Towfish altitude is the placement of the towfish with in the water column.

To achieve maximum sonar image quality it is recommended that the towfish be towed at an altitude that is 10% of the sonar operating range. The 10% rule optimizes sonar geometry, maximizes range performance, improves the reception of sound backscatter thus improving image quality, and shows more bottom and less water column therefore improving bottom coverage.

For example.

If your sonar operating range is 100 meters, then the optimum towfish placement within the water column would be an altitude of 10 meters.
If you had an operating range of 200 meters, then a towfish altitude of 20 meters would advised.

If you are using a sensored towfish, the altitude can be determined directly by reading the altitude from the sensor information.
If you are using an unsensored towfish, the altitude can be determined from the displayed record on the sonar read-out. This can be done by measuring the size of the water column with scale lines, cursor, or rough approximation based on the operating sonar range.

When running survey operations in an area, care should be taken to have a rough idea of the bottom you are working over and towfish altitude should be adjusted as needed.

Towfish operating on a 100 meter range and being towed at an altitude of 10 meters