A Generic Framework for Library-based Mapping

of Urban Areas


A spectral library?

A spectral library is a collection of labelled spectra that is usually accompanied by at least some metadata, e.g. on how, when and where these spectra were acquired.


Spectra or spectral profiles describe the amount of sunlight that is reflected/absorbed by different surfaces. Reflectance can vary over wavelengths depending on the (bio)physical or chemical properties of the surface being measured, resulting in unique signatures that allow us to identify those surfaces (and sometimes even their condition).

If a surface type is pure, meaning it consists of only 1 material, we call it an endmember. Spectral libraries typically contain only pure endmember spectra labelled with their corresponding cover type. Labelled endmember spectra are essential for many urban mapping applications.

Endmember spectra can be measured with laboratory/field spectrometers (near or on the ground) or sampled from remote sensing imagery (from the sky). While ground measurements are typically more accurate, they are also more expensive to acquire.


High-resolution imagery, i.e. having both small pixels (<5m) and many narrow bands, is most suited for building spectral libraries. For now, such imagery is acquired with airborne sensors, although in the near future this may also become possible with spaceborne sensors ...

Check out the next page to learn how spectral libraries are used in general, and why we think it's a problem.

Or you can skip to how we think spectral libraries should be used.