add comments with info and links for ISG

Author: jonathonpsmith

geodepy/constants.py

@@ -66,6 +66,10 @@ def __init__(self, falseeast, falsenorth, cmscale, zonewidth, initialcm):
 
 
 utm = Projection(500000, 10000000, 0.9996, 6, -177)
+
+# Integrated Survey Grid - used in NSW as the projection for AGD66
+# Spatial Services projections page - https://www.spatial.nsw.gov.au/surveying/geodesy/projections
+# ISG Technical Manual - https://www.spatial.nsw.gov.au/__data/assets/pdf_file/0017/25730/ISG.pdf
 isg = Projection(300000, 5000000, 0.99994, 2, -177)