As part of our project on spatial behavior in tropical frogs, we are trying to relate rainforest understory openness/visibility with homing orientation accuracy. We are using a photographic "cover board" method adapted from vegetation density literature to quantify the visibility (i.e., the percentage of the reference board covered by vegetation at a given distance). However, I'm having hard time finding any references using this (or similar) approaches for studying animal navigation. Has anyone tried something similar or knows of good references? Are there good reasons why this method doesn't seem to be often used for studying animal orientation? Or do I simply have my keywords wrong?... I'd appreciate any suggestions. Thanks!