Coordinate system¶

The Mars CRS hub¶

All MarsRecon datasets expose a single common CRS:

+proj=longlat +a=3396190 +b=3376200

This is the IAU 2000 Mars geographic CRS — ellipsoidal latitude/longitude on the IAU 2000 reference ellipsoid (semi-major 3 396 190 m, semi-minor 3 376 200 m). Longitudes are normalized to [-180°, 180°] (positive east).

Per-observation projections¶

Each individual JP2 or IMG raster uses its own per-observation Equirectangular projection centred on the local target. Reprojection into the common hub is handled automatically inside __getitem__ via rasterio.warp.transform_bounds, so calling code never needs to deal with per-observation CRSs.

Why a geographic hub?¶

Strips span large enough swaths that any single projected CRS introduces unacceptable distortion.
A geographic hub lets HiRISEGeoSampler reason about strip polygons in a single coordinate system across the planet.
The cost — non-uniform metric pixel sizes near the poles — is acceptable because HiRISE coverage at high latitudes is sparse.

Patch units¶

When using HiRISEGeoSampler, the size parameter is in CRS units:

`units`	Interpretation
`Units.CRS`	Size in degrees (geographic hub).
`Units.PIX`	Size in pixels of the source raster.

A typical training patch uses size=0.018 CRS units (~1 km at the equator).