What does the word 'heath' mean in context?

Directions: Some words in the verse below have been highlighted. Select the best meaning for the corresponding word.

A little black thing in the snow,
Crying "weep! weep!" in notes of woe!
"Where are thy father and mother? Say!"
"They are both gone up to the church to pray.
"Because I was happy upon the heath,
And smiled among the winter's snow,
They clothed me in the clothes of death,
And taught me to sing the notes of woe.
"And because I am happy and dance and sing,
They think they have done me no injury,
And are gone to praise God and his priest and king,
Who make up a heaven of our misery."