# Spectral Angle Mapper (SAM) SAM measures how closely each pixel's spectrum matches one or more **reference spectra** (e.g. known minerals), by treating spectra as vectors and comparing their *angle* rather than their brightness. ## How it works Picture each spectrum — a pixel's, and a reference's — as a vector in *n*-dimensional space, where *n* is the number of bands. SAM computes the **angle between those two vectors**: ```text angle = arccos( (pixel · reference) / (‖pixel‖ ‖reference‖) ) ``` reported in degrees. Because it depends only on the *direction* of the vectors, not their length, SAM is **insensitive to illumination and brightness** — a brightly lit and a shadowed pixel of the same material point the same way and give the same (small) angle. A **smaller angle means a better match**; an angle below the per-reference threshold marks the pixel as a detection. Before comparison, each reference is interpolated onto the target's wavelength grid (linearly), bad bands are dropped, and only the chosen wavelength range is used. In **Image Cube** mode each run produces two datasets, one band per reference spectrum: - `SAM Angle, Img: ` — the angle (degrees) at every pixel. - `SAM CLS, Img: ` — a boolean classification (angle < threshold). ## Using the tool Inputs work the same way as [Spectral Feature Fitting](spectral-feature-fitting.md): 1. **Target** — choose **Image Cube** (or **Spectrum**) and select the dataset (or spectrum) to analyze. 2. **Wavelength range** — set the min/max and units to restrict the comparison. 3. **References** — check one or more reference spectra. Add them via **Add Library**, **Add Spectrum** (from a text file), or **Add Collected Spectrum**; a default USGS mineral library is preloaded to get you started. Each reference has its own threshold (**Initial Angle (°)**, default 5°), seeded from the method threshold but individually overridable. 4. Click **Run SAM**. Image-cube runs proceed in the background; spectrum runs open a ranked details table with a comparison plot.