# Observation Levels

MAG layers are not interchangeable just because they cover the same longitude and latitude. Magnetic anomalies attenuate with distance: short-wavelength, local features fade faster with height than broad regional structures. A point on Earth can therefore have several honest values depending on the observation level.

<svg width="640" height="260" viewBox="0 0 640 260" xmlns="http://www.w3.org/2000/svg" role="img" aria-label="Not to scale observation level cross-section">
  <rect x="0" y="0" width="640" height="260" fill="#f7f7f7"/>
  <path d="M0 215 C90 190 155 235 245 208 C335 180 400 230 500 198 C565 176 610 198 640 188 L640 260 L0 260 Z" fill="#d2c4a3"/>
  <path d="M0 218 C90 193 155 238 245 211 C335 183 400 233 500 201 C565 179 610 201 640 191" fill="none" stroke="#7b6d54" stroke-width="3"/>
  <line x1="80" y1="188" x2="80" y2="215" stroke="#555" stroke-dasharray="4 4"/>
  <circle cx="80" cy="188" r="5" fill="#777"/>
  <text x="100" y="192" font-family="Arial" font-size="14" fill="#333">street level ~1.2 m</text>
  <line x1="230" y1="145" x2="230" y2="210" stroke="#555" stroke-dasharray="4 4"/>
  <path d="M216 145 L230 137 L244 145 L230 153 Z" fill="#777"/>
  <text x="252" y="149" font-family="Arial" font-size="14" fill="#333">drone survey ~30-150 m</text>
  <line x1="390" y1="82" x2="390" y2="205" stroke="#2d6a4f" stroke-dasharray="4 4"/>
  <path d="M360 82 L420 82 L405 96 L375 96 Z" fill="#2d6a4f"/>
  <text x="432" y="91" font-family="Arial" font-size="14" fill="#333">aerial survey compilation</text>
  <line x1="520" y1="34" x2="520" y2="196" stroke="#1f4f9a" stroke-dasharray="4 4"/>
  <circle cx="520" cy="34" r="11" fill="#1f4f9a"/>
  <text x="540" y="39" font-family="Arial" font-size="14" fill="#333">global compilation ~4 km</text>
  <text x="20" y="28" font-family="Arial" font-size="13" fill="#666">not to scale</text>
</svg>

MAG does not blend layers across observation levels. A global compilation, an aerial survey, a future drone survey, and a future handheld street-level package are different physical views of the field. They can be compared as separate evidence, but they should not be silently merged into one value.

Aerial survey compilations usually do not have one honest height. They merge many surveys acquired with different flight plans, terrain-clearance behavior, barometric elevations, sensors, years, and re-levelling steps. When source metadata gives a component survey table, MAG records the source-backed range with a caveat. When source metadata does not state a product-wide acquisition-height range, MAG labels the range as estimated instead of inventing a source-backed number.

The current catalog has global-compilation and aerial-survey packages. No street-level or drone-level package exists yet in this catalog. Coverage at these levels is buildable - see contribution roadmap.