Land Surface Temperature (LST) Calculator

Geospatial Tools & Analysis

This tool estimates Land Surface Temperature (LST) from At-Satellite Brightness Temperature, a key step in workflows that calculate LST using ENVI or other remote sensing software. Enter the values derived from your satellite imagery’s thermal band to begin.

This guide provides a deep dive into the theory and practice of how to calculate LST using ENVI, one of the premier software solutions for processing and analyzing geospatial imagery. Understanding Land Surface Temperature (LST) is critical for numerous applications, from climate change studies to urban planning and agricultural monitoring.

A) What is Land Surface Temperature (LST)?

Land Surface Temperature (LST) is the radiative temperature of the Earth’s surface. It’s a measure of how hot the ‘skin’ of the Earth would feel to the touch. This is different from air temperature, which is what weather forecasts typically report. LST is a key variable in the physics of land-surface processes, governing the balance of energy between the Earth’s surface and the atmosphere. Scientists and GIS professionals use software like ENVI to process thermal data from satellites (such as Landsat or ASTER) to create LST maps. These maps are invaluable for identifying urban heat islands, monitoring drought conditions, assessing vegetation health, and much more. The process of deriving LST is a core function of thermal remote sensing.

B) The Formula to Calculate LST and its Explanation

While ENVI can automate many steps, understanding the core formula is crucial for accurate analysis. After converting the raw digital numbers (DN) from a satellite’s thermal band to At-Satellite Brightness Temperature (BT), the final step to get LST involves correcting for surface emissivity. The most widely used formula for this conversion is:

LST = BT / (1 + (λ * BT / ρ) * ln(ε))

This equation, often applied using band math in ENVI or other GIS software, adjusts the satellite-measured temperature based on the specific wavelength of the sensor and the emissivity properties of the surface material. For more details on geospatial analysis, see our guide on {related_keywords}.

Description of variables used in the LST calculation.

Variable	Meaning	Unit (Auto-Inferred)	Typical Range
LST	Land Surface Temperature	Kelvin (K) or Celsius (°C)	250 K to 350 K
BT	At-Satellite Brightness Temperature	Kelvin (K)	250 K to 350 K
λ	Center Wavelength of the Thermal Band	Meters (m)	10.0 x 10^-6 to 12.5 x 10^-6
ε	Land Surface Emissivity	Unitless	0.90 to 0.99
ρ	Constant derived from Planck’s Constant, Boltzmann Constant, and speed of light	Meters-Kelvin (m·K)	1.438 x 10^-2

C) Practical Examples

Example 1: Calculating LST for an Urban Area

An analyst using ENVI is studying the urban heat island effect. They have already calculated the Brightness Temperature for a pixel in a dense downtown area.

• Inputs:
  • At-Satellite Brightness Temperature (BT): 310 K
  • Sensor Wavelength (λ): 10.8 µm (Landsat 8, Band 10)
  • Surface Emissivity (ε): 0.94 (typical for asphalt and concrete)
• Result:
  • The calculated LST would be approximately 315.4 K or 42.2 °C, showing a significant increase from the initial brightness temperature due to the emissivity of urban materials.

Example 2: Calculating LST for a Forested Area

In the same image, the analyst examines a pixel over a dense forest, a topic related to {related_keywords}.

• Inputs:
  • At-Satellite Brightness Temperature (BT): 298 K
  • Sensor Wavelength (λ): 10.8 µm (Landsat 8, Band 10)
  • Surface Emissivity (ε): 0.98 (typical for dense vegetation)
• Result:
  • The calculated LST would be approximately 299.1 K or 25.9 °C. The correction is smaller because vegetation has a high emissivity, meaning it radiates energy more efficiently and its brightness temperature is closer to its true kinetic temperature.

D) How to Use This LST Calculator

This calculator simplifies the final step of the LST calculation process, often performed in ENVI’s band math tool.

1. Obtain Brightness Temperature (BT): First, use ENVI’s Radiometric Calibration tool to convert the raw DN of your thermal band (e.g., Landsat Band 10) to radiance, and then to Brightness Temperature in Kelvin.
2. Enter BT: Input this Kelvin value into the “At-Satellite Brightness Temperature (BT)” field.
3. Enter Wavelength: Input the central wavelength of the thermal band you used. This can be found in the satellite’s metadata file.
4. Determine Emissivity (ε): This is the most complex input. Emissivity can be estimated using an NDVI-based method within ENVI, or by using a land cover classification to assign emissivity values to different surface types (e.g., water, forest, urban). Enter this value.
5. Interpret Results: The calculator instantly provides the final LST, corrected for emissivity. You can switch between Celsius and Kelvin to suit your analysis needs. This process is a fundamental part of a {related_keywords} workflow.

E) Key Factors That Affect LST

An accurate effort to calculate LST using ENVI must account for several influencing factors:

• Surface Emissivity: As demonstrated, this is a critical factor. Different materials emit thermal energy with different efficiencies. An incorrect emissivity value is a primary source of error in LST calculations.
• Atmospheric Correction: The atmosphere absorbs and re-radiates thermal energy. For highly accurate research, atmospheric correction models (like MODTRAN, available via tools in ENVI) should be used to correct the initial radiance values before they are converted to BT.
• Land Cover Type: Vegetation, water, soil, and man-made structures all have unique thermal properties that directly impact LST.
• Solar Radiation: The amount of incoming solar energy significantly affects the surface temperature, causing LST to vary greatly between day and night.
• Topography: The slope and aspect (the direction a slope faces) influence the amount of solar radiation received, causing variations in LST. North-facing slopes are generally cooler than south-facing slopes in the Northern Hemisphere.
• Soil Moisture: Wet soil has a higher thermal inertia and will heat up slower than dry soil, resulting in a lower daytime LST. This is an important consideration in {related_keywords} studies.

F) Frequently Asked Questions (FAQ)

What is ENVI software?

ENVI (Environment for Visualizing Images) is a specialized software used for the processing and analysis of geospatial imagery, including satellite and airborne sensor data. It is widely used in remote sensing to perform tasks like image correction, classification, and feature extraction, including workflows to calculate LST.

Why is emissivity so important when I calculate LST?

Emissivity defines how efficiently a surface radiates thermal energy compared to a perfect blackbody. Satellites measure radiance, not temperature directly. Since different materials (like water vs. asphalt) radiate differently even at the same true temperature, you must correct for emissivity to convert brightness temperature into an accurate LST.

What’s the difference between LST and air temperature?

LST is the temperature of the ground surface, while air temperature is typically measured 2 meters above the ground. LST often shows much greater extremes. On a sunny day, an asphalt parking lot can have an LST over 50°C (122°F) while the air temperature is only 30°C (86°F).

How do I get the Brightness Temperature value in ENVI?

You can calculate it from a satellite image’s thermal band. The typical workflow in ENVI is: 1) Open the image file (with its metadata). 2) Use the ‘Radiometric Calibration’ tool. 3) Select the thermal band and choose ‘Brightness Temperature’ as the output. The result will be a new raster file where pixel values represent BT, usually in Kelvin.

Can I calculate LST without ENVI?

Yes, other GIS and remote sensing software like ArcGIS, QGIS, or even programming languages like Python with libraries (GDAL, Rasterio) can perform the necessary raster calculations (band math) to derive LST. However, ENVI provides specialized, streamlined tools for these workflows.

Which Landsat band should I use?

For Landsat 8 and 9, you should use the Thermal Infrared (TIRS) Band 10. While Band 11 also exists, it has historically suffered from calibration issues (stray light) and USGS recommends using Band 10 for single-band LST algorithms.

Why does the LST result change when I select different units?

The underlying calculation is always performed in Kelvin, the absolute scale of temperature required for physics-based formulas. The unit selector is a post-calculation conversion for your convenience, changing the final Kelvin value to Celsius (K – 273.15) for easier interpretation, but it doesn’t alter the core scientific result.

What is a typical emissivity value for a city?

There is no single value. A city is a mosaic of surfaces. Emissivity values would range from ~0.92 for asphalt/concrete to ~0.96 for rooftops, and ~0.98 for parks and vegetation. For a large-scale view, an average emissivity of 0.93-0.95 is often used as a starting point for mixed urban areas.

G) Related Tools and Internal Resources

Explore more of our tools and guides to enhance your geospatial analysis skills.

• {related_keywords}: Analyze vegetation health and density.
• {related_keywords}: Learn about a different satellite-based temperature metric.
• {related_keywords}: A guide for another essential remote sensing index.