ICS is a fundamental parameter in the fort.15 file that defines the coordinate system and the desired projection. The value of ICS also has an important consequence for the choice of the Coriolis CORI parameter of the fort.15 file.
Available ICS Values
ICS Value

Shortname

Description

1

Cartesian

Points in the fort.14 are already mapped onto an arbitrary Cartesian coordinate system, e.g., UTM. Also useful for idealized problems.

2

Geographic, CPP, no curvature

Points in the fort.14 are specified in geographic coordinates, which will be projected using the CPP (equidistant) cylindrical mapping by ADCIRC. The curvature of the Earth is not accounted for.

20

Geographic, Equalarea

Points in the fort.14 are specified in geographic coordinates, which will be projected using the Equalarea cylindrical mapping by ADCIRC. The curvature of the Earth is correctly accounted for.

21

Geographic, CPP

Points in the fort.14 are specified in geographic coordinates, which will be projected using the CPP (equidistant) cylindrical mapping by ADCIRC. The curvature of the Earth is correctly accounted for.

22

Geographic, Mercator

Points in the fort.14 are specified in geographic coordinates, which will be projected using the Mercator (conformal) cylindrical mapping by ADCIRC. The curvature of the Earth is correctly accounted for.

23

Geographic, Miller

Points in the fort.14 are specified in geographic coordinates, which will be projected using the Miller cylindrical mapping by ADCIRC. The curvature of the Earth is correctly accounted for.

24

Geographic, GallStereographic

Points in the fort.14 are specified in geographic coordinates, which will be projected using the GallStereographic cylindrical mapping by ADCIRC. The curvature of the Earth is correctly accounted for.

Implications
Beginning from Version 55, ICS values equal to 2024 will be possible. These are intended to replace the old method of specifying ICS = 2, but this option is retained for regression testing. When ICS = 2, the curvature of the Earth is not correctly accounted for in the Spherical coordinate form of the governing equations which becomes more important as the geographic size of the computational domain size increases. In recent years global modeling using ADCIRC has been successful^{[1]}, e.g., [1] where it was found that the old method was deficient. The new options using values ICS = 2024 now account for the curvature correctly, and should in general be always used on geographical domains (ICS = 1 should still be used for Cartesian coordinate domains). ICS = 22 is particularly attractive because it uses a conformal mapping (Mercator) that conserve the angles on the spherical Earth, but testing has generally found that all choices of ICS = 2024 give effectively the same answers.
Negative ICS Value for Rotation
The values 2024 can also be set to a negative value (i.e, 20, 21, ...) to implement an arbitrary rotation of the geographic coordinates by ADCIRC. This is primarily used to ensure that Earth's poles are rotated onto land to eliminate the singularity in the Spherical coordinate form of the governing equations. ADCIRC's outputs will be displayed on the original unrotated coordinate system. The rotation is set by the fort.rotm input file (see the link for example formats).
References
 ↑ Pringle et al., Global OceantoCoastal Storm Tide Modeling in ADCIRC v55: Unstructured Mesh Design, in preparation (2020)