# Why feature maps are indexed by two indices?

I'm reading about convolutional neural networks. As I understood a feature map is a set of neurons (i.e like a single hidden layer in traditional ANN). So why feature maps are indexed by (i,j)? Shouldn't a feature map be indexed only by one index because it's just a set of neurons?

In the following picture layer C1 has 6 feature maps. The picture says that a feature map is 28 by 28. But why?! Why 2D?!