Are 'Nigerian Colony Coordinate System', MTM (Modified Transverse Mercator) and NTM (Nigerian Transverse Mercator) one and the same? How can I transform coordinates on 'Nigerian Colony Coordinate System' to UTM/WGS84?
The Nigerian Transverse Mercator system combines three UTM zones to a contiguous coordinate system:
NTM uses meters as units, so you have to convert your feet data to meters before. The result 326262 North 572475 East can be found in the Minna Mid Belt zone. As an alternative, you could create a custom CRS with feet as units:
+proj=tmerc +lat_0=4 +lon_0=8.5 +k=0.99975 +x_0=670553.98 +y_0=0 +ellps=clrk80 +towgs84=-92,-93,122,0,0,0,0 +units=us-ft +no_defs
As a first guess, I have chosen US survey feet, but British surveyors may have taken a different conversion factor.
More information on Nigerian surveying can be found in http://www.asprs.org/a/resources/grids/02-2009-nigeria.pdf and http://icaci.org/files/documents/ICC_proceedings/ICC2005/htm/pdf/oral/TEMA2/Session%201/DR.%20M.E.%20UFUAH%202.pdf
Use the details for MTM in the document at the link above. You will need to create a transformation from MTM to UTM using the details of each projection.