I bet it means channel
A map or diagram starts with ch is a chart.
"Ch" on an Ordnance Survey map typically stands for "church." It marks the location of a church or a place of worship on the map.
Church
Ch can stand for channel as in television channel.
CH is the international badge for Switzerland, or Helvetia.
marks on a map that stand for real places or real features on a map
It is country domain, like "us" is for United States, "CH" is for Switzerland
Danzig helvetia
Measure of Academic Progress
n on a map stands for north on a compass
On an OS map, it's a Public House