The map array is a key concept in many cases when developing for OpenTTD.
The GitHub docs folder contains two documents which show the arrangement of bytes in the map array, and the features they represent.
These are html documents, which GitHub does not (as of October 2019) display as html in the browser.
If you have a git checkout of OpenTTD source, the landscape docs can be opened locally in a browser from the /docs folder.
(At your own risk) they can also be viewed in the browser with a github->html converter service: