Note: Information moved to: https://larsoft.org/important-concepts-in-larsoft/geometry/ on 12/13/21.

Thanks to the help of Erica Snider, Gianluca Petrillo and Thomas Junk, we have an updated Geometry description available in the LArSoft wiki here.  The geometry package contains classes related to the geometry representation such as planes, TPCs, cryostats, etc. The LArSoft geometry provides descriptions of the physical structures and materials in the detector. Some important specifiable parameters in the detector geometry include:

• the position of the detector relative to the beam
• the structure and material properties of the cathode planes
• where individual photon detectors are
• the placement of wires and the distance between them within a plane
• the distance between the wire planes
• the details of the material surrounding the cryostat
• the composition of the overburden
• transformations between coordinate systems attached to various elements and global coordinates

The geometry also provides a mapping between sensing elements such as wires or strips and DAQ channels.

LArSoft release 6.28 changed the geometry to support dual-phase TPCs, which caused several assumptions to be removed or to change:

• the drift direction is no longer assumed to be along x, but can be on any axis
• the projection of a point on a plane is no longer assumed to have coordinates (y,z)
• views no longer are assumed to measure a coordinate growing with z
• the outer plane cannot be assumed to have drift coordinate 0 (same as drift distance)

When updating code, understanding the assumptions at the time the code was written may help explain why certain options were chosen.

For more information, please go to the LArSoft wiki here.

–Katherine Lato