The Urban Index: Built-Up Mapping With the Far-SWIR Band
The Urban Index is NDBI's close cousin, reaching for the longer SWIR2 band to emphasise the built environment. Where SWIR2 imagery is clean, it can sharpen the contrast between city and countryside.
The physical basis
UI plays the far-shortwave band against NIR: dry, man-made materials reflect strongly in SWIR2 while vegetation does not, so built-up areas rise and vegetation falls. Conceptually it is NDBI shifted to a longer wavelength.
How it is calculated
where SWIR2 is shortwave-infrared 2 reflectance (~2.2 µm), NIR is near-infrared reflectance.
What the numbers mean
Higher = more built-up. Interpreted like NDBI but using the longer SWIR2 band.
Band configuration
| Sentinel-2 | SWIR2 B12 · NIR B8 |
| Landsat 8/9 | SWIR2 B7 · NIR B5 |
Where it is used
It appears in urban-growth studies and land-cover classifications as a complementary built-up feature, often stacked alongside NDBI and NDVI to improve class separability.
Limitations to know
It inherits NDBI's confusion between built-up and bare soil, and SWIR2 is noisier and coarser than the visible bands — treat UI as one feature among several, not a standalone urban map.
Compute UI on your own study area
Skip the code. Draw or upload a boundary and Spatial Research Suite runs UI on live Sentinel-2 or Landsat imagery — with cloud masking, exports and citations built in.
Run this analysis in GISforus →Frequently asked
How is the Urban Index different from NDBI?
It uses the far SWIR2 band rather than SWIR1, which can sharpen built-up contrast but adds noise; the two are often used together as classification features.
Can UI be used alone to map cities?
Better not — like NDBI it confuses bare soil with built-up, so combine it with vegetation and water indices.