# Altimetry or Bathymetry File

The format is the same for top altimetry or bottom bathymetry. Only BELLHOP and SimplePE use these files. (SimplePE allows bathymetry variations, but not altimetry variations.)

Syntax:

TYPE
NPOINTS
R(1) Z(1)
R(2) Z(2)
.
.
.
R(NPOINTS) Z(NPOINTS)

Description:

TYPE(1:1)
'L' for piecewise linear
'C' for curvilinear
TYPE(2:2)
'S' Short format (bathymetry only) (default)
'L' Long format (bathymetry and geoacoustic parameters)

NPOINTS: Number of r-z points to follow.
R(): Range (km)
Z(): Depth (m).

Example (bathymetry only):

'L'
5
0 3000
10 3000
20 500
30 3000
100 3000

Example (bathymetry and geoacoustics):

'LL'
3
0 100 1700 0.0 1.2 0.5 0
2.5 100 1550 0.0 1.2 0.5 0
5.0 100 1550 0.0 1.2 0.5 0

Generally the piecewise linear option should be selected. However, if the boundaries are sampled on a very fine scale (a few wavelengths) then the curvilinear option produces a smoother fit. That in turn improves the results obtained with geometric ray tracing. The curvilinear option was implemented to treat effects of fine-scale scattering.
Note that depths, z, are positive downward, following a convention used throughout the models. This is a bit counter-intuitive for altimetry; however, a different convention for that would also have been counter-intuitive for others.
The vector of range values must increase strictly monotonically.
The second letter of the TYPE string is used to select a range-dependent bottom type. If you set it 'L' additional entries will be read with that geoacoustic information given in the same format as used for halfspaces in the environmental file (compressional wave speed, compressional wave attenuation, density, shear wave speed, shear wave attenuation).