Heightfields use pixel values over the white value to represent height. So what you are seeing in your attachment is the first meter of the terrain above 0. There's probably information below zero too.
I've tended to not use Heightfields these days. I generally use a Get Altitude plugged into a Linear Step, with two constants, one for the lowest level of terrain, say -200m and the second for the highest level in the terrain, say 2000m. What this outputs is a flat gradient between black and white for the full range I've indicated by the two constants. I then use this to illuminate the terrain by plugging it into the Luminosity input of a surface layer with its colour set to full white. Turn off all lighting and render an orthographic top down view of the area I want to export. Render to the rez I want, then save as an EXR which is linear in the way it holds it's colour information. Why? Because I can take this into Photoshop and visually edit it, bring it back into TG as an image map in a single colour channel, with say erosion and river masks in the other two, which can cut out a lot of render processing when you are rendering sequences.