Extract pixel value with libpng in C++

Ok, so I have a squared (1024x1024) pixel 8-bit grayscale image I'm trying to use as the "landscape" (it's a terrain height image) and all I'm trying to do is just get something like

byte = pngimg.at(x, y)

I've been scouring l;ibpng's pages and googling but nothing is showing up. In java it's as simple as a buffered image. Where is the C++ equivalent for a png?