Is TVG based on range or depth??

[DeepSouthTX]

See below screen shot. I was in manual depth coming back inshore from the shelf. The range was 1200ft then I changed it to 500. And just changing that changed the return. I no longer showed any return in the upper part of the water column. And the bottom return thinned a little. I would not think just changing the effecting range while being in the exact same depth of water would do this?? I was on manual gain as well.

 

[Snips]

DeepSouthTX,
The sounders depth range and pulse length are tied together. The pulse length and TVG are also linked. Below is a screen shot how changing the depth changes the pulse length which in turn affects TVG in a test tank.

Snips

 

[DeepSouthTX]

Okay not what I expected but obviously something was up. No big deal I will just need to make sure I take that into account and adjust range consisitently