Vertical CRS

DEM and world coordinate heights may have different vertical coordinate systems e.g. heights may be referenced to different datums (ellipsoid or geoid) and have different units. Accurate orthorectification requires conversion of heights into a common system. When both the DEM and world / ortho CRS have a defined vertical system (as a 3D CRS or a 2D+1D compound CRS), this conversion is performed automatically. Otherwise, DEM and world vertical systems are assumed the same, and no conversion is performed.

Note

Public and commercial DEMs seldom have a defined vertical coordinate system.

As a brief introduction to defining 3D or 2D+1D CRSs with vertical coordinate systems, it is easiest to use EPSG codes when they exist for the CRS. The basic form for an EPSG CRS string is EPSG:<code>. Compound 2D+1D CRSs can be defined with EPSG:<horizontal code>+<vertical code>. CRSs can be specified like this with the Orthority command line and API, or with third party tools for editing a raster’s metadata e.g. Rasterio’s rio edit-info. Equivalent proj4 or WKT strings can also be used.

Note

Orthority uses the network capabilities of the PROJ library to transform vertical CRSs by setting the PROJ_NETWORK environment variable to 'ON'. See the PROJ documentation for details.