.
Run this first. Both retrieval tools take its output.
Land Surface Temperature Split Window
Section titled “Land Surface Temperature Split Window”The preferred method when your sensor has two thermal bands, as Landsat 8 and 9 do.
Water vapour absorbs slightly differently in the two bands, so the difference between them carries the information needed to cancel the atmospheric effect out. That is what makes the split window robust without needing atmospheric measurements.
Use it whenever two thermal bands are available.
Land Surface Temperature Single Channel
Section titled “Land Surface Temperature Single Channel”For sensors with a single thermal band. It inverts the radiative transfer equation from that one channel, which means the atmospheric contribution has to come from elsewhere, typically from water vapour content estimates.
More sensitive to that assumption than the split window, and perfectly usable when it is the only option.
All the tools
Section titled “All the tools”| Tool | What it does |
|---|---|
| Ndvi Based Emissivity | Estimates broadband emissivity from the vegetation fraction derived from NDVI. |
| Land Surface Temperature Split Window | Retrieves temperature from two thermal bands, cancelling the atmospheric effect. |
| Land Surface Temperature Single Channel | Retrieves temperature from a single thermal band. |
Surface temperature is not air temperature. A dry asphalt car park can reach 50 degrees while the air above it is at 30. These tools measure the skin of the ground, which is what matters for urban heat and for water stress, and which is not what a weather station reports.
Time of acquisition matters. Surface temperature swings enormously through the day. Comparing two dates only makes sense when both were acquired at the same local solar time, which is normally the case for a given satellite.
Thermal bands are coarse. Landsat thermal data is delivered at 100 metres, resampled to 30. Do not expect building level detail from it.