I'm looking at an 1851 map showing Montgomery County Pennsylvania, which shows areas in acres and perches. Seems to me that the perch should be added to the calculation table for area.
Here's the definition, from Russ Rowlett's web page:
perch [2]
a unit of area equal to one square perch [1]. A perch of area covers exactly 272.25 square feet or about 25.292 85 square meters. There are 40 perches in a rood and 160 perches in an acre.
- Max